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NAME
perlfunc - Perl 內部函式
描述 DESCRIPTION
這一章裡的函式可以作為表示式來使用。 Perl 中的函式主要分為兩類：陣列運算子和命名的一元運算子。 不同之處在
於他們的優先順序關係。(參閱 perlop 中的優先順序表 ) 陣列運算子需要一個以上的引數，而一元運算子不能超過一
個引數。 因此，一個逗號將結束一個一元運算子, 但對於陣列運算子，只是起到分隔的作用。 一元運算子一般只提供
一個標量作為引數，而陣列運算子可能會提供標量或者陣列作為引數。 如果二者都有，標量引數一般在前面，陣列引數
跟在後面。 (注意，可以只有一個數組變數) 例如, splice() 有三個標量變數，後面加上一個陣列, 相反
gethostbyname() 有四個標量變數。
在語法描述中,陣列運算子需要一個用LIST標識的陣列作為引數。 這些 陣列可能由標量引數和陣列值混合組成; 陣列值
將包含在陣列中，每個元素被插入陣列中, 形成一個更長一維的陣列值。 陣列的元素應該用逗號分開。
下面列出的任何函式可以在引數兩邊有括號，也可以沒有。(語法描述中省略括號) 如果你使用括號，一個簡單的規則是
(偶爾會令人吃驚): 如果是函式,沒有優先順序的問題；如果它是一個數組運算子或者一元運算子 那麼就要考慮優先順
序。並且，函式兩邊的空白和 "（" 是不計算的－－因此， 有時候需要小心行事。看看下面的幾個例子：
print 1+2+4; # Prints 7.
print(1+2) + 4; # Prints 3.
print (1+2)+4; # Also prints 3!
print +(1+2)+4; # Prints 7.
print ((1+2)+4); # Prints 7.
前面說得似乎有點抽象，那麼你在執行PERL時帶上-w開關你將得到一些 警告資訊，您可以根據這些資訊再體會一下。例
如，上面的例子會產生如下資訊：
print (...) interpreted as function at - line 1.
Useless use of integer addition in void context at - line 1.
有些函式根本不需要引數，因此工作方式和一元運算子和陣列運算子都不同， "time" 和 "endpwent" 算是兩個典型吧.
如, "time+86_400" 實際上是 "time() + 86_400".
對於可以用在標量或者陣列的上下文中的函式,非失敗性的錯誤在標量環境下 通常暗示返回了未定義的值或在陣列環境
下返回一個空的陣列。
記住下面的重要原則： 沒有規則和陣列環境下的表示式的行為和他的標量環境的行為有關係，反之亦然。 這可能產生
兩種完全不同的情況。在標量環境下，每個運算子和函式決定怎樣以最恰當的次序返回值。 有些運算子在陣列環境下返
回陣列的長度.，有些運算子返回的一個元素，有些返回陣列中的最後一個元素，有些返回成功執行的操作的語句數。通
常，他們返回一些你需要的值，除非你需要連續性。
在標量環境下的命名陣列在第一眼看上去時和在標量環境下的列表有很大的不同。 在標量環境下，你不能得到一個像
"(1,2,3)" 的列表,因為在編譯時，編譯器是知道當前環境的，它將在那裡產生標量的逗號運算子， 而不是用於分隔陣
列元素的逗號. 也就是說，它永遠不會以一個數組開始。
一般說來， PERL中的函式對應相應的系統呼叫 (如chown(2), fork(2), closedir(2), 等等.) 成功呼叫後返回真
值，否則返回 "undef" , 下面將會提到。這一點和C的介面不一樣，C中出錯時將返回"-1" .但是也有幾個例外，他們是
"wait", "waitpid", 和 "syscall" 。 系統調用出錯時出錯資訊將透過特殊變數$!返回。其他的函式則不會，除非發生
意外。
函式分類 Perl Functions by Category
下面是Perl中的函式(包括看起來像函式的，如某些關鍵詞，命名運算子)的分類. 有些函式在多處出現了。
標量和字串函式 Functions for SCALARs or strings
"chomp", "chop", "chr", "crypt", "hex", "index", "lc", "lcfirst", "length", "oct", "ord", "pack",
"q/STRING/", "qq/STRING/", "reverse", "rindex", "sprintf", "substr", "tr///", "uc", "ucfirst", "y///"
正則表示式和模式匹配 Regular expressions and pattern matching
"m//", "pos", "quotemeta", "s///", "split", "study", "qr//"
數字運算 Numeric functions
"abs", "atan2", "cos", "exp", "hex", "int", "log", "oct", "rand", "sin", "sqrt", "srand"
真實陣列函式 Functions for real @ARRAYs
"pop", "push", "shift", "splice", "unshift"
列表資料函式 Functions for list data
"grep", "join", "map", "qw/STRING/", "reverse", "sort", "unpack"
真實雜湊函式 Functions for real %HASHes
"delete", "each", "exists", "keys", "values"
輸入輸出 Input and output functions
"binmode", "close", "closedir", "dbmclose", "dbmopen", "die", "eof", "fileno", "flock", "format",
"getc", "print", "printf", "read", "readdir", "rewinddir", "seek", "seekdir", "select", "syscall",
"sysread", "sysseek", "syswrite", "tell", "telldir", "truncate", "warn", "write"
定長的資料或記錄 Functions for fixed length data or records
"pack", "read", "syscall", "sysread", "syswrite", "unpack", "vec"
檔案目錄控制 Functions for filehandles, files, or directories
"-X", "chdir", "chmod", "chown", "chroot", "fcntl", "glob", "ioctl", "link", "lstat", "mkdir",
"open", "opendir", "readlink", "rename", "rmdir", "stat", "symlink", "sysopen", "umask", "unlink",
"utime"
流控制關鍵詞 Keywords related to the control flow of your perl program
"caller", "continue", "die", "do", "dump", "eval", "exit", "goto", "last", "next", "redo", "return",
"sub", "wantarray"
作用域關鍵詞 Keywords related to scoping
"caller", "import", "local", "my", "our", "package", "use"
雜項 Miscellaneous functions
"defined", "dump", "eval", "formline", "local", "my", "our", "reset", "scalar", "undef", "wantarray"
程序和程序組 Functions for processes and process groups
"alarm", "exec", "fork", "getpgrp", "getppid", "getpriority", "kill", "pipe", "qx/STRING/",
"setpgrp", "setpriority", "sleep", "system", "times", "wait", "waitpid"
模組關鍵詞 Keywords related to perl modules
"do", "import", "no", "package", "require", "use"
類和麵向物件關鍵詞 Keywords related to classes and object-orientedness
"bless", "dbmclose", "dbmopen", "package", "ref", "tie", "tied", "untie", "use"
底層 socket 函式 Low-level socket functions
"accept", "bind", "connect", "getpeername", "getsockname", "getsockopt", "listen", "recv", "send",
"setsockopt", "shutdown", "socket", "socketpair"
SysV 程序間通訊 System V interprocess communication functions
"msgctl", "msgget", "msgrcv", "msgsnd", "semctl", "semget", "semop", "shmctl", "shmget", "shmread",
"shmwrite"
獲取使用者資訊 Fetching user and group info
"endgrent", "endhostent", "endnetent", "endpwent", "getgrent", "getgrgid", "getgrnam", "getlogin",
"getpwent", "getpwnam", "getpwuid", "setgrent", "setpwent"
獲取網路資訊 Fetching network info
"endprotoent", "endservent", "gethostbyaddr", "gethostbyname", "gethostent", "getnetbyaddr",
"getnetbyname", "getnetent", "getprotobyname", "getprotobynumber", "getprotoent", "getservbyname",
"getservbyport", "getservent", "sethostent", "setnetent", "setprotoent", "setservent"
時間函式 Time-related functions
"gmtime", "localtime", "time", "times"
PERL5中的新函式 Functions new in perl5
"abs", "bless", "chomp", "chr", "exists", "formline", "glob", "import", "lc", "lcfirst", "map", "my",
"no", "our", "prototype", "qx", "qw", "readline", "readpipe", "ref", "sub*", "sysopen", "tie",
"tied", "uc", "ucfirst", "untie", "use"
* - "sub" was a keyword in perl4, but in perl5 it is an operator, which can be used in expressions.
過時的函式 Functions obsoleted in perl5
"dbmclose", "dbmopen"
可移植性 Portability
Perl 誕生於UNIX，因此可以訪問所有的一般系統呼叫。 在非UNIX環境中，某些UNIX下有的呼叫是沒有實現的，或者有
輕微的區別。受到影響的有：
"-X", "binmode", "chmod", "chown", "chroot", "crypt", "dbmclose", "dbmopen", "dump", "endgrent",
"endhostent", "endnetent", "endprotoent", "endpwent", "endservent", "exec", "fcntl", "flock", "fork",
"getgrent", "getgrgid", "gethostbyname", "gethostent", "getlogin", "getnetbyaddr", "getnetbyname",
"getnetent", "getppid", "getprgp", "getpriority", "getprotobynumber", "getprotoent", "getpwent",
"getpwnam", "getpwuid", "getservbyport", "getservent", "getsockopt", "glob", "ioctl", "kill", "link",
"lstat", "msgctl", "msgget", "msgrcv", "msgsnd", "open", "pipe", "readlink", "rename", "select",
"semctl", "semget", "semop", "setgrent", "sethostent", "setnetent", "setpgrp", "setpriority",
"setprotoent", "setpwent", "setservent", "setsockopt", "shmctl", "shmget", "shmread", "shmwrite",
"socket", "socketpair", "stat", "symlink", "syscall", "sysopen", "system", "times", "truncate", "umask",
"unlink", "utime", "wait", "waitpid"
參見 perlport 和其他平臺的說明文件以獲得更多關於移植性的資料
按字母順序排列的PERL函式 Alphabetical Listing of Perl Functions
-X FILEHANDLE
-X EXPR
-X A file test, where X is one of the letters listed below. This unary operator takes one argument,
either a filename or a filehandle, and tests the associated file to see if something is true
about it. If the argument is omitted, tests $_, except for "-t", which tests STDIN. Unless
otherwise documented, it returns 1 for true and '' for false, or the undefined value if the file
doesn't exist. Despite the funny names, precedence is the same as any other named unary
operator, and the argument may be parenthesized like any other unary operator. The operator may
be any of:
-r File is readable by effective uid/gid.
-w File is writable by effective uid/gid.
-x File is executable by effective uid/gid.
-o File is owned by effective uid.
-R File is readable by real uid/gid.
-W File is writable by real uid/gid.
-X File is executable by real uid/gid.
-O File is owned by real uid.
-e File exists.
-z File has zero size (is empty).
-s File has nonzero size (returns size in bytes).
-f File is a plain file.
-d File is a directory.
-l File is a symbolic link.
-p File is a named pipe (FIFO), or Filehandle is a pipe.
-S File is a socket.
-b File is a block special file.
-c File is a character special file.
-t Filehandle is opened to a tty.
-u File has setuid bit set.
-g File has setgid bit set.
-k File has sticky bit set.
-T File is an ASCII text file (heuristic guess).
-B File is a "binary" file (opposite of -T).
-M Script start time minus file modification time, in days.
-A Same for access time.
-C Same for inode change time (Unix, may differ for other platforms)
Example:
while (<>) {
chomp;
next unless -f $_; # ignore specials
#...
}
The interpretation of the file permission operators "-r", "-R", "-w", "-W", "-x", and "-X" is by
default based solely on the mode of the file and the uids and gids of the user. There may be
other reasons you can't actually read, write, or execute the file. Such reasons may be for
example network filesystem access controls, ACLs (access control lists), read-only filesystems,
and unrecognized executable formats.
Also note that, for the superuser on the local filesystems, the "-r", "-R", "-w", and "-W" tests
always return 1, and "-x" and "-X" return 1 if any execute bit is set in the mode. Scripts run
by the superuser may thus need to do a stat() to determine the actual mode of the file, or
temporarily set their effective uid to something else.
If you are using ACLs, there is a pragma called "filetest" that may produce more accurate results
than the bare stat() mode bits. When under the "use filetest 'access'" the above-mentioned
filetests will test whether the permission can (not) be granted using the access() family of
system calls. Also note that the "-x" and "-X" may under this pragma return true even if there
are no execute permission bits set (nor any extra execute permission ACLs). This strangeness is
due to the underlying system calls' definitions. Read the documentation for the "filetest"
pragma for more information.
Note that "-s/a/b/" does not do a negated substitution. Saying "-exp($foo)" still works as
expected, however--only single letters following a minus are interpreted as file tests.
The "-T" and "-B" switches work as follows. The first block or so of the file is examined for
odd characters such as strange control codes or characters with the high bit set. If too many
strange characters (>30%) are found, it's a "-B" file, otherwise it's a "-T" file. Also, any
file containing null in the first block is considered a binary file. If "-T" or "-B" is used on
a filehandle, the current IO buffer is examined rather than the first block. Both "-T" and "-B"
return true on a null file, or a file at EOF when testing a filehandle. Because you have to read
a file to do the "-T" test, on most occasions you want to use a "-f" against the file first, as
in "next unless -f $file && -T $file".
If any of the file tests (or either the "stat" or "lstat" operators) are given the special
filehandle consisting of a solitary underline, then the stat structure of the previous file test
(or stat operator) is used, saving a system call. (This doesn't work with "-t", and you need to
remember that lstat() and "-l" will leave values in the stat structure for the symbolic link, not
the real file.) (Also, if the stat buffer was filled by a "lstat" call, "-T" and "-B" will reset
it with the results of "stat _"). Example:
print "Can do.\n" if -r $a ⎪⎪ -w _ ⎪⎪ -x _;
stat($filename);
print "Readable\n" if -r _;
print "Writable\n" if -w _;
print "Executable\n" if -x _;
print "Setuid\n" if -u _;
print "Setgid\n" if -g _;
print "Sticky\n" if -k _;
print "Text\n" if -T _;
print "Binary\n" if -B _;
abs VALUE
abs Returns the absolute value of its argument. If VALUE is omitted, uses $_.
accept NEWSOCKET,GENERICSOCKET
Accepts an incoming socket connect, just as the accept(2) system call does. Returns the packed
address if it succeeded, false otherwise. See the example in "Sockets: Client/Server
Communication" in perlipc.
On systems that support a close-on-exec flag on files, the flag will be set for the newly opened
file descriptor, as determined by the value of $^F. See "$^F" in perlvar.
alarm SECONDS
alarm Arranges to have a SIGALRM delivered to this process after the specified number of wallclock
seconds have elapsed. If SECONDS is not specified, the value stored in $_ is used. (On some
machines, unfortunately, the elapsed time may be up to one second less or more than you specified
because of how seconds are counted, and process scheduling may delay the delivery of the signal
even further.)
Only one timer may be counting at once. Each call disables the previous timer, and an argument
of 0 may be supplied to cancel the previous timer without starting a new one. The returned value
is the amount of time remaining on the previous timer.
For delays of finer granularity than one second, you may use Perl's four-argument version of
select() leaving the first three arguments undefined, or you might be able to use the "syscall"
interface to access setitimer(2) if your system supports it. The Time::HiRes module (from CPAN,
and starting from Perl 5.8 part of the standard distribution) may also prove useful.
It is usually a mistake to intermix "alarm" and "sleep" calls. ("sleep" may be internally
implemented in your system with "alarm")
If you want to use "alarm" to time out a system call you need to use an "eval"/"die" pair. You
can't rely on the alarm causing the system call to fail with $! set to "EINTR" because Perl sets
up signal handlers to restart system calls on some systems. Using "eval"/"die" always works,
modulo the caveats given in "Signals" in perlipc.
eval {
local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
alarm $timeout;
$nread = sysread SOCKET, $buffer, $size;
alarm 0;
};
if ($@) {
die unless $@ eq "alarm\n"; # propagate unexpected errors
# timed out
}
else {
# didn't
}
For more information see perlipc.
atan2 Y,X
Returns the arctangent of Y/X in the range -PI to PI.
For the tangent operation, you may use the "Math::Trig::tan" function, or use the familiar
relation:
sub tan { sin($_[0]) / cos($_[0]) }
bind SOCKET,NAME
Binds a network address to a socket, just as the bind system call does. Returns true if it
succeeded, false otherwise. NAME should be a packed address of the appropriate type for the
socket. See the examples in "Sockets: Client/Server Communication" in perlipc.
binmode FILEHANDLE, LAYER
binmode FILEHANDLE
Arranges for FILEHANDLE to be read or written in "binary" or "text" mode on systems where the
run-time libraries distinguish between binary and text files. If FILEHANDLE is an expression,
the value is taken as the name of the filehandle. Returns true on success, otherwise it returns
"undef" and sets $! (errno).
On some systems (in general, DOS and Windows-based systems) binmode() is necessary when you're
not working with a text file. For the sake of portability it is a good idea to always use it
when appropriate, and to never use it when it isn't appropriate. Also, people can set their I/O
to be by default UTF-8 encoded Unicode, not bytes.
In other words: regardless of platform, use binmode() on binary data, like for example images.
If LAYER is present it is a single string, but may contain multiple directives. The directives
alter the behaviour of the file handle. When LAYER is present using binmode on text file makes
sense.
If LAYER is omitted or specified as ":raw" the filehandle is made suitable for passing binary
data. This includes turning off possible CRLF translation and marking it as bytes (as opposed to
Unicode characters). Note that as despite what may be implied in "Programming Perl" (the Camel)
or elsewhere ":raw" is not the simply inverse of ":crlf" -- other layers which would affect
binary nature of the stream are also disabled. See PerlIO, perlrun and the discussion about the
PERLIO environment variable.
The ":bytes", ":crlf", and ":utf8", and any other directives of the form ":...", are called I/O
layers. The "open" pragma can be used to establish default I/O layers. See open.
The LAYER parameter of the binmode() function is described as "DISCIPLINE" in "Programming Perl,
3rd Edition". However, since the publishing of this book, by many known as "Camel III", the
consensus of the naming of this functionality has moved from "discipline" to "layer". All
documentation of this version of Perl therefore refers to "layers" rather than to "disciplines".
Now back to the regularly scheduled documentation...
To mark FILEHANDLE as UTF-8, use ":utf8".
In general, binmode() should be called after open() but before any I/O is done on the filehandle.
Calling binmode() will normally flush any pending buffered output data (and perhaps pending input
data) on the handle. An exception to this is the ":encoding" layer that changes the default
character encoding of the handle, see open. The ":encoding" layer sometimes needs to be called
in mid-stream, and it doesn't flush the stream. The ":encoding" also implicitly pushes on top of
itself the ":utf8" layer because internally Perl will operate on UTF-8 encoded Unicode
characters.
The operating system, device drivers, C libraries, and Perl run-time system all work together to
let the programmer treat a single character ("\n") as the line terminator, irrespective of the
external representation. On many operating systems, the native text file representation matches
the internal representation, but on some platforms the external representation of "\n" is made up
of more than one character.
Mac OS, all variants of Unix, and Stream_LF files on VMS use a single character to end each line
in the external representation of text (even though that single character is CARRIAGE RETURN on
Mac OS and LINE FEED on Unix and most VMS files). In other systems like OS/2, DOS and the various
flavors of MS-Windows your program sees a "\n" as a simple "\cJ", but what's stored in text files
are the two characters "\cM\cJ". That means that, if you don't use binmode() on these systems,
"\cM\cJ" sequences on disk will be converted to "\n" on input, and any "\n" in your program will
be converted back to "\cM\cJ" on output. This is what you want for text files, but it can be
disastrous for binary files.
Another consequence of using binmode() (on some systems) is that special end-of-file markers will
be seen as part of the data stream. For systems from the Microsoft family this means that if
your binary data contains "\cZ", the I/O subsystem will regard it as the end of the file, unless
you use binmode().
binmode() is not only important for readline() and print() operations, but also when using
read(), seek(), sysread(), syswrite() and tell() (see perlport for more details). See the $/ and
"$\" variables in perlvar for how to manually set your input and output line-termination
sequences.
bless REF,CLASSNAME
bless REF
This function tells the thingy referenced by REF that it is now an object in the CLASSNAME
package. If CLASSNAME is omitted, the current package is used. Because a "bless" is often the
last thing in a constructor, it returns the reference for convenience. Always use the two-
argument version if the function doing the blessing might be inherited by a derived class. See
perltoot and perlobj for more about the blessing (and blessings) of objects.
Consider always blessing objects in CLASSNAMEs that are mixed case. Namespaces with all
lowercase names are considered reserved for Perl pragmata. Builtin types have all uppercase
names, so to prevent confusion, you may wish to avoid such package names as well. Make sure that
CLASSNAME is a true value.
See "Perl Modules" in perlmod.
caller EXPR
caller Returns the context of the current subroutine call. In scalar context, returns the caller's
package name if there is a caller, that is, if we're in a subroutine or "eval" or "require", and
the undefined value otherwise. In list context, returns
($package, $filename, $line) = caller;
With EXPR, it returns some extra information that the debugger uses to print a stack trace. The
value of EXPR indicates how many call frames to go back before the current one.
($package, $filename, $line, $subroutine, $hasargs,
$wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
Here $subroutine may be "(eval)" if the frame is not a subroutine call, but an "eval". In such a
case additional elements $evaltext and $is_require are set: $is_require is true if the frame is
created by a "require" or "use" statement, $evaltext contains the text of the "eval EXPR"
statement. In particular, for an "eval BLOCK" statement, $filename is "(eval)", but $evaltext is
undefined. (Note also that each "use" statement creates a "require" frame inside an "eval EXPR"
frame.) $subroutine may also be "(unknown)" if this particular subroutine happens to have been
deleted from the symbol table. $hasargs is true if a new instance of @_ was set up for the
frame. $hints and $bitmask contain pragmatic hints that the caller was compiled with. The
$hints and $bitmask values are subject to change between versions of Perl, and are not meant for
external use.
Furthermore, when called from within the DB package, caller returns more detailed information: it
sets the list variable @DB::args to be the arguments with which the subroutine was invoked.
Be aware that the optimizer might have optimized call frames away before "caller" had a chance to
get the information. That means that caller(N) might not return information about the call frame
you expect it do, for "N > 1". In particular, @DB::args might have information from the previous
time "caller" was called.
chdir EXPR
Changes the working directory to EXPR, if possible. If EXPR is omitted, changes to the directory
specified by $ENV{HOME}, if set; if not, changes to the directory specified by $ENV{LOGDIR}.
(Under VMS, the variable $ENV{SYS$LOGIN} is also checked, and used if it is set.) If neither is
set, "chdir" does nothing. It returns true upon success, false otherwise. See the example under
"die".
chmod LIST
Changes the permissions of a list of files. The first element of the list must be the numerical
mode, which should probably be an octal number, and which definitely should not a string of octal
digits: 0644 is okay, '0644' is not. Returns the number of files successfully changed. See also
"oct", if all you have is a string.
$cnt = chmod 0755, 'foo', 'bar';
chmod 0755, @executables;
$mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
# --w----r-T
$mode = '0644'; chmod oct($mode), 'foo'; # this is better
$mode = 0644; chmod $mode, 'foo'; # this is best
You can also import the symbolic "S_I*" constants from the Fcntl module:
use Fcntl ':mode';
chmod S_IRWXU⎪S_IRGRP⎪S_IXGRP⎪S_IROTH⎪S_IXOTH, @executables;
# This is identical to the chmod 0755 of the above example.
chomp VARIABLE
chomp( LIST )
chomp This safer version of "chop" removes any trailing string that corresponds to the current value of
$/ (also known as $INPUT_RECORD_SEPARATOR in the "English" module). It returns the total number
of characters removed from all its arguments. It's often used to remove the newline from the end
of an input record when you're worried that the final record may be missing its newline. When in
paragraph mode ("$/ = """), it removes all trailing newlines from the string. When in slurp mode
("$/ = undef") or fixed-length record mode ($/ is a reference to an integer or the like, see
perlvar) chomp() won't remove anything. If VARIABLE is omitted, it chomps $_. Example:
while (<>) {
chomp; # avoid \n on last field
@array = split(/:/);
# ...
}
If VARIABLE is a hash, it chomps the hash's values, but not its keys.
You can actually chomp anything that's an lvalue, including an assignment:
chomp($cwd = `pwd`);
chomp($answer = <STDIN>);
If you chomp a list, each element is chomped, and the total number of characters removed is
returned.
Note that parentheses are necessary when you're chomping anything that is not a simple variable.
This is because "chomp $cwd = `pwd`;" is interpreted as "(chomp $cwd) = `pwd`;", rather than as
"chomp( $cwd = `pwd` )" which you might expect. Similarly, "chomp $a, $b" is interpreted as
"chomp($a), $b" rather than as "chomp($a, $b)".
chop VARIABLE
chop( LIST )
chop Chops off the last character of a string and returns the character chopped. It is much more
efficient than "s/.$//s" because it neither scans nor copies the string. If VARIABLE is omitted,
chops $_. If VARIABLE is a hash, it chops the hash's values, but not its keys.
You can actually chop anything that's an lvalue, including an assignment.
If you chop a list, each element is chopped. Only the value of the last "chop" is returned.
Note that "chop" returns the last character. To return all but the last character, use
"substr($string, 0, -1)".
See also "chomp".
chown LIST
Changes the owner (and group) of a list of files. The first two elements of the list must be the
numeric uid and gid, in that order. A value of -1 in either position is interpreted by most
systems to leave that value unchanged. Returns the number of files successfully changed.
$cnt = chown $uid, $gid, 'foo', 'bar';
chown $uid, $gid, @filenames;
Here's an example that looks up nonnumeric uids in the passwd file:
print "User: ";
chomp($user = <STDIN>);
print "Files: ";
chomp($pattern = <STDIN>);
($login,$pass,$uid,$gid) = getpwnam($user)
or die "$user not in passwd file";
@ary = glob($pattern); # expand filenames
chown $uid, $gid, @ary;
On most systems, you are not allowed to change the ownership of the file unless you're the
superuser, although you should be able to change the group to any of your secondary groups. On
insecure systems, these restrictions may be relaxed, but this is not a portable assumption. On
POSIX systems, you can detect this condition this way:
use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
$can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
chr NUMBER
chr Returns the character represented by that NUMBER in the character set. For example, "chr(65)" is
"A" in either ASCII or Unicode, and chr(0x263a) is a Unicode smiley face. Note that characters
from 128 to 255 (inclusive) are by default not encoded in UTF-8 Unicode for backward
compatibility reasons (but see encoding).
If NUMBER is omitted, uses $_.
For the reverse, use "ord".
Note that under the "bytes" pragma the NUMBER is masked to the low eight bits.
See perlunicode and encoding for more about Unicode.
chroot FILENAME
chroot This function works like the system call by the same name: it makes the named directory the new
root directory for all further pathnames that begin with a "/" by your process and all its
children. (It doesn't change your current working directory, which is unaffected.) For security
reasons, this call is restricted to the superuser. If FILENAME is omitted, does a "chroot" to
$_.
close FILEHANDLE
close Closes the file or pipe associated with the file handle, returning true only if IO buffers are
successfully flushed and closes the system file descriptor. Closes the currently selected
filehandle if the argument is omitted.
You don't have to close FILEHANDLE if you are immediately going to do another "open" on it,
because "open" will close it for you. (See "open".) However, an explicit "close" on an input
file resets the line counter ($.), while the implicit close done by "open" does not.
If the file handle came from a piped open "close" will additionally return false if one of the
other system calls involved fails or if the program exits with non-zero status. (If the only
problem was that the program exited non-zero $! will be set to 0.) Closing a pipe also waits for
the process executing on the pipe to complete, in case you want to look at the output of the pipe
afterwards, and implicitly puts the exit status value of that command into $?.
Prematurely closing the read end of a pipe (i.e. before the process writing to it at the other
end has closed it) will result in a SIGPIPE being delivered to the writer. If the other end
can't handle that, be sure to read all the data before closing the pipe.
Example:
open(OUTPUT, '⎪sort >foo') # pipe to sort
or die "Can't start sort: $!";
#... # print stuff to output
close OUTPUT # wait for sort to finish
or warn $! ? "Error closing sort pipe: $!"
: "Exit status $? from sort";
open(INPUT, 'foo') # get sort's results
or die "Can't open 'foo' for input: $!";
FILEHANDLE may be an expression whose value can be used as an indirect filehandle, usually the
real filehandle name.
closedir DIRHANDLE
Closes a directory opened by "opendir" and returns the success of that system call.
connect SOCKET,NAME
Attempts to connect to a remote socket, just as the connect system call does. Returns true if it
succeeded, false otherwise. NAME should be a packed address of the appropriate type for the
socket. See the examples in "Sockets: Client/Server Communication" in perlipc.
continue BLOCK
Actually a flow control statement rather than a function. If there is a "continue" BLOCK
attached to a BLOCK (typically in a "while" or "foreach"), it is always executed just before the
conditional is about to be evaluated again, just like the third part of a "for" loop in C. Thus
it can be used to increment a loop variable, even when the loop has been continued via the "next"
statement (which is similar to the C "continue" statement).
"last", "next", or "redo" may appear within a "continue" block. "last" and "redo" will behave as
if they had been executed within the main block. So will "next", but since it will execute a
"continue" block, it may be more entertaining.
while (EXPR) {
### redo always comes here
do_something;
} continue {
### next always comes here
do_something_else;
# then back the top to re-check EXPR
}
### last always comes here
Omitting the "continue" section is semantically equivalent to using an empty one, logically
enough. In that case, "next" goes directly back to check the condition at the top of the loop.
cos EXPR
cos Returns the cosine of EXPR (expressed in radians). If EXPR is omitted, takes cosine of $_.
For the inverse cosine operation, you may use the "Math::Trig::acos()" function, or use this
relation:
sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
crypt PLAINTEXT,SALT
Encrypts a string exactly like the crypt(3) function in the C library (assuming that you actually
have a version there that has not been extirpated as a potential munition). This can prove
useful for checking the password file for lousy passwords, amongst other things. Only the guys
wearing white hats should do this.
Note that crypt is intended to be a one-way function, much like breaking eggs to make an
omelette. There is no (known) corresponding decrypt function (in other words, the crypt() is a
one-way hash function). As a result, this function isn't all that useful for cryptography. (For
that, see your nearby CPAN mirror.)
When verifying an existing encrypted string you should use the encrypted text as the salt (like
"crypt($plain, $crypted) eq $crypted"). This allows your code to work with the standard crypt
and with more exotic implementations. In other words, do not assume anything about the returned
string itself, or how many bytes in the encrypted string matter.
Traditionally the result is a string of 13 bytes: two first bytes of the salt, followed by 11
bytes from the set "[./0-9A-Za-z]", and only the first eight bytes of the encrypted string
mattered, but alternative hashing schemes (like MD5), higher level security schemes (like C2),
and implementations on non-UNIX platforms may produce different strings.
When choosing a new salt create a random two character string whose characters come from the set
"[./0-9A-Za-z]" (like "join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]"). This
set of characters is just a recommendation; the characters allowed in the salt depend solely on
your system's crypt library, and Perl can't restrict what salts "crypt()" accepts.
Here's an example that makes sure that whoever runs this program knows their own password:
$pwd = (getpwuid($<))[1];
system "stty -echo";
print "Password: ";
chomp($word = <STDIN>);
print "\n";
system "stty echo";
if (crypt($word, $pwd) ne $pwd) {
die "Sorry...\n";
} else {
print "ok\n";
}
Of course, typing in your own password to whoever asks you for it is unwise.
The crypt function is unsuitable for encrypting large quantities of data, not least of all
because you can't get the information back. Look at the by-module/Crypt and by-module/PGP
directories on your favorite CPAN mirror for a slew of potentially useful modules.
If using crypt() on a Unicode string (which potentially has characters with codepoints above
255), Perl tries to make sense of the situation by trying to downgrade (a copy of the string) the
string back to an eight-bit byte string before calling crypt() (on that copy). If that works,
good. If not, crypt() dies with "Wide character in crypt".
dbmclose HASH
[This function has been largely superseded by the "untie" function.]
Breaks the binding between a DBM file and a hash.
dbmopen HASH,DBNAME,MASK
[This function has been largely superseded by the "tie" function.]
This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a hash. HASH is the name
of the hash. (Unlike normal "open", the first argument is not a filehandle, even though it looks
like one). DBNAME is the name of the database (without the .dir or .pag extension if any). If
the database does not exist, it is created with protection specified by MASK (as modified by the
"umask"). If your system supports only the older DBM functions, you may perform only one
"dbmopen" in your program. In older versions of Perl, if your system had neither DBM nor ndbm,
calling "dbmopen" produced a fatal error; it now falls back to sdbm(3).
If you don't have write access to the DBM file, you can only read hash variables, not set them.
If you want to test whether you can write, either use file tests or try setting a dummy hash
entry inside an "eval", which will trap the error.
Note that functions such as "keys" and "values" may return huge lists when used on large DBM
files. You may prefer to use the "each" function to iterate over large DBM files. Example:
# print out history file offsets
dbmopen(%HIST,'/usr/lib/news/history',0666);
while (($key,$val) = each %HIST) {
print $key, ' = ', unpack('L',$val), "\n";
}
dbmclose(%HIST);
See also AnyDBM_File for a more general description of the pros and cons of the various dbm
approaches, as well as DB_File for a particularly rich implementation.
You can control which DBM library you use by loading that library before you call dbmopen():
use DB_File;
dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
or die "Can't open netscape history file: $!";
defined EXPR
defined Returns a Boolean value telling whether EXPR has a value other than the undefined value "undef".
If EXPR is not present, $_ will be checked.
Many operations return "undef" to indicate failure, end of file, system error, uninitialized
variable, and other exceptional conditions. This function allows you to distinguish "undef" from
other values. (A simple Boolean test will not distinguish among "undef", zero, the empty string,
and "0", which are all equally false.) Note that since "undef" is a valid scalar, its presence
doesn't necessarily indicate an exceptional condition: "pop" returns "undef" when its argument is
an empty array, or when the element to return happens to be "undef".
You may also use "defined(&func)" to check whether subroutine &func has ever been defined. The
return value is unaffected by any forward declarations of &func. Note that a subroutine which is
not defined may still be callable: its package may have an "AUTOLOAD" method that makes it spring
into existence the first time that it is called -- see perlsub.
Use of "defined" on aggregates (hashes and arrays) is deprecated. It used to report whether
memory for that aggregate has ever been allocated. This behavior may disappear in future
versions of Perl. You should instead use a simple test for size:
if (@an_array) { print "has array elements\n" }
if (%a_hash) { print "has hash members\n" }
When used on a hash element, it tells you whether the value is defined, not whether the key
exists in the hash. Use "exists" for the latter purpose.
Examples:
print if defined $switch{'D'};
print "$val\n" while defined($val = pop(@ary));
die "Can't readlink $sym: $!"
unless defined($value = readlink $sym);
sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
$debugging = 0 unless defined $debugging;
Note: Many folks tend to overuse "defined", and then are surprised to discover that the number 0
and "" (the zero-length string) are, in fact, defined values. For example, if you say
"ab" =~ /a(.*)b/;
The pattern match succeeds, and $1 is defined, despite the fact that it matched "nothing". But
it didn't really match nothing--rather, it matched something that happened to be zero characters
long. This is all very above-board and honest. When a function returns an undefined value, it's
an admission that it couldn't give you an honest answer. So you should use "defined" only when
you're questioning the integrity of what you're trying to do. At other times, a simple
comparison to 0 or "" is what you want.
See also "undef", "exists", "ref".
delete EXPR
Given an expression that specifies a hash element, array element, hash slice, or array slice,
deletes the specified element(s) from the hash or array. In the case of an array, if the array
elements happen to be at the end, the size of the array will shrink to the highest element that
tests true for exists() (or 0 if no such element exists).
Returns each element so deleted or the undefined value if there was no such element. Deleting
from $ENV{} modifies the environment. Deleting from a hash tied to a DBM file deletes the entry
from the DBM file. Deleting from a "tie"d hash or array may not necessarily return anything.
Deleting an array element effectively returns that position of the array to its initial,
uninitialized state. Subsequently testing for the same element with exists() will return false.
Note that deleting array elements in the middle of an array will not shift the index of the ones
after them down--use splice() for that. See "exists".
The following (inefficiently) deletes all the values of %HASH and @ARRAY:
foreach $key (keys %HASH) {
delete $HASH{$key};
}
foreach $index (0 .. $#ARRAY) {
delete $ARRAY[$index];
}
And so do these:
delete @HASH{keys %HASH};
delete @ARRAY[0 .. $#ARRAY];
But both of these are slower than just assigning the empty list or undefining %HASH or @ARRAY:
%HASH = (); # completely empty %HASH
undef %HASH; # forget %HASH ever existed
@ARRAY = (); # completely empty @ARRAY
undef @ARRAY; # forget @ARRAY ever existed
Note that the EXPR can be arbitrarily complicated as long as the final operation is a hash
element, array element, hash slice, or array slice lookup:
delete $ref->[$x][$y]{$key};
delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
delete $ref->[$x][$y][$index];
delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
die LIST
Outside an "eval", prints the value of LIST to "STDERR" and exits with the current value of $!
(errno). If $! is 0, exits with the value of "($? >> 8)" (backtick `command` status). If "($?
>> 8)" is 0, exits with 255. Inside an "eval()," the error message is stuffed into $@ and the
"eval" is terminated with the undefined value. This makes "die" the way to raise an exception.
Equivalent examples:
die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
If the last element of LIST does not end in a newline, the current script line number and input
line number (if any) are also printed, and a newline is supplied. Note that the "input line
number" (also known as "chunk") is subject to whatever notion of "line" happens to be currently
in effect, and is also available as the special variable $.. See "$/" in perlvar and "$." in
perlvar.
Hint: sometimes appending ", stopped" to your message will cause it to make better sense when the
string "at foo line 123" is appended. Suppose you are running script "canasta".
die "/etc/games is no good";
die "/etc/games is no good, stopped";
produce, respectively
/etc/games is no good at canasta line 123.
/etc/games is no good, stopped at canasta line 123.
See also exit(), warn(), and the Carp module.
If LIST is empty and $@ already contains a value (typically from a previous eval) that value is
reused after appending "\t...propagated". This is useful for propagating exceptions:
eval { ... };
die unless $@ =~ /Expected exception/;
If LIST is empty and $@ contains an object reference that has a "PROPAGATE" method, that method
will be called with additional file and line number parameters. The return value replaces the
value in $@. ie. as if "$@ = eval { $@->PROPAGATE(__FILE__, __LINE__) };" were called.
If $@ is empty then the string "Died" is used.
die() can also be called with a reference argument. If this happens to be trapped within an
eval(), $@ contains the reference. This behavior permits a more elaborate exception handling
implementation using objects that maintain arbitrary state about the nature of the exception.
Such a scheme is sometimes preferable to matching particular string values of $@ using regular
expressions. Here's an example:
eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
if ($@) {
if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
# handle Some::Module::Exception
}
else {
# handle all other possible exceptions
}
}
Because perl will stringify uncaught exception messages before displaying them, you may want to
overload stringification operations on such custom exception objects. See overload for details
about that.
You can arrange for a callback to be run just before the "die" does its deed, by setting the
$SIG{__DIE__} hook. The associated handler will be called with the error text and can change the
error message, if it sees fit, by calling "die" again. See "$SIG{expr}" in perlvar for details
on setting %SIG entries, and "eval BLOCK" for some examples. Although this feature was meant to
be run only right before your program was to exit, this is not currently the case--the
$SIG{__DIE__} hook is currently called even inside eval()ed blocks/strings! If one wants the
hook to do nothing in such situations, put
die @_ if $^S;
as the first line of the handler (see "$^S" in perlvar). Because this promotes strange action at
a distance, this counterintuitive behavior may be fixed in a future release.
do BLOCK
Not really a function. Returns the value of the last command in the sequence of commands
indicated by BLOCK. When modified by a loop modifier, executes the BLOCK once before testing the
loop condition. (On other statements the loop modifiers test the conditional first.)
"do BLOCK" does not count as a loop, so the loop control statements "next", "last", or "redo"
cannot be used to leave or restart the block. See perlsyn for alternative strategies.
do SUBROUTINE(LIST)
A deprecated form of subroutine call. See perlsub.
do EXPR Uses the value of EXPR as a filename and executes the contents of the file as a Perl script. Its
primary use is to include subroutines from a Perl subroutine library.
do 'stat.pl';
is just like
eval `cat stat.pl`;
except that it's more efficient and concise, keeps track of the current filename for error
messages, searches the @INC libraries, and updates %INC if the file is found. See "Predefined
Names" in perlvar for these variables. It also differs in that code evaluated with "do FILENAME"
cannot see lexicals in the enclosing scope; "eval STRING" does. It's the same, however, in that
it does reparse the file every time you call it, so you probably don't want to do this inside a
loop.
If "do" cannot read the file, it returns undef and sets $! to the error. If "do" can read the
file but cannot compile it, it returns undef and sets an error message in $@. If the file is
successfully compiled, "do" returns the value of the last expression evaluated.
Note that inclusion of library modules is better done with the "use" and "require" operators,
which also do automatic error checking and raise an exception if there's a problem.
You might like to use "do" to read in a program configuration file. Manual error checking can be
done this way:
# read in config files: system first, then user
for $file ("/share/prog/defaults.rc",
"$ENV{HOME}/.someprogrc")
{
unless ($return = do $file) {
warn "couldn't parse $file: $@" if $@;
warn "couldn't do $file: $!" unless defined $return;
warn "couldn't run $file" unless $return;
}
}
dump LABEL
dump This function causes an immediate core dump. See also the -u command-line switch in perlrun,
which does the same thing. Primarily this is so that you can use the undump program (not
supplied) to turn your core dump into an executable binary after having initialized all your
variables at the beginning of the program. When the new binary is executed it will begin by
executing a "goto LABEL" (with all the restrictions that "goto" suffers). Think of it as a goto
with an intervening core dump and reincarnation. If "LABEL" is omitted, restarts the program
from the top.
WARNING: Any files opened at the time of the dump will not be open any more when the program is
reincarnated, with possible resulting confusion on the part of Perl.
This function is now largely obsolete, partly because it's very hard to convert a core file into
an executable, and because the real compiler backends for generating portable bytecode and
compilable C code have superseded it. That's why you should now invoke it as "CORE::dump()", if
you don't want to be warned against a possible typo.
If you're looking to use dump to speed up your program, consider generating bytecode or native C
code as described in perlcc. If you're just trying to accelerate a CGI script, consider using
the "mod_perl" extension to Apache, or the CPAN module, CGI::Fast. You might also consider
autoloading or selfloading, which at least make your program appear to run faster.
each HASH
When called in list context, returns a 2-element list consisting of the key and value for the
next element of a hash, so that you can iterate over it. When called in scalar context, returns
only the key for the next element in the hash.
Entries are returned in an apparently random order. The actual random order is subject to change
in future versions of perl, but it is guaranteed to be in the same order as either the "keys" or
"values" function would produce on the same (unmodified) hash. Since Perl 5.8.1 the ordering is
different even between different runs of Perl for security reasons (see "Algorithmic Complexity
Attacks" in perlsec).
When the hash is entirely read, a null array is returned in list context (which when assigned
produces a false (0) value), and "undef" in scalar context. The next call to "each" after that
will start iterating again. There is a single iterator for each hash, shared by all "each",
"keys", and "values" function calls in the program; it can be reset by reading all the elements
from the hash, or by evaluating "keys HASH" or "values HASH". If you add or delete elements of a
hash while you're iterating over it, you may get entries skipped or duplicated, so don't.
Exception: It is always safe to delete the item most recently returned by "each()", which means
that the following code will work:
while (($key, $value) = each %hash) {
print $key, "\n";
delete $hash{$key}; # This is safe
}
The following prints out your environment like the printenv(1) program, only in a different
order:
while (($key,$value) = each %ENV) {
print "$key=$value\n";
}
See also "keys", "values" and "sort".
eof FILEHANDLE
eof ()
eof Returns 1 if the next read on FILEHANDLE will return end of file, or if FILEHANDLE is not open.
FILEHANDLE may be an expression whose value gives the real filehandle. (Note that this function
actually reads a character and then "ungetc"s it, so isn't very useful in an interactive
context.) Do not read from a terminal file (or call "eof(FILEHANDLE)" on it) after end-of-file
is reached. File types such as terminals may lose the end-of-file condition if you do.
An "eof" without an argument uses the last file read. Using "eof()" with empty parentheses is
very different. It refers to the pseudo file formed from the files listed on the command line
and accessed via the "<>" operator. Since "<>" isn't explicitly opened, as a normal filehandle
is, an "eof()" before "<>" has been used will cause @ARGV to be examined to determine if input is
available. Similarly, an "eof()" after "<>" has returned end-of-file will assume you are
processing another @ARGV list, and if you haven't set @ARGV, will read input from "STDIN"; see
"I/O Operators" in perlop.
In a "while (<>)" loop, "eof" or "eof(ARGV)" can be used to detect the end of each file, "eof()"
will only detect the end of the last file. Examples:
# reset line numbering on each input file
while (<>) {
next if /^\s*#/; # skip comments
print "$.\t$_";
} continue {
close ARGV if eof; # Not eof()!
}
# insert dashes just before last line of last file
while (<>) {
if (eof()) { # check for end of last file
print "--------------\n";
}
print;
last if eof(); # needed if we're reading from a terminal
}
Practical hint: you almost never need to use "eof" in Perl, because the input operators typically
return "undef" when they run out of data, or if there was an error.
eval EXPR
eval BLOCK
In the first form, the return value of EXPR is parsed and executed as if it were a little Perl
program. The value of the expression (which is itself determined within scalar context) is first
parsed, and if there weren't any errors, executed in the lexical context of the current Perl
program, so that any variable settings or subroutine and format definitions remain afterwards.
Note that the value is parsed every time the eval executes. If EXPR is omitted, evaluates $_.
This form is typically used to delay parsing and subsequent execution of the text of EXPR until
run time.
In the second form, the code within the BLOCK is parsed only once--at the same time the code
surrounding the eval itself was parsed--and executed within the context of the current Perl
program. This form is typically used to trap exceptions more efficiently than the first (see
below), while also providing the benefit of checking the code within BLOCK at compile time.
The final semicolon, if any, may be omitted from the value of EXPR or within the BLOCK.
In both forms, the value returned is the value of the last expression evaluated inside the
mini-program; a return statement may be also used, just as with subroutines. The expression
providing the return value is evaluated in void, scalar, or list context, depending on the
context of the eval itself. See "wantarray" for more on how the evaluation context can be
determined.
If there is a syntax error or runtime error, or a "die" statement is executed, an undefined value
is returned by "eval", and $@ is set to the error message. If there was no error, $@ is
guaranteed to be a null string. Beware that using "eval" neither silences perl from printing
warnings to STDERR, nor does it stuff the text of warning messages into $@. To do either of
those, you have to use the $SIG{__WARN__} facility, or turn off warnings inside the BLOCK or EXPR
using "no warnings 'all'". See "warn", perlvar, warnings and perllexwarn.
Note that, because "eval" traps otherwise-fatal errors, it is useful for determining whether a
particular feature (such as "socket" or "symlink") is implemented. It is also Perl's exception
trapping mechanism, where the die operator is used to raise exceptions.
If the code to be executed doesn't vary, you may use the eval-BLOCK form to trap run-time errors
without incurring the penalty of recompiling each time. The error, if any, is still returned in
$@. Examples:
# make divide-by-zero nonfatal
eval { $answer = $a / $b; }; warn $@ if $@;
# same thing, but less efficient
eval '$answer = $a / $b'; warn $@ if $@;
# a compile-time error
eval { $answer = }; # WRONG
# a run-time error
eval '$answer ='; # sets $@
Due to the current arguably broken state of "__DIE__" hooks, when using the "eval{}" form as an
exception trap in libraries, you may wish not to trigger any "__DIE__" hooks that user code may
have installed. You can use the "local $SIG{__DIE__}" construct for this purpose, as shown in
this example:
# a very private exception trap for divide-by-zero
eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
warn $@ if $@;
This is especially significant, given that "__DIE__" hooks can call "die" again, which has the
effect of changing their error messages:
# __DIE__ hooks may modify error messages
{
local $SIG{'__DIE__'} =
sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
eval { die "foo lives here" };
print $@ if $@; # prints "bar lives here"
}
Because this promotes action at a distance, this counterintuitive behavior may be fixed in a
future release.
With an "eval", you should be especially careful to remember what's being looked at when:
eval $x; # CASE 1
eval "$x"; # CASE 2
eval '$x'; # CASE 3
eval { $x }; # CASE 4
eval "\$$x++"; # CASE 5
$$x++; # CASE 6
Cases 1 and 2 above behave identically: they run the code contained in the variable $x.
(Although case 2 has misleading double quotes making the reader wonder what else might be
happening (nothing is).) Cases 3 and 4 likewise behave in the same way: they run the code '$x',
which does nothing but return the value of $x. (Case 4 is preferred for purely visual reasons,
but it also has the advantage of compiling at compile-time instead of at run-time.) Case 5 is a
place where normally you would like to use double quotes, except that in this particular
situation, you can just use symbolic references instead, as in case 6.
"eval BLOCK" does not count as a loop, so the loop control statements "next", "last", or "redo"
cannot be used to leave or restart the block.
Note that as a very special case, an "eval ''" executed within the "DB" package doesn't see the
usual surrounding lexical scope, but rather the scope of the first non-DB piece of code that
called it. You don't normally need to worry about this unless you are writing a Perl debugger.
exec LIST
exec PROGRAM LIST
The "exec" function executes a system command and never returns-- use "system" instead of "exec"
if you want it to return. It fails and returns false only if the command does not exist and it
is executed directly instead of via your system's command shell (see below).
Since it's a common mistake to use "exec" instead of "system", Perl warns you if there is a
following statement which isn't "die", "warn", or "exit" (if "-w" is set - but you always do
that). If you really want to follow an "exec" with some other statement, you can use one of
these styles to avoid the warning:
exec ('foo') or print STDERR "couldn't exec foo: $!";
{ exec ('foo') }; print STDERR "couldn't exec foo: $!";
If there is more than one argument in LIST, or if LIST is an array with more than one value,
calls execvp(3) with the arguments in LIST. If there is only one scalar argument or an array
with one element in it, the argument is checked for shell metacharacters, and if there are any,
the entire argument is passed to the system's command shell for parsing (this is "/bin/sh -c" on
Unix platforms, but varies on other platforms). If there are no shell metacharacters in the
argument, it is split into words and passed directly to "execvp", which is more efficient.
Examples:
exec '/bin/echo', 'Your arguments are: ', @ARGV;
exec "sort $outfile ⎪ uniq";
If you don't really want to execute the first argument, but want to lie to the program you are
executing about its own name, you can specify the program you actually want to run as an
"indirect object" (without a comma) in front of the LIST. (This always forces interpretation of
the LIST as a multivalued list, even if there is only a single scalar in the list.) Example:
$shell = '/bin/csh';
exec $shell '-sh'; # pretend it's a login shell
or, more directly,
exec {'/bin/csh'} '-sh'; # pretend it's a login shell
When the arguments get executed via the system shell, results will be subject to its quirks and
capabilities. See "`STRING`" in perlop for details.
Using an indirect object with "exec" or "system" is also more secure. This usage (which also
works fine with system()) forces interpretation of the arguments as a multivalued list, even if
the list had just one argument. That way you're safe from the shell expanding wildcards or
splitting up words with whitespace in them.
@args = ( "echo surprise" );
exec @args; # subject to shell escapes
# if @args == 1
exec { $args[0] } @args; # safe even with one-arg list
The first version, the one without the indirect object, ran the echo program, passing it
"surprise" an argument. The second version didn't--it tried to run a program literally called
"echo surprise", didn't find it, and set $? to a non-zero value indicating failure.
Beginning with v5.6.0, Perl will attempt to flush all files opened for output before the exec,
but this may not be supported on some platforms (see perlport). To be safe, you may need to set
$⎪ ($AUTOFLUSH in English) or call the "autoflush()" method of "IO::Handle" on any open handles
in order to avoid lost output.
Note that "exec" will not call your "END" blocks, nor will it call any "DESTROY" methods in your
objects.
exists EXPR
Given an expression that specifies a hash element or array element, returns true if the specified
element in the hash or array has ever been initialized, even if the corresponding value is
undefined. The element is not autovivified if it doesn't exist.
print "Exists\n" if exists $hash{$key};
print "Defined\n" if defined $hash{$key};
print "True\n" if $hash{$key};
print "Exists\n" if exists $array[$index];
print "Defined\n" if defined $array[$index];
print "True\n" if $array[$index];
A hash or array element can be true only if it's defined, and defined if it exists, but the
reverse doesn't necessarily hold true.
Given an expression that specifies the name of a subroutine, returns true if the specified
subroutine has ever been declared, even if it is undefined. Mentioning a subroutine name for
exists or defined does not count as declaring it. Note that a subroutine which does not exist
may still be callable: its package may have an "AUTOLOAD" method that makes it spring into
existence the first time that it is called -- see perlsub.
print "Exists\n" if exists &subroutine;
print "Defined\n" if defined &subroutine;
Note that the EXPR can be arbitrarily complicated as long as the final operation is a hash or
array key lookup or subroutine name:
if (exists $ref->{A}->{B}->{$key}) { }
if (exists $hash{A}{B}{$key}) { }
if (exists $ref->{A}->{B}->[$ix]) { }
if (exists $hash{A}{B}[$ix]) { }
if (exists &{$ref->{A}{B}{$key}}) { }
Although the deepest nested array or hash will not spring into existence just because its
existence was tested, any intervening ones will. Thus "$ref->{"A"}" and "$ref->{"A"}->{"B"}"
will spring into existence due to the existence test for the $key element above. This happens
anywhere the arrow operator is used, including even:
undef $ref;
if (exists $ref->{"Some key"}) { }
print $ref; # prints HASH(0x80d3d5c)
This surprising autovivification in what does not at first--or even second--glance appear to be
an lvalue context may be fixed in a future release.
See "Pseudo-hashes: Using an array as a hash" in perlref for specifics on how exists() acts when
used on a pseudo-hash.
Use of a subroutine call, rather than a subroutine name, as an argument to exists() is an error.
exists ⊂ # OK
exists &sub(); # Error
exit EXPR
Evaluates EXPR and exits immediately with that value. Example:
$ans = <STDIN>;
exit 0 if $ans =~ /^[Xx]/;
See also "die". If EXPR is omitted, exits with 0 status. The only universally recognized values
for EXPR are 0 for success and 1 for error; other values are subject to interpretation depending
on the environment in which the Perl program is running. For example, exiting 69
(EX_UNAVAILABLE) from a sendmail incoming-mail filter will cause the mailer to return the item
undelivered, but that's not true everywhere.
Don't use "exit" to abort a subroutine if there's any chance that someone might want to trap
whatever error happened. Use "die" instead, which can be trapped by an "eval".
The exit() function does not always exit immediately. It calls any defined "END" routines first,
but these "END" routines may not themselves abort the exit. Likewise any object destructors that
need to be called are called before the real exit. If this is a problem, you can call
"POSIX:_exit($status)" to avoid END and destructor processing. See perlmod for details.
exp EXPR
exp Returns e (the natural logarithm base) to the power of EXPR. If EXPR is omitted, gives
"exp($_)".
fcntl FILEHANDLE,FUNCTION,SCALAR
Implements the fcntl(2) function. You'll probably have to say
use Fcntl;
first to get the correct constant definitions. Argument processing and value return works just
like "ioctl" below. For example:
use Fcntl;
fcntl($filehandle, F_GETFL, $packed_return_buffer)
or die "can't fcntl F_GETFL: $!";
You don't have to check for "defined" on the return from "fcntl". Like "ioctl", it maps a 0
return from the system call into "0 but true" in Perl. This string is true in boolean context
and 0 in numeric context. It is also exempt from the normal -w warnings on improper numeric
conversions.
Note that "fcntl" will produce a fatal error if used on a machine that doesn't implement
fcntl(2). See the Fcntl module or your fcntl(2) manpage to learn what functions are available on
your system.
fileno FILEHANDLE
Returns the file descriptor for a filehandle, or undefined if the filehandle is not open. This
is mainly useful for constructing bitmaps for "select" and low-level POSIX tty-handling
operations. If FILEHANDLE is an expression, the value is taken as an indirect filehandle,
generally its name.
You can use this to find out whether two handles refer to the same underlying descriptor:
if (fileno(THIS) == fileno(THAT)) {
print "THIS and THAT are dups\n";
}
(Filehandles connected to memory objects via new features of "open" may return undefined even
though they are open.)
flock FILEHANDLE,OPERATION
Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true for success, false on
failure. Produces a fatal error if used on a machine that doesn't implement flock(2), fcntl(2)
locking, or lockf(3). "flock" is Perl's portable file locking interface, although it locks only
entire files, not records.
Two potentially non-obvious but traditional "flock" semantics are that it waits indefinitely
until the lock is granted, and that its locks merely advisory. Such discretionary locks are more
flexible, but offer fewer guarantees. This means that files locked with "flock" may be modified
by programs that do not also use "flock". See perlport, your port's specific documentation, or
your system-specific local manpages for details. It's best to assume traditional behavior if
you're writing portable programs. (But if you're not, you should as always feel perfectly free
to write for your own system's idiosyncrasies (sometimes called "features"). Slavish adherence
to portability concerns shouldn't get in the way of your getting your job done.)
OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with LOCK_NB. These
constants are traditionally valued 1, 2, 8 and 4, but you can use the symbolic names if you
import them from the Fcntl module, either individually, or as a group using the ':flock' tag.
LOCK_SH requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN releases a
previously requested lock. If LOCK_NB is bitwise-or'ed with LOCK_SH or LOCK_EX then "flock" will
return immediately rather than blocking waiting for the lock (check the return status to see if
you got it).
To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE before locking or
unlocking it.
Note that the emulation built with lockf(3) doesn't provide shared locks, and it requires that
FILEHANDLE be open with write intent. These are the semantics that lockf(3) implements. Most if
not all systems implement lockf(3) in terms of fcntl(2) locking, though, so the differing
semantics shouldn't bite too many people.
Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE be open with read intent to
use LOCK_SH and requires that it be open with write intent to use LOCK_EX.
Note also that some versions of "flock" cannot lock things over the network; you would need to
use the more system-specific "fcntl" for that. If you like you can force Perl to ignore your
system's flock(2) function, and so provide its own fcntl(2)-based emulation, by passing the
switch "-Ud_flock" to the Configure program when you configure perl.
Here's a mailbox appender for BSD systems.
use Fcntl ':flock'; # import LOCK_* constants
sub lock {
flock(MBOX,LOCK_EX);
# and, in case someone appended
# while we were waiting...
seek(MBOX, 0, 2);
}
sub unlock {
flock(MBOX,LOCK_UN);
}
open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
or die "Can't open mailbox: $!";
lock();
print MBOX $msg,"\n\n";
unlock();
On systems that support a real flock(), locks are inherited across fork() calls, whereas those
that must resort to the more capricious fcntl() function lose the locks, making it harder to
write servers.
See also DB_File for other flock() examples.
fork Does a fork(2) system call to create a new process running the same program at the same point.
It returns the child pid to the parent process, 0 to the child process, or "undef" if the fork is
unsuccessful. File descriptors (and sometimes locks on those descriptors) are shared, while
everything else is copied. On most systems supporting fork(), great care has gone into making it
extremely efficient (for example, using copy-on-write technology on data pages), making it the
dominant paradigm for multitasking over the last few decades.
Beginning with v5.6.0, Perl will attempt to flush all files opened for output before forking the
child process, but this may not be supported on some platforms (see perlport). To be safe, you
may need to set $⎪ ($AUTOFLUSH in English) or call the "autoflush()" method of "IO::Handle" on
any open handles in order to avoid duplicate output.
If you "fork" without ever waiting on your children, you will accumulate zombies. On some
systems, you can avoid this by setting $SIG{CHLD} to "IGNORE". See also perlipc for more
examples of forking and reaping moribund children.
Note that if your forked child inherits system file descriptors like STDIN and STDOUT that are
actually connected by a pipe or socket, even if you exit, then the remote server (such as, say, a
CGI script or a backgrounded job launched from a remote shell) won't think you're done. You
should reopen those to /dev/null if it's any issue.
format Declare a picture format for use by the "write" function. For example:
format Something =
Test: @<<<<<<<< @⎪⎪⎪⎪⎪ @>>>>>
$str, $%, '$' . int($num)
.
$str = "widget";
$num = $cost/$quantity;
$~ = 'Something';
write;
See perlform for many details and examples.
formline PICTURE,LIST
This is an internal function used by "format"s, though you may call it, too. It formats (see
perlform) a list of values according to the contents of PICTURE, placing the output into the
format output accumulator, $^A (or $ACCUMULATOR in English). Eventually, when a "write" is done,
the contents of $^A are written to some filehandle, but you could also read $^A yourself and then
set $^A back to "". Note that a format typically does one "formline" per line of form, but the
"formline" function itself doesn't care how many newlines are embedded in the PICTURE. This
means that the "~" and "~~" tokens will treat the entire PICTURE as a single line. You may
therefore need to use multiple formlines to implement a single record format, just like the
format compiler.
Be careful if you put double quotes around the picture, because an "@" character may be taken to
mean the beginning of an array name. "formline" always returns true. See perlform for other
examples.
getc FILEHANDLE
getc Returns the next character from the input file attached to FILEHANDLE, or the undefined value at
end of file, or if there was an error (in the latter case $! is set). If FILEHANDLE is omitted,
reads from STDIN. This is not particularly efficient. However, it cannot be used by itself to
fetch single characters without waiting for the user to hit enter. For that, try something more
like:
if ($BSD_STYLE) {
system "stty cbreak </dev/tty >/dev/tty 2>&1";
}
else {
system "stty", '-icanon', 'eol', "\001";
}
$key = getc(STDIN);
if ($BSD_STYLE) {
system "stty -cbreak </dev/tty >/dev/tty 2>&1";
}
else {
system "stty", 'icanon', 'eol', '^@'; # ASCII null
}
print "\n";
Determination of whether $BSD_STYLE should be set is left as an exercise to the reader.
The "POSIX::getattr" function can do this more portably on systems purporting POSIX compliance.
See also the "Term::ReadKey" module from your nearest CPAN site; details on CPAN can be found on
"CPAN" in perlmodlib.
getlogin
Implements the C library function of the same name, which on most systems returns the current
login from /etc/utmp, if any. If null, use "getpwuid".
$login = getlogin ⎪⎪ getpwuid($<) ⎪⎪ "Kilroy";
Do not consider "getlogin" for authentication: it is not as secure as "getpwuid".
getpeername SOCKET
Returns the packed sockaddr address of other end of the SOCKET connection.
use Socket;
$hersockaddr = getpeername(SOCK);
($port, $iaddr) = sockaddr_in($hersockaddr);
$herhostname = gethostbyaddr($iaddr, AF_INET);
$herstraddr = inet_ntoa($iaddr);
getpgrp PID
Returns the current process group for the specified PID. Use a PID of 0 to get the current
process group for the current process. Will raise an exception if used on a machine that doesn't
implement getpgrp(2). If PID is omitted, returns process group of current process. Note that
the POSIX version of "getpgrp" does not accept a PID argument, so only "PID==0" is truly
portable.
getppid Returns the process id of the parent process.
Note for Linux users: on Linux, the C functions "getpid()" and "getppid()" return different
values from different threads. In order to be portable, this behavior is not reflected by the
perl-level function "getppid()", that returns a consistent value across threads. If you want to
call the underlying "getppid()", you may use the CPAN module "Linux::Pid".
getpriority WHICH,WHO
Returns the current priority for a process, a process group, or a user. (See getpriority(2).)
Will raise a fatal exception if used on a machine that doesn't implement getpriority(2).
getpwnam NAME
getgrnam NAME
gethostbyname NAME
getnetbyname NAME
getprotobyname NAME
getpwuid UID
getgrgid GID
getservbyname NAME,PROTO
gethostbyaddr ADDR,ADDRTYPE
getnetbyaddr ADDR,ADDRTYPE
getprotobynumber NUMBER
getservbyport PORT,PROTO
getpwent
getgrent
gethostent
getnetent
getprotoent
getservent
setpwent
setgrent
sethostent STAYOPEN
setnetent STAYOPEN
setprotoent STAYOPEN
setservent STAYOPEN
endpwent
endgrent
endhostent
endnetent
endprotoent
endservent
These routines perform the same functions as their counterparts in the system library. In list
context, the return values from the various get routines are as follows:
($name,$passwd,$uid,$gid,
$quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
($name,$passwd,$gid,$members) = getgr*
($name,$aliases,$addrtype,$length,@addrs) = gethost*
($name,$aliases,$addrtype,$net) = getnet*
($name,$aliases,$proto) = getproto*
($name,$aliases,$port,$proto) = getserv*
(If the entry doesn't exist you get a null list.)
The exact meaning of the $gcos field varies but it usually contains the real name of the user (as
opposed to the login name) and other information pertaining to the user. Beware, however, that
in many system users are able to change this information and therefore it cannot be trusted and
therefore the $gcos is tainted (see perlsec). The $passwd and $shell, user's encrypted password
and login shell, are also tainted, because of the same reason.
In scalar context, you get the name, unless the function was a lookup by name, in which case you
get the other thing, whatever it is. (If the entry doesn't exist you get the undefined value.)
For example:
$uid = getpwnam($name);
$name = getpwuid($num);
$name = getpwent();
$gid = getgrnam($name);
$name = getgrgid($num);
$name = getgrent();
#etc.
In getpw*() the fields $quota, $comment, and $expire are special cases in the sense that in many
systems they are unsupported. If the $quota is unsupported, it is an empty scalar. If it is
supported, it usually encodes the disk quota. If the $comment field is unsupported, it is an
empty scalar. If it is supported it usually encodes some administrative comment about the user.
In some systems the $quota field may be $change or $age, fields that have to do with password
aging. In some systems the $comment field may be $class. The $expire field, if present, encodes
the expiration period of the account or the password. For the availability and the exact meaning
of these fields in your system, please consult your getpwnam(3) documentation and your pwd.h
file. You can also find out from within Perl what your $quota and $comment fields mean and
whether you have the $expire field by using the "Config" module and the values "d_pwquota",
"d_pwage", "d_pwchange", "d_pwcomment", and "d_pwexpire". Shadow password files are only
supported if your vendor has implemented them in the intuitive fashion that calling the regular C
library routines gets the shadow versions if you're running under privilege or if there exists
the shadow(3) functions as found in System V ( this includes Solaris and Linux.) Those systems
which implement a proprietary shadow password facility are unlikely to be supported.
The $members value returned by getgr*() is a space separated list of the login names of the
members of the group.
For the gethost*() functions, if the "h_errno" variable is supported in C, it will be returned to
you via $? if the function call fails. The @addrs value returned by a successful call is a list
of the raw addresses returned by the corresponding system library call. In the Internet domain,
each address is four bytes long and you can unpack it by saying something like:
($a,$b,$c,$d) = unpack('C4',$addr[0]);
The Socket library makes this slightly easier:
use Socket;
$iaddr = inet_aton("127.1"); # or whatever address
$name = gethostbyaddr($iaddr, AF_INET);
# or going the other way
$straddr = inet_ntoa($iaddr);
If you get tired of remembering which element of the return list contains which return value, by-
name interfaces are provided in standard modules: "File::stat", "Net::hostent", "Net::netent",
"Net::protoent", "Net::servent", "Time::gmtime", "Time::localtime", and "User::grent". These
override the normal built-ins, supplying versions that return objects with the appropriate names
for each field. For example:
use File::stat;
use User::pwent;
$is_his = (stat($filename)->uid == pwent($whoever)->uid);
Even though it looks like they're the same method calls (uid), they aren't, because a
"File::stat" object is different from a "User::pwent" object.
getsockname SOCKET
Returns the packed sockaddr address of this end of the SOCKET connection, in case you don't know
the address because you have several different IPs that the connection might have come in on.
use Socket;
$mysockaddr = getsockname(SOCK);
($port, $myaddr) = sockaddr_in($mysockaddr);
printf "Connect to %s [%s]\n",
scalar gethostbyaddr($myaddr, AF_INET),
inet_ntoa($myaddr);
getsockopt SOCKET,LEVEL,OPTNAME
Returns the socket option requested, or undef if there is an error.
glob EXPR
glob In list context, returns a (possibly empty) list of filename expansions on the value of EXPR such
as the standard Unix shell /bin/csh would do. In scalar context, glob iterates through such
filename expansions, returning undef when the list is exhausted. This is the internal function
implementing the "<*.c>" operator, but you can use it directly. If EXPR is omitted, $_ is used.
The "<*.c>" operator is discussed in more detail in "I/O Operators" in perlop.
Beginning with v5.6.0, this operator is implemented using the standard "File::Glob" extension.
See File::Glob for details.
gmtime EXPR
Converts a time as returned by the time function to an 8-element list with the time localized for
the standard Greenwich time zone. Typically used as follows:
# 0 1 2 3 4 5 6 7
($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
gmtime(time);
All list elements are numeric, and come straight out of the C `struct tm'. $sec, $min, and $hour
are the seconds, minutes, and hours of the specified time. $mday is the day of the month, and
$mon is the month itself, in the range 0..11 with 0 indicating January and 11 indicating
December. $year is the number of years since 1900. That is, $year is 123 in year 2023. $wday
is the day of the week, with 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
the year, in the range 0..364 (or 0..365 in leap years.)
Note that the $year element is not simply the last two digits of the year. If you assume it is,
then you create non-Y2K-compliant programs--and you wouldn't want to do that, would you?
The proper way to get a complete 4-digit year is simply:
$year += 1900;
And to get the last two digits of the year (e.g., '01' in 2001) do:
$year = sprintf("%02d", $year % 100);
If EXPR is omitted, "gmtime()" uses the current time ("gmtime(time)").
In scalar context, "gmtime()" returns the ctime(3) value:
$now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
Also see the "timegm" function provided by the "Time::Local" module, and the strftime(3) function
available via the POSIX module.
This scalar value is not locale dependent (see perllocale), but is instead a Perl builtin. Also
see the "Time::Local" module, and the strftime(3) and mktime(3) functions available via the POSIX
module. To get somewhat similar but locale dependent date strings, set up your locale
environment variables appropriately (please see perllocale) and try for example:
use POSIX qw(strftime);
$now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
Note that the %a and %b escapes, which represent the short forms of the day of the week and the
month of the year, may not necessarily be three characters wide in all locales.
goto LABEL
goto EXPR
goto &NAME
The "goto-LABEL" form finds the statement labeled with LABEL and resumes execution there. It may
not be used to go into any construct that requires initialization, such as a subroutine or a
"foreach" loop. It also can't be used to go into a construct that is optimized away, or to get
out of a block or subroutine given to "sort". It can be used to go almost anywhere else within
the dynamic scope, including out of subroutines, but it's usually better to use some other
construct such as "last" or "die". The author of Perl has never felt the need to use this form
of "goto" (in Perl, that is--C is another matter). (The difference being that C does not offer
named loops combined with loop control. Perl does, and this replaces most structured uses of
"goto" in other languages.)
The "goto-EXPR" form expects a label name, whose scope will be resolved dynamically. This allows
for computed "goto"s per FORTRAN, but isn't necessarily recommended if you're optimizing for
maintainability:
goto ("FOO", "BAR", "GLARCH")[$i];
The "goto-&NAME" form is quite different from the other forms of "goto". In fact, it isn't a
goto in the normal sense at all, and doesn't have the stigma associated with other gotos.
Instead, it exits the current subroutine (losing any changes set by local()) and immediately
calls in its place the named subroutine using the current value of @_. This is used by
"AUTOLOAD" subroutines that wish to load another subroutine and then pretend that the other
subroutine had been called in the first place (except that any modifications to @_ in the current
subroutine are propagated to the other subroutine.) After the "goto", not even "caller" will be
able to tell that this routine was called first.
NAME needn't be the name of a subroutine; it can be a scalar variable containing a code
reference, or a block which evaluates to a code reference.
grep BLOCK LIST
grep EXPR,LIST
This is similar in spirit to, but not the same as, grep(1) and its relatives. In particular, it
is not limited to using regular expressions.
Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each element) and
returns the list value consisting of those elements for which the expression evaluated to true.
In scalar context, returns the number of times the expression was true.
@foo = grep(!/^#/, @bar); # weed out comments
or equivalently,
@foo = grep {!/^#/} @bar; # weed out comments
Note that $_ is an alias to the list value, so it can be used to modify the elements of the LIST.
While this is useful and supported, it can cause bizarre results if the elements of LIST are not
variables. Similarly, grep returns aliases into the original list, much as a for loop's index
variable aliases the list elements. That is, modifying an element of a list returned by grep
(for example, in a "foreach", "map" or another "grep") actually modifies the element in the
original list. This is usually something to be avoided when writing clear code.
See also "map" for a list composed of the results of the BLOCK or EXPR.
hex EXPR
hex Interprets EXPR as a hex string and returns the corresponding value. (To convert strings that
might start with either 0, 0x, or 0b, see "oct".) If EXPR is omitted, uses $_.
print hex '0xAf'; # prints '175'
print hex 'aF'; # same
Hex strings may only represent integers. Strings that would cause integer overflow trigger a
warning. Leading whitespace is not stripped, unlike oct().
import There is no builtin "import" function. It is just an ordinary method (subroutine) defined (or
inherited) by modules that wish to export names to another module. The "use" function calls the
"import" method for the package used. See also "use", perlmod, and Exporter.
index STR,SUBSTR,POSITION
index STR,SUBSTR
The index function searches for one string within another, but without the wildcard-like behavior
of a full regular-expression pattern match. It returns the position of the first occurrence of
SUBSTR in STR at or after POSITION. If POSITION is omitted, starts searching from the beginning
of the string. The return value is based at 0 (or whatever you've set the $[ variable to--but
don't do that). If the substring is not found, returns one less than the base, ordinarily "-1".
int EXPR
int Returns the integer portion of EXPR. If EXPR is omitted, uses $_. You should not use this
function for rounding: one because it truncates towards 0, and two because machine
representations of floating point numbers can sometimes produce counterintuitive results. For
example, "int(-6.725/0.025)" produces -268 rather than the correct -269; that's because it's
really more like -268.99999999999994315658 instead. Usually, the "sprintf", "printf", or the
"POSIX::floor" and "POSIX::ceil" functions will serve you better than will int().
ioctl FILEHANDLE,FUNCTION,SCALAR
Implements the ioctl(2) function. You'll probably first have to say
require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
to get the correct function definitions. If ioctl.ph doesn't exist or doesn't have the correct
definitions you'll have to roll your own, based on your C header files such as <sys/ioctl.h>.
(There is a Perl script called h2ph that comes with the Perl kit that may help you in this, but
it's nontrivial.) SCALAR will be read and/or written depending on the FUNCTION--a pointer to the
string value of SCALAR will be passed as the third argument of the actual "ioctl" call. (If
SCALAR has no string value but does have a numeric value, that value will be passed rather than a
pointer to the string value. To guarantee this to be true, add a 0 to the scalar before using
it.) The "pack" and "unpack" functions may be needed to manipulate the values of structures used
by "ioctl".
The return value of "ioctl" (and "fcntl") is as follows:
if OS returns: then Perl returns:
-1 undefined value
0 string "0 but true"
anything else that number
Thus Perl returns true on success and false on failure, yet you can still easily determine the
actual value returned by the operating system:
$retval = ioctl(...) ⎪⎪ -1;
printf "System returned %d\n", $retval;
The special string "0 but true" is exempt from -w complaints about improper numeric conversions.
Here's an example of setting a filehandle named "REMOTE" to be non-blocking at the system level.
You'll have to negotiate $⎪ on your own, though.
use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
$flags = fcntl(REMOTE, F_GETFL, 0)
or die "Can't get flags for the socket: $!\n";
$flags = fcntl(REMOTE, F_SETFL, $flags ⎪ O_NONBLOCK)
or die "Can't set flags for the socket: $!\n";
join EXPR,LIST
Joins the separate strings of LIST into a single string with fields separated by the value of
EXPR, and returns that new string. Example:
$rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
Beware that unlike "split", "join" doesn't take a pattern as its first argument. Compare
"split".
keys HASH
Returns a list consisting of all the keys of the named hash. (In scalar context, returns the
number of keys.)
The keys are returned in an apparently random order. The actual random order is subject to
change in future versions of perl, but it is guaranteed to be the same order as either the
"values" or "each" function produces (given that the hash has not been modified). Since Perl
5.8.1 the ordering is different even between different runs of Perl for security reasons (see
"Algorithmic Complexity Attacks" in perlsec).
As a side effect, calling keys() resets the HASH's internal iterator, see "each".
Here is yet another way to print your environment:
@keys = keys %ENV;
@values = values %ENV;
while (@keys) {
print pop(@keys), '=', pop(@values), "\n";
}
or how about sorted by key:
foreach $key (sort(keys %ENV)) {
print $key, '=', $ENV{$key}, "\n";
}
The returned values are copies of the original keys in the hash, so modifying them will not
affect the original hash. Compare "values".
To sort a hash by value, you'll need to use a "sort" function. Here's a descending numeric sort
of a hash by its values:
foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
printf "%4d %s\n", $hash{$key}, $key;
}
As an lvalue "keys" allows you to increase the number of hash buckets allocated for the given
hash. This can gain you a measure of efficiency if you know the hash is going to get big. (This
is similar to pre-extending an array by assigning a larger number to $#array.) If you say
keys %hash = 200;
then %hash will have at least 200 buckets allocated for it--256 of them, in fact, since it rounds
up to the next power of two. These buckets will be retained even if you do "%hash = ()", use
"undef %hash" if you want to free the storage while %hash is still in scope. You can't shrink
the number of buckets allocated for the hash using "keys" in this way (but you needn't worry
about doing this by accident, as trying has no effect).
See also "each", "values" and "sort".
kill SIGNAL, LIST
Sends a signal to a list of processes. Returns the number of processes successfully signaled
(which is not necessarily the same as the number actually killed).
$cnt = kill 1, $child1, $child2;
kill 9, @goners;
If SIGNAL is zero, no signal is sent to the process. This is a useful way to check that a child
process is alive and hasn't changed its UID. See perlport for notes on the portability of this
construct.
Unlike in the shell, if SIGNAL is negative, it kills process groups instead of processes. (On
System V, a negative PROCESS number will also kill process groups, but that's not portable.)
That means you usually want to use positive not negative signals. You may also use a signal name
in quotes.
See "Signals" in perlipc for more details.
last LABEL
last The "last" command is like the "break" statement in C (as used in loops); it immediately exits
the loop in question. If the LABEL is omitted, the command refers to the innermost enclosing
loop. The "continue" block, if any, is not executed:
LINE: while (<STDIN>) {
last LINE if /^$/; # exit when done with header
#...
}
"last" cannot be used to exit a block which returns a value such as "eval {}", "sub {}" or "do
{}", and should not be used to exit a grep() or map() operation.
Note that a block by itself is semantically identical to a loop that executes once. Thus "last"
can be used to effect an early exit out of such a block.
See also "continue" for an illustration of how "last", "next", and "redo" work.
lc EXPR
lc Returns a lowercased version of EXPR. This is the internal function implementing the "\L" escape
in double-quoted strings. Respects current LC_CTYPE locale if "use locale" in force. See
perllocale and perlunicode for more details about locale and Unicode support.
If EXPR is omitted, uses $_.
lcfirst EXPR
lcfirst Returns the value of EXPR with the first character lowercased. This is the internal function
implementing the "\l" escape in double-quoted strings. Respects current LC_CTYPE locale if "use
locale" in force. See perllocale and perlunicode for more details about locale and Unicode
support.
If EXPR is omitted, uses $_.
length EXPR
length Returns the length in characters of the value of EXPR. If EXPR is omitted, returns length of $_.
Note that this cannot be used on an entire array or hash to find out how many elements these
have. For that, use "scalar @array" and "scalar keys %hash" respectively.
Note the characters: if the EXPR is in Unicode, you will get the number of characters, not the
number of bytes. To get the length in bytes, use "do { use bytes; length(EXPR) }", see bytes.
link OLDFILE,NEWFILE
Creates a new filename linked to the old filename. Returns true for success, false otherwise.
listen SOCKET,QUEUESIZE
Does the same thing that the listen system call does. Returns true if it succeeded, false
otherwise. See the example in "Sockets: Client/Server Communication" in perlipc.
local EXPR
You really probably want to be using "my" instead, because "local" isn't what most people think
of as "local". See "Private Variables via my()" in perlsub for details.
A local modifies the listed variables to be local to the enclosing block, file, or eval. If more
than one value is listed, the list must be placed in parentheses. See "Temporary Values via
local()" in perlsub for details, including issues with tied arrays and hashes.
localtime EXPR
Converts a time as returned by the time function to a 9-element list with the time analyzed for
the local time zone. Typically used as follows:
# 0 1 2 3 4 5 6 7 8
($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
localtime(time);
All list elements are numeric, and come straight out of the C `struct tm'. $sec, $min, and $hour
are the seconds, minutes, and hours of the specified time. $mday is the day of the month, and
$mon is the month itself, in the range 0..11 with 0 indicating January and 11 indicating
December. $year is the number of years since 1900. That is, $year is 123 in year 2023. $wday
is the day of the week, with 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
the year, in the range 0..364 (or 0..365 in leap years.) $isdst is true if the specified time
occurs during daylight savings time, false otherwise.
Note that the $year element is not simply the last two digits of the year. If you assume it is,
then you create non-Y2K-compliant programs--and you wouldn't want to do that, would you?
The proper way to get a complete 4-digit year is simply:
$year += 1900;
And to get the last two digits of the year (e.g., '01' in 2001) do:
$year = sprintf("%02d", $year % 100);
If EXPR is omitted, "localtime()" uses the current time ("localtime(time)").
In scalar context, "localtime()" returns the ctime(3) value:
$now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
This scalar value is not locale dependent, see perllocale, but instead a Perl builtin. Also see
the "Time::Local" module (to convert the second, minutes, hours, ... back to seconds since the
stroke of midnight the 1st of January 1970, the value returned by time()), and the strftime(3)
and mktime(3) functions available via the POSIX module. To get somewhat similar but locale
dependent date strings, set up your locale environment variables appropriately (please see
perllocale) and try for example:
use POSIX qw(strftime);
$now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
Note that the %a and %b, the short forms of the day of the week and the month of the year, may
not necessarily be three characters wide.
lock THING
This function places an advisory lock on a shared variable, or referenced object contained in
THING until the lock goes out of scope.
lock() is a "weak keyword" : this means that if you've defined a function by this name (before
any calls to it), that function will be called instead. (However, if you've said "use threads",
lock() is always a keyword.) See threads.
log EXPR
log Returns the natural logarithm (base e) of EXPR. If EXPR is omitted, returns log of $_. To get
the log of another base, use basic algebra: The base-N log of a number is equal to the natural
log of that number divided by the natural log of N. For example:
sub log10 {
my $n = shift;
return log($n)/log(10);
}
See also "exp" for the inverse operation.
lstat EXPR
lstat Does the same thing as the "stat" function (including setting the special "_" filehandle) but
stats a symbolic link instead of the file the symbolic link points to. If symbolic links are
unimplemented on your system, a normal "stat" is done. For much more detailed information,
please see the documentation for "stat".
If EXPR is omitted, stats $_.
m// The match operator. See perlop.
map BLOCK LIST
map EXPR,LIST
Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each element) and
returns the list value composed of the results of each such evaluation. In scalar context,
returns the total number of elements so generated. Evaluates BLOCK or EXPR in list context, so
each element of LIST may produce zero, one, or more elements in the returned value.
@chars = map(chr, @nums);
translates a list of numbers to the corresponding characters. And
%hash = map { getkey($_) => $_ } @array;
is just a funny way to write
%hash = ();
foreach $_ (@array) {
$hash{getkey($_)} = $_;
}
Note that $_ is an alias to the list value, so it can be used to modify the elements of the LIST.
While this is useful and supported, it can cause bizarre results if the elements of LIST are not
variables. Using a regular "foreach" loop for this purpose would be clearer in most cases. See
also "grep" for an array composed of those items of the original list for which the BLOCK or EXPR
evaluates to true.
"{" starts both hash references and blocks, so "map { ..." could be either the start of map BLOCK
LIST or map EXPR, LIST. Because perl doesn't look ahead for the closing "}" it has to take a
guess at which its dealing with based what it finds just after the "{". Usually it gets it right,
but if it doesn't it won't realize something is wrong until it gets to the "}" and encounters the
missing (or unexpected) comma. The syntax error will be reported close to the "}" but you'll need
to change something near the "{" such as using a unary "+" to give perl some help:
%hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
%hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
%hash = map { ("\L$_", 1) } @array # this also works
%hash = map { lc($_), 1 } @array # as does this.
%hash = map +( lc($_), 1 ), @array # this is EXPR and works!
%hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
or to force an anon hash constructor use "+{"
@hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
and you get list of anonymous hashes each with only 1 entry.
mkdir FILENAME,MASK
mkdir FILENAME
Creates the directory specified by FILENAME, with permissions specified by MASK (as modified by
"umask"). If it succeeds it returns true, otherwise it returns false and sets $! (errno). If
omitted, MASK defaults to 0777.
In general, it is better to create directories with permissive MASK, and let the user modify that
with their "umask", than it is to supply a restrictive MASK and give the user no way to be more
permissive. The exceptions to this rule are when the file or directory should be kept private
(mail files, for instance). The perlfunc(1) entry on "umask" discusses the choice of MASK in
more detail.
Note that according to the POSIX 1003.1-1996 the FILENAME may have any number of trailing
slashes. Some operating and filesystems do not get this right, so Perl automatically removes all
trailing slashes to keep everyone happy.
msgctl ID,CMD,ARG
Calls the System V IPC function msgctl(2). You'll probably have to say
use IPC::SysV;
first to get the correct constant definitions. If CMD is "IPC_STAT", then ARG must be a variable
which will hold the returned "msqid_ds" structure. Returns like "ioctl": the undefined value for
error, "0 but true" for zero, or the actual return value otherwise. See also "SysV IPC" in
perlipc, "IPC::SysV", and "IPC::Semaphore" documentation.
msgget KEY,FLAGS
Calls the System V IPC function msgget(2). Returns the message queue id, or the undefined value
if there is an error. See also "SysV IPC" in perlipc and "IPC::SysV" and "IPC::Msg"
documentation.
msgrcv ID,VAR,SIZE,TYPE,FLAGS
Calls the System V IPC function msgrcv to receive a message from message queue ID into variable
VAR with a maximum message size of SIZE. Note that when a message is received, the message type
as a native long integer will be the first thing in VAR, followed by the actual message. This
packing may be opened with "unpack("l! a*")". Taints the variable. Returns true if successful,
or false if there is an error. See also "SysV IPC" in perlipc, "IPC::SysV", and "IPC::SysV::Msg"
documentation.
msgsnd ID,MSG,FLAGS
Calls the System V IPC function msgsnd to send the message MSG to the message queue ID. MSG must
begin with the native long integer message type, and be followed by the length of the actual
message, and finally the message itself. This kind of packing can be achieved with "pack("l!
a*", $type, $message)". Returns true if successful, or false if there is an error. See also
"IPC::SysV" and "IPC::SysV::Msg" documentation.
my EXPR
my TYPE EXPR
my EXPR : ATTRS
my TYPE EXPR : ATTRS
A "my" declares the listed variables to be local (lexically) to the enclosing block, file, or
"eval". If more than one value is listed, the list must be placed in parentheses.
The exact semantics and interface of TYPE and ATTRS are still evolving. TYPE is currently bound
to the use of "fields" pragma, and attributes are handled using the "attributes" pragma, or
starting from Perl 5.8.0 also via the "Attribute::Handlers" module. See "Private Variables via
my()" in perlsub for details, and fields, attributes, and Attribute::Handlers.
next LABEL
next The "next" command is like the "continue" statement in C; it starts the next iteration of the
loop:
LINE: while (<STDIN>) {
next LINE if /^#/; # discard comments
#...
}
Note that if there were a "continue" block on the above, it would get executed even on discarded
lines. If the LABEL is omitted, the command refers to the innermost enclosing loop.
"next" cannot be used to exit a block which returns a value such as "eval {}", "sub {}" or "do
{}", and should not be used to exit a grep() or map() operation.
Note that a block by itself is semantically identical to a loop that executes once. Thus "next"
will exit such a block early.
See also "continue" for an illustration of how "last", "next", and "redo" work.
no Module VERSION LIST
no Module VERSION
no Module LIST
no Module
See the "use" function, which "no" is the opposite of.
oct EXPR
oct Interprets EXPR as an octal string and returns the corresponding value. (If EXPR happens to
start off with "0x", interprets it as a hex string. If EXPR starts off with "0b", it is
interpreted as a binary string. Leading whitespace is ignored in all three cases.) The
following will handle decimal, binary, octal, and hex in the standard Perl or C notation:
$val = oct($val) if $val =~ /^0/;
If EXPR is omitted, uses $_. To go the other way (produce a number in octal), use sprintf() or
printf():
$perms = (stat("filename"))[2] & 07777;
$oct_perms = sprintf "%lo", $perms;
The oct() function is commonly used when a string such as 644 needs to be converted into a file
mode, for example. (Although perl will automatically convert strings into numbers as needed, this
automatic conversion assumes base 10.)
open FILEHANDLE,EXPR
open FILEHANDLE,MODE,EXPR
open FILEHANDLE,MODE,EXPR,LIST
open FILEHANDLE,MODE,REFERENCE
open FILEHANDLE
Opens the file whose filename is given by EXPR, and associates it with FILEHANDLE.
(The following is a comprehensive reference to open(): for a gentler introduction you may
consider perlopentut.)
If FILEHANDLE is an undefined scalar variable (or array or hash element) the variable is assigned
a reference to a new anonymous filehandle, otherwise if FILEHANDLE is an expression, its value is
used as the name of the real filehandle wanted. (This is considered a symbolic reference, so
"use strict 'refs'" should not be in effect.)
If EXPR is omitted, the scalar variable of the same name as the FILEHANDLE contains the filename.
(Note that lexical variables--those declared with "my"--will not work for this purpose; so if
you're using "my", specify EXPR in your call to open.)
If three or more arguments are specified then the mode of opening and the file name are separate.
If MODE is '<' or nothing, the file is opened for input. If MODE is '>', the file is truncated
and opened for output, being created if necessary. If MODE is '>>', the file is opened for
appending, again being created if necessary.
You can put a '+' in front of the '>' or '<' to indicate that you want both read and write access
to the file; thus '+<' is almost always preferred for read/write updates--the '+>' mode would
clobber the file first. You can't usually use either read-write mode for updating textfiles,
since they have variable length records. See the -i switch in perlrun for a better approach.
The file is created with permissions of 0666 modified by the process' "umask" value.
These various prefixes correspond to the fopen(3) modes of 'r', 'r+', 'w', 'w+', 'a', and 'a+'.
In the 2-arguments (and 1-argument) form of the call the mode and filename should be concatenated
(in this order), possibly separated by spaces. It is possible to omit the mode in these forms if
the mode is '<'.
If the filename begins with '⎪', the filename is interpreted as a command to which output is to
be piped, and if the filename ends with a '⎪', the filename is interpreted as a command which
pipes output to us. See "Using open() for IPC" in perlipc for more examples of this. (You are
not allowed to "open" to a command that pipes both in and out, but see IPC::Open2, IPC::Open3,
and "Bidirectional Communication with Another Process" in perlipc for alternatives.)
For three or more arguments if MODE is '⎪-', the filename is interpreted as a command to which
output is to be piped, and if MODE is '-⎪', the filename is interpreted as a command which pipes
output to us. In the 2-arguments (and 1-argument) form one should replace dash ('-') with the
command. See "Using open() for IPC" in perlipc for more examples of this. (You are not allowed
to "open" to a command that pipes both in and out, but see IPC::Open2, IPC::Open3, and
"Bidirectional Communication" in perlipc for alternatives.)
In the three-or-more argument form of pipe opens, if LIST is specified (extra arguments after the
command name) then LIST becomes arguments to the command invoked if the platform supports it.
The meaning of "open" with more than three arguments for non-pipe modes is not yet specified.
Experimental "layers" may give extra LIST arguments meaning.
In the 2-arguments (and 1-argument) form opening '-' opens STDIN and opening '>-' opens STDOUT.
You may use the three-argument form of open to specify IO "layers" (sometimes also referred to as
"disciplines") to be applied to the handle that affect how the input and output are processed
(see open and PerlIO for more details). For example
open(FH, "<:utf8", "file")
will open the UTF-8 encoded file containing Unicode characters, see perluniintro. (Note that if
layers are specified in the three-arg form then default layers set by the "open" pragma are
ignored.)
Open returns nonzero upon success, the undefined value otherwise. If the "open" involved a pipe,
the return value happens to be the pid of the subprocess.
If you're running Perl on a system that distinguishes between text files and binary files, then
you should check out "binmode" for tips for dealing with this. The key distinction between
systems that need "binmode" and those that don't is their text file formats. Systems like Unix,
Mac OS, and Plan 9, which delimit lines with a single character, and which encode that character
in C as "\n", do not need "binmode". The rest need it.
When opening a file, it's usually a bad idea to continue normal execution if the request failed,
so "open" is frequently used in connection with "die". Even if "die" won't do what you want
(say, in a CGI script, where you want to make a nicely formatted error message (but there are
modules that can help with that problem)) you should always check the return value from opening a
file. The infrequent exception is when working with an unopened filehandle is actually what you
want to do.
As a special case the 3 arg form with a read/write mode and the third argument being "undef":
open(TMP, "+>", undef) or die ...
opens a filehandle to an anonymous temporary file. Also using "+<" works for symmetry, but you
really should consider writing something to the temporary file first. You will need to seek() to
do the reading.
File handles can be opened to "in memory" files held in Perl scalars via:
open($fh, '>', \$variable) ⎪⎪ ..
Though if you try to re-open "STDOUT" or "STDERR" as an "in memory" file, you have to close it
first:
close STDOUT;
open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
Examples:
$ARTICLE = 100;
open ARTICLE or die "Can't find article $ARTICLE: $!\n";
while (<ARTICLE>) {...
open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
# if the open fails, output is discarded
open(DBASE, '+<', 'dbase.mine') # open for update
or die "Can't open 'dbase.mine' for update: $!";
open(DBASE, '+<dbase.mine') # ditto
or die "Can't open 'dbase.mine' for update: $!";
open(ARTICLE, '-⎪', "caesar <$article") # decrypt article
or die "Can't start caesar: $!";
open(ARTICLE, "caesar <$article ⎪") # ditto
or die "Can't start caesar: $!";
open(EXTRACT, "⎪sort >/tmp/Tmp$$") # $$ is our process id
or die "Can't start sort: $!";
# in memory files
open(MEMORY,'>', \$var)
or die "Can't open memory file: $!";
print MEMORY "foo!\n"; # output will end up in $var
# process argument list of files along with any includes
foreach $file (@ARGV) {
process($file, 'fh00');
}
sub process {
my($filename, $input) = @_;
$input++; # this is a string increment
unless (open($input, $filename)) {
print STDERR "Can't open $filename: $!\n";
return;
}
local $_;
while (<$input>) { # note use of indirection
if (/^#include "(.*)"/) {
process($1, $input);
next;
}
#... # whatever
}
}
You may also, in the Bourne shell tradition, specify an EXPR beginning with '>&', in which case
the rest of the string is interpreted as the name of a filehandle (or file descriptor, if
numeric) to be duped (as dup(2)) and opened. You may use "&" after ">", ">>", "<", "+>", "+>>",
and "+<". The mode you specify should match the mode of the original filehandle. (Duping a
filehandle does not take into account any existing contents of IO buffers.) If you use the 3 arg
form then you can pass either a number, the name of a filehandle or the normal "reference to a
glob".
Here is a script that saves, redirects, and restores "STDOUT" and "STDERR" using various methods:
#!/usr/bin/perl
open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
select STDERR; $⎪ = 1; # make unbuffered
select STDOUT; $⎪ = 1; # make unbuffered
print STDOUT "stdout 1\n"; # this works for
print STDERR "stderr 1\n"; # subprocesses too
close STDOUT;
close STDERR;
open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
print STDOUT "stdout 2\n";
print STDERR "stderr 2\n";
If you specify '<&=X', where "X" is a file descriptor number or a filehandle, then Perl will do
an equivalent of C's "fdopen" of that file descriptor (and not call dup(2)); this is more
parsimonious of file descriptors. For example:
# open for input, reusing the fileno of $fd
open(FILEHANDLE, "<&=$fd")
or
open(FILEHANDLE, "<&=", $fd)
or
# open for append, using the fileno of OLDFH
open(FH, ">>&=", OLDFH)
or
open(FH, ">>&=OLDFH")
Being parsimonious on filehandles is also useful (besides being parsimonious) for example when
something is dependent on file descriptors, like for example locking using flock(). If you do
just "open(A, '>>&B')", the filehandle A will not have the same file descriptor as B, and
therefore flock(A) will not flock(B), and vice versa. But with "open(A, '>>&=B')" the
filehandles will share the same file descriptor.
Note that if you are using Perls older than 5.8.0, Perl will be using the standard C libraries'
fdopen() to implement the "=" functionality. On many UNIX systems fdopen() fails when file
descriptors exceed a certain value, typically 255. For Perls 5.8.0 and later, PerlIO is most
often the default.
You can see whether Perl has been compiled with PerlIO or not by running "perl -V" and looking
for "useperlio=" line. If "useperlio" is "define", you have PerlIO, otherwise you don't.
If you open a pipe on the command '-', i.e., either '⎪-' or '-⎪' with 2-arguments (or 1-argument)
form of open(), then there is an implicit fork done, and the return value of open is the pid of
the child within the parent process, and 0 within the child process. (Use "defined($pid)" to
determine whether the open was successful.) The filehandle behaves normally for the parent, but
i/o to that filehandle is piped from/to the STDOUT/STDIN of the child process. In the child
process the filehandle isn't opened--i/o happens from/to the new STDOUT or STDIN. Typically this
is used like the normal piped open when you want to exercise more control over just how the pipe
command gets executed, such as when you are running setuid, and don't want to have to scan shell
commands for metacharacters. The following triples are more or less equivalent:
open(FOO, "⎪tr '[a-z]' '[A-Z]'");
open(FOO, '⎪-', "tr '[a-z]' '[A-Z]'");
open(FOO, '⎪-') ⎪⎪ exec 'tr', '[a-z]', '[A-Z]';
open(FOO, '⎪-', "tr", '[a-z]', '[A-Z]');
open(FOO, "cat -n '$file'⎪");
open(FOO, '-⎪', "cat -n '$file'");
open(FOO, '-⎪') ⎪⎪ exec 'cat', '-n', $file;
open(FOO, '-⎪', "cat", '-n', $file);
The last example in each block shows the pipe as "list form", which is not yet supported on all
platforms. A good rule of thumb is that if your platform has true "fork()" (in other words, if
your platform is UNIX) you can use the list form.
See "Safe Pipe Opens" in perlipc for more examples of this.
Beginning with v5.6.0, Perl will attempt to flush all files opened for output before any
operation that may do a fork, but this may not be supported on some platforms (see perlport). To
be safe, you may need to set $⎪ ($AUTOFLUSH in English) or call the "autoflush()" method of
"IO::Handle" on any open handles.
On systems that support a close-on-exec flag on files, the flag will be set for the newly opened
file descriptor as determined by the value of $^F. See "$^F" in perlvar.
Closing any piped filehandle causes the parent process to wait for the child to finish, and
returns the status value in $?.
The filename passed to 2-argument (or 1-argument) form of open() will have leading and trailing
whitespace deleted, and the normal redirection characters honored. This property, known as
"magic open", can often be used to good effect. A user could specify a filename of "rsh cat file
⎪", or you could change certain filenames as needed:
$filename =~ s/(.*\.gz)\s*$/gzip -dc < $1⎪/;
open(FH, $filename) or die "Can't open $filename: $!";
Use 3-argument form to open a file with arbitrary weird characters in it,
open(FOO, '<', $file);
otherwise it's necessary to protect any leading and trailing whitespace:
$file =~ s#^(\s)#./$1#;
open(FOO, "< $file\0");
(this may not work on some bizarre filesystems). One should conscientiously choose between the
magic and 3-arguments form of open():
open IN, $ARGV[0];
will allow the user to specify an argument of the form "rsh cat file ⎪", but will not work on a
filename which happens to have a trailing space, while
open IN, '<', $ARGV[0];
will have exactly the opposite restrictions.
If you want a "real" C "open" (see open(2) on your system), then you should use the "sysopen"
function, which involves no such magic (but may use subtly different filemodes than Perl open(),
which is mapped to C fopen()). This is another way to protect your filenames from
interpretation. For example:
use IO::Handle;
sysopen(HANDLE, $path, O_RDWR⎪O_CREAT⎪O_EXCL)
or die "sysopen $path: $!";
$oldfh = select(HANDLE); $⎪ = 1; select($oldfh);
print HANDLE "stuff $$\n";
seek(HANDLE, 0, 0);
print "File contains: ", <HANDLE>;
Using the constructor from the "IO::Handle" package (or one of its subclasses, such as "IO::File"
or "IO::Socket"), you can generate anonymous filehandles that have the scope of whatever
variables hold references to them, and automatically close whenever and however you leave that
scope:
use IO::File;
#...
sub read_myfile_munged {
my $ALL = shift;
my $handle = new IO::File;
open($handle, "myfile") or die "myfile: $!";
$first = <$handle>
or return (); # Automatically closed here.
mung $first or die "mung failed"; # Or here.
return $first, <$handle> if $ALL; # Or here.
$first; # Or here.
}
See "seek" for some details about mixing reading and writing.
opendir DIRHANDLE,EXPR
Opens a directory named EXPR for processing by "readdir", "telldir", "seekdir", "rewinddir", and
"closedir". Returns true if successful. DIRHANDLE may be an expression whose value can be used
as an indirect dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined scalar
variable (or array or hash element), the variable is assigned a reference to a new anonymous
dirhandle. DIRHANDLEs have their own namespace separate from FILEHANDLEs.
ord EXPR
ord Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC, or Unicode) value of the
first character of EXPR. If EXPR is omitted, uses $_.
For the reverse, see "chr". See perlunicode and encoding for more about Unicode.
our EXPR
our EXPR TYPE
our EXPR : ATTRS
our TYPE EXPR : ATTRS
An "our" declares the listed variables to be valid globals within the enclosing block, file, or
"eval". That is, it has the same scoping rules as a "my" declaration, but does not create a
local variable. If more than one value is listed, the list must be placed in parentheses. The
"our" declaration has no semantic effect unless "use strict vars" is in effect, in which case it
lets you use the declared global variable without qualifying it with a package name. (But only
within the lexical scope of the "our" declaration. In this it differs from "use vars", which is
package scoped.)
An "our" declaration declares a global variable that will be visible across its entire lexical
scope, even across package boundaries. The package in which the variable is entered is
determined at the point of the declaration, not at the point of use. This means the following
behavior holds:
package Foo;
our $bar; # declares $Foo::bar for rest of lexical scope
$bar = 20;
package Bar;
print $bar; # prints 20
Multiple "our" declarations in the same lexical scope are allowed if they are in different
packages. If they happened to be in the same package, Perl will emit warnings if you have asked
for them.
use warnings;
package Foo;
our $bar; # declares $Foo::bar for rest of lexical scope
$bar = 20;
package Bar;
our $bar = 30; # declares $Bar::bar for rest of lexical scope
print $bar; # prints 30
our $bar; # emits warning
An "our" declaration may also have a list of attributes associated with it.
The exact semantics and interface of TYPE and ATTRS are still evolving. TYPE is currently bound
to the use of "fields" pragma, and attributes are handled using the "attributes" pragma, or
starting from Perl 5.8.0 also via the "Attribute::Handlers" module. See "Private Variables via
my()" in perlsub for details, and fields, attributes, and Attribute::Handlers.
The only currently recognized "our()" attribute is "unique" which indicates that a single copy of
the global is to be used by all interpreters should the program happen to be running in a multi-
interpreter environment. (The default behaviour would be for each interpreter to have its own
copy of the global.) Examples:
our @EXPORT : unique = qw(foo);
our %EXPORT_TAGS : unique = (bar => [qw(aa bb cc)]);
our $VERSION : unique = "1.00";
Note that this attribute also has the effect of making the global readonly when the first new
interpreter is cloned (for example, when the first new thread is created).
Multi-interpreter environments can come to being either through the fork() emulation on Windows
platforms, or by embedding perl in a multi-threaded application. The "unique" attribute does
nothing in all other environments.
pack TEMPLATE,LIST
Takes a LIST of values and converts it into a string using the rules given by the TEMPLATE. The
resulting string is the concatenation of the converted values. Typically, each converted value
looks like its machine-level representation. For example, on 32-bit machines a converted integer
may be represented by a sequence of 4 bytes.
The TEMPLATE is a sequence of characters that give the order and type of values, as follows:
a A string with arbitrary binary data, will be null padded.
A A text (ASCII) string, will be space padded.
Z A null terminated (ASCIZ) string, will be null padded.
b A bit string (ascending bit order inside each byte, like vec()).
B A bit string (descending bit order inside each byte).
h A hex string (low nybble first).
H A hex string (high nybble first).
c A signed char value.
C An unsigned char value. Only does bytes. See U for Unicode.
s A signed short value.
S An unsigned short value.
(This 'short' is _exactly_ 16 bits, which may differ from
what a local C compiler calls 'short'. If you want
native-length shorts, use the '!' suffix.)
i A signed integer value.
I An unsigned integer value.
(This 'integer' is _at_least_ 32 bits wide. Its exact
size depends on what a local C compiler calls 'int',
and may even be larger than the 'long' described in
the next item.)
l A signed long value.
L An unsigned long value.
(This 'long' is _exactly_ 32 bits, which may differ from
what a local C compiler calls 'long'. If you want
native-length longs, use the '!' suffix.)
n An unsigned short in "network" (big-endian) order.
N An unsigned long in "network" (big-endian) order.
v An unsigned short in "VAX" (little-endian) order.
V An unsigned long in "VAX" (little-endian) order.
(These 'shorts' and 'longs' are _exactly_ 16 bits and
_exactly_ 32 bits, respectively.)
q A signed quad (64-bit) value.
Q An unsigned quad value.
(Quads are available only if your system supports 64-bit
integer values _and_ if Perl has been compiled to support those.
Causes a fatal error otherwise.)
j A signed integer value (a Perl internal integer, IV).
J An unsigned integer value (a Perl internal unsigned integer, UV).
f A single-precision float in the native format.
d A double-precision float in the native format.
F A floating point value in the native native format
(a Perl internal floating point value, NV).
D A long double-precision float in the native format.
(Long doubles are available only if your system supports long
double values _and_ if Perl has been compiled to support those.
Causes a fatal error otherwise.)
p A pointer to a null-terminated string.
P A pointer to a structure (fixed-length string).
u A uuencoded string.
U A Unicode character number. Encodes to UTF-8 internally
(or UTF-EBCDIC in EBCDIC platforms).
w A BER compressed integer. Its bytes represent an unsigned
integer in base 128, most significant digit first, with as
few digits as possible. Bit eight (the high bit) is set
on each byte except the last.
x A null byte.
X Back up a byte.
@ Null fill to absolute position, counted from the start of
the innermost ()-group.
( Start of a ()-group.
The following rules apply:
* Each letter may optionally be followed by a number giving a repeat count. With all types
except "a", "A", "Z", "b", "B", "h", "H", "@", "x", "X" and "P" the pack function will
gobble up that many values from the LIST. A "*" for the repeat count means to use
however many items are left, except for "@", "x", "X", where it is equivalent to 0, and
"u", where it is equivalent to 1 (or 45, what is the same). A numeric repeat count may
optionally be enclosed in brackets, as in "pack 'C[80]', @arr".
One can replace the numeric repeat count by a template enclosed in brackets; then the
packed length of this template in bytes is used as a count. For example, "x[L]" skips a
long (it skips the number of bytes in a long); the template "$t X[$t] $t" unpack()s twice
what $t unpacks. If the template in brackets contains alignment commands (such as
"x![d]"), its packed length is calculated as if the start of the template has the maximal
possible alignment.
When used with "Z", "*" results in the addition of a trailing null byte (so the packed
result will be one longer than the byte "length" of the item).
The repeat count for "u" is interpreted as the maximal number of bytes to encode per line
of output, with 0 and 1 replaced by 45.
* The "a", "A", and "Z" types gobble just one value, but pack it as a string of length
count, padding with nulls or spaces as necessary. When unpacking, "A" strips trailing
spaces and nulls, "Z" strips everything after the first null, and "a" returns data
verbatim. When packing, "a", and "Z" are equivalent.
If the value-to-pack is too long, it is truncated. If too long and an explicit count is
provided, "Z" packs only "$count-1" bytes, followed by a null byte. Thus "Z" always
packs a trailing null byte under all circumstances.
* Likewise, the "b" and "B" fields pack a string that many bits long. Each byte of the
input field of pack() generates 1 bit of the result. Each result bit is based on the
least-significant bit of the corresponding input byte, i.e., on "ord($byte)%2". In
particular, bytes "0" and "1" generate bits 0 and 1, as do bytes "\0" and "\1".
Starting from the beginning of the input string of pack(), each 8-tuple of bytes is
converted to 1 byte of output. With format "b" the first byte of the 8-tuple determines
the least-significant bit of a byte, and with format "B" it determines the most-
significant bit of a byte.
If the length of the input string is not exactly divisible by 8, the remainder is packed
as if the input string were padded by null bytes at the end. Similarly, during
unpack()ing the "extra" bits are ignored.
If the input string of pack() is longer than needed, extra bytes are ignored. A "*" for
the repeat count of pack() means to use all the bytes of the input field. On unpack()ing
the bits are converted to a string of "0"s and "1"s.
* The "h" and "H" fields pack a string that many nybbles (4-bit groups, representable as
hexadecimal digits, 0-9a-f) long.
Each byte of the input field of pack() generates 4 bits of the result. For non-
alphabetical bytes the result is based on the 4 least-significant bits of the input byte,
i.e., on "ord($byte)%16". In particular, bytes "0" and "1" generate nybbles 0 and 1, as
do bytes "\0" and "\1". For bytes "a".."f" and "A".."F" the result is compatible with
the usual hexadecimal digits, so that "a" and "A" both generate the nybble "0xa==10".
The result for bytes "g".."z" and "G".."Z" is not well-defined.
Starting from the beginning of the input string of pack(), each pair of bytes is
converted to 1 byte of output. With format "h" the first byte of the pair determines the
least-significant nybble of the output byte, and with format "H" it determines the most-
significant nybble.
If the length of the input string is not even, it behaves as if padded by a null byte at
the end. Similarly, during unpack()ing the "extra" nybbles are ignored.
If the input string of pack() is longer than needed, extra bytes are ignored. A "*" for
the repeat count of pack() means to use all the bytes of the input field. On unpack()ing
the bits are converted to a string of hexadecimal digits.
* The "p" type packs a pointer to a null-terminated string. You are responsible for
ensuring the string is not a temporary value (which can potentially get deallocated
before you get around to using the packed result). The "P" type packs a pointer to a
structure of the size indicated by the length. A NULL pointer is created if the
corresponding value for "p" or "P" is "undef", similarly for unpack().
* The "/" template character allows packing and unpacking of strings where the packed
structure contains a byte count followed by the string itself. You write length-
item"/"string-item.
The length-item can be any "pack" template letter, and describes how the length value is
packed. The ones likely to be of most use are integer-packing ones like "n" (for Java
strings), "w" (for ASN.1 or SNMP) and "N" (for Sun XDR).
For "pack", the string-item must, at present, be "A*", "a*" or "Z*". For "unpack" the
length of the string is obtained from the length-item, but if you put in the '*' it will
be ignored. For all other codes, "unpack" applies the length value to the next item,
which must not have a repeat count.
unpack 'C/a', "\04Gurusamy"; gives 'Guru'
unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
The length-item is not returned explicitly from "unpack".
Adding a count to the length-item letter is unlikely to do anything useful, unless that
letter is "A", "a" or "Z". Packing with a length-item of "a" or "Z" may introduce "\000"
characters, which Perl does not regard as legal in numeric strings.
* The integer types "s", "S", "l", and "L" may be immediately followed by a "!" suffix to
signify native shorts or longs--as you can see from above for example a bare "l" does
mean exactly 32 bits, the native "long" (as seen by the local C compiler) may be larger.
This is an issue mainly in 64-bit platforms. You can see whether using "!" makes any
difference by
print length(pack("s")), " ", length(pack("s!")), "\n";
print length(pack("l")), " ", length(pack("l!")), "\n";
"i!" and "I!" also work but only because of completeness; they are identical to "i" and
"I".
The actual sizes (in bytes) of native shorts, ints, longs, and long longs on the platform
where Perl was built are also available via Config:
use Config;
print $Config{shortsize}, "\n";
print $Config{intsize}, "\n";
print $Config{longsize}, "\n";
print $Config{longlongsize}, "\n";
(The $Config{longlongsize} will be undefined if your system does not support long longs.)
* The integer formats "s", "S", "i", "I", "l", "L", "j", and "J" are inherently non-
portable between processors and operating systems because they obey the native byteorder
and endianness. For example a 4-byte integer 0x12345678 (305419896 decimal) would be
ordered natively (arranged in and handled by the CPU registers) into bytes as
0x12 0x34 0x56 0x78 # big-endian
0x78 0x56 0x34 0x12 # little-endian
Basically, the Intel and VAX CPUs are little-endian, while everybody else, for example
Motorola m68k/88k, PPC, Sparc, HP PA, Power, and Cray are big-endian. Alpha and MIPS can
be either: Digital/Compaq used/uses them in little-endian mode; SGI/Cray uses them in
big-endian mode.
The names `big-endian' and `little-endian' are comic references to the classic
"Gulliver's Travels" (via the paper "On Holy Wars and a Plea for Peace" by Danny Cohen,
USC/ISI IEN 137, April 1, 1980) and the egg-eating habits of the Lilliputians.
Some systems may have even weirder byte orders such as
0x56 0x78 0x12 0x34
0x34 0x12 0x78 0x56
You can see your system's preference with
print join(" ", map { sprintf "%#02x", $_ }
unpack("C*",pack("L",0x12345678))), "\n";
The byteorder on the platform where Perl was built is also available via Config:
use Config;
print $Config{byteorder}, "\n";
Byteorders '1234' and '12345678' are little-endian, '4321' and '87654321' are big-endian.
If you want portable packed integers use the formats "n", "N", "v", and "V", their byte
endianness and size are known. See also perlport.
* Real numbers (floats and doubles) are in the native machine format only; due to the
multiplicity of floating formats around, and the lack of a standard "network"
representation, no facility for interchange has been made. This means that packed
floating point data written on one machine may not be readable on another - even if both
use IEEE floating point arithmetic (as the endian-ness of the memory representation is
not part of the IEEE spec). See also perlport.
Note that Perl uses doubles internally for all numeric calculation, and converting from
double into float and thence back to double again will lose precision (i.e., "unpack("f",
pack("f", $foo)") will not in general equal $foo).
* If the pattern begins with a "U", the resulting string will be treated as UTF-8-encoded
Unicode. You can force UTF-8 encoding on in a string with an initial "U0", and the bytes
that follow will be interpreted as Unicode characters. If you don't want this to happen,
you can begin your pattern with "C0" (or anything else) to force Perl not to UTF-8 encode
your string, and then follow this with a "U*" somewhere in your pattern.
* You must yourself do any alignment or padding by inserting for example enough 'x'es while
packing. There is no way to pack() and unpack() could know where the bytes are going to
or coming from. Therefore "pack" (and "unpack") handle their output and input as flat
sequences of bytes.
* A ()-group is a sub-TEMPLATE enclosed in parentheses. A group may take a repeat count,
both as postfix, and for unpack() also via the "/" template character. Within each
repetition of a group, positioning with "@" starts again at 0. Therefore, the result of
pack( '@1A((@2A)@3A)', 'a', 'b', 'c' )
is the string "\0a\0\0bc".
* "x" and "X" accept "!" modifier. In this case they act as alignment commands: they jump
forward/back to the closest position aligned at a multiple of "count" bytes. For
example, to pack() or unpack() C's "struct {char c; double d; char cc[2]}" one may need
to use the template "C x![d] d C[2]"; this assumes that doubles must be aligned on the
double's size.
For alignment commands "count" of 0 is equivalent to "count" of 1; both result in no-ops.
* A comment in a TEMPLATE starts with "#" and goes to the end of line. White space may be
used to separate pack codes from each other, but a "!" modifier and a repeat count must
follow immediately.
* If TEMPLATE requires more arguments to pack() than actually given, pack() assumes
additional "" arguments. If TEMPLATE requires less arguments to pack() than actually
given, extra arguments are ignored.
Examples:
$foo = pack("CCCC",65,66,67,68);
# foo eq "ABCD"
$foo = pack("C4",65,66,67,68);
# same thing
$foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
# same thing with Unicode circled letters
$foo = pack("ccxxcc",65,66,67,68);
# foo eq "AB\0\0CD"
# note: the above examples featuring "C" and "c" are true
# only on ASCII and ASCII-derived systems such as ISO Latin 1
# and UTF-8. In EBCDIC the first example would be
# $foo = pack("CCCC",193,194,195,196);
$foo = pack("s2",1,2);
# "\1\0\2\0" on little-endian
# "\0\1\0\2" on big-endian
$foo = pack("a4","abcd","x","y","z");
# "abcd"
$foo = pack("aaaa","abcd","x","y","z");
# "axyz"
$foo = pack("a14","abcdefg");
# "abcdefg\0\0\0\0\0\0\0"
$foo = pack("i9pl", gmtime);
# a real struct tm (on my system anyway)
$utmp_template = "Z8 Z8 Z16 L";
$utmp = pack($utmp_template, @utmp1);
# a struct utmp (BSDish)
@utmp2 = unpack($utmp_template, $utmp);
# "@utmp1" eq "@utmp2"
sub bintodec {
unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
}
$foo = pack('sx2l', 12, 34);
# short 12, two zero bytes padding, long 34
$bar = pack('s@4l', 12, 34);
# short 12, zero fill to position 4, long 34
# $foo eq $bar
The same template may generally also be used in unpack().
package NAMESPACE
package Declares the compilation unit as being in the given namespace. The scope of the package
declaration is from the declaration itself through the end of the enclosing block, file, or eval
(the same as the "my" operator). All further unqualified dynamic identifiers will be in this
namespace. A package statement affects only dynamic variables--including those you've used
"local" on--but not lexical variables, which are created with "my". Typically it would be the
first declaration in a file to be included by the "require" or "use" operator. You can switch
into a package in more than one place; it merely influences which symbol table is used by the
compiler for the rest of that block. You can refer to variables and filehandles in other
packages by prefixing the identifier with the package name and a double colon:
$Package::Variable. If the package name is null, the "main" package as assumed. That is,
$::sail is equivalent to $main::sail (as well as to $main'sail, still seen in older code).
If NAMESPACE is omitted, then there is no current package, and all identifiers must be fully
qualified or lexicals. However, you are strongly advised not to make use of this feature. Its
use can cause unexpected behaviour, even crashing some versions of Perl. It is deprecated, and
will be removed from a future release.
See "Packages" in perlmod for more information about packages, modules, and classes. See perlsub
for other scoping issues.
pipe READHANDLE,WRITEHANDLE
Opens a pair of connected pipes like the corresponding system call. Note that if you set up a
loop of piped processes, deadlock can occur unless you are very careful. In addition, note that
Perl's pipes use IO buffering, so you may need to set $⎪ to flush your WRITEHANDLE after each
command, depending on the application.
See IPC::Open2, IPC::Open3, and "Bidirectional Communication" in perlipc for examples of such
things.
On systems that support a close-on-exec flag on files, the flag will be set for the newly opened
file descriptors as determined by the value of $^F. See "$^F" in perlvar.
pop ARRAY
pop Pops and returns the last value of the array, shortening the array by one element. Has an effect
similar to
$ARRAY[$#ARRAY--]
If there are no elements in the array, returns the undefined value (although this may happen at
other times as well). If ARRAY is omitted, pops the @ARGV array in the main program, and the @_
array in subroutines, just like "shift".
pos SCALAR
pos Returns the offset of where the last "m//g" search left off for the variable in question ($_ is
used when the variable is not specified). May be modified to change that offset. Such
modification will also influence the "\G" zero-width assertion in regular expressions. See
perlre and perlop.
print FILEHANDLE LIST
print LIST
print Prints a string or a list of strings. Returns true if successful. FILEHANDLE may be a scalar
variable name, in which case the variable contains the name of or a reference to the filehandle,
thus introducing one level of indirection. (NOTE: If FILEHANDLE is a variable and the next token
is a term, it may be misinterpreted as an operator unless you interpose a "+" or put parentheses
around the arguments.) If FILEHANDLE is omitted, prints by default to standard output (or to the
last selected output channel--see "select"). If LIST is also omitted, prints $_ to the currently
selected output channel. To set the default output channel to something other than STDOUT use
the select operation. The current value of $, (if any) is printed between each LIST item. The
current value of "$\" (if any) is printed after the entire LIST has been printed. Because print
takes a LIST, anything in the LIST is evaluated in list context, and any subroutine that you call
will have one or more of its expressions evaluated in list context. Also be careful not to
follow the print keyword with a left parenthesis unless you want the corresponding right
parenthesis to terminate the arguments to the print--interpose a "+" or put parentheses around
all the arguments.
Note that if you're storing FILEHANDLES in an array or other expression, you will have to use a
block returning its value instead:
print { $files[$i] } "stuff\n";
print { $OK ? STDOUT : STDERR } "stuff\n";
printf FILEHANDLE FORMAT, LIST
printf FORMAT, LIST
Equivalent to "print FILEHANDLE sprintf(FORMAT, LIST)", except that "$\" (the output record
separator) is not appended. The first argument of the list will be interpreted as the "printf"
format. See "sprintf" for an explanation of the format argument. If "use locale" is in effect,
the character used for the decimal point in formatted real numbers is affected by the LC_NUMERIC
locale. See perllocale.
Don't fall into the trap of using a "printf" when a simple "print" would do. The "print" is more
efficient and less error prone.
prototype FUNCTION
Returns the prototype of a function as a string (or "undef" if the function has no prototype).
FUNCTION is a reference to, or the name of, the function whose prototype you want to retrieve.
If FUNCTION is a string starting with "CORE::", the rest is taken as a name for Perl builtin. If
the builtin is not overridable (such as "qw//") or its arguments cannot be expressed by a
prototype (such as "system") returns "undef" because the builtin does not really behave like a
Perl function. Otherwise, the string describing the equivalent prototype is returned.
push ARRAY,LIST
Treats ARRAY as a stack, and pushes the values of LIST onto the end of ARRAY. The length of
ARRAY increases by the length of LIST. Has the same effect as
for $value (LIST) {
$ARRAY[++$#ARRAY] = $value;
}
but is more efficient. Returns the new number of elements in the array.
q/STRING/
qq/STRING/
qr/STRING/
qx/STRING/
qw/STRING/
Generalized quotes. See "Regexp Quote-Like Operators" in perlop.
quotemeta EXPR
quotemeta
Returns the value of EXPR with all non-"word" characters backslashed. (That is, all characters
not matching "/[A-Za-z_0-9]/" will be preceded by a backslash in the returned string, regardless
of any locale settings.) This is the internal function implementing the "\Q" escape in double-
quoted strings.
If EXPR is omitted, uses $_.
rand EXPR
rand Returns a random fractional number greater than or equal to 0 and less than the value of EXPR.
(EXPR should be positive.) If EXPR is omitted, the value 1 is used. Currently EXPR with the
value 0 is also special-cased as 1 - this has not been documented before perl 5.8.0 and is
subject to change in future versions of perl. Automatically calls "srand" unless "srand" has
already been called. See also "srand".
Apply "int()" to the value returned by "rand()" if you want random integers instead of random
fractional numbers. For example,
int(rand(10))
returns a random integer between 0 and 9, inclusive.
(Note: If your rand function consistently returns numbers that are too large or too small, then
your version of Perl was probably compiled with the wrong number of RANDBITS.)
read FILEHANDLE,SCALAR,LENGTH,OFFSET
read FILEHANDLE,SCALAR,LENGTH
Attempts to read LENGTH characters of data into variable SCALAR from the specified FILEHANDLE.
Returns the number of characters actually read, 0 at end of file, or undef if there was an error
(in the latter case $! is also set). SCALAR will be grown or shrunk so that the last character
actually read is the last character of the scalar after the read.
An OFFSET may be specified to place the read data at some place in the string other than the
beginning. A negative OFFSET specifies placement at that many characters counting backwards from
the end of the string. A positive OFFSET greater than the length of SCALAR results in the string
being padded to the required size with "\0" bytes before the result of the read is appended.
The call is actually implemented in terms of either Perl's or system's fread() call. To get a
true read(2) system call, see "sysread".
Note the characters: depending on the status of the filehandle, either (8-bit) bytes or
characters are read. By default all filehandles operate on bytes, but for example if the
filehandle has been opened with the ":utf8" I/O layer (see "open", and the "open" pragma, open),
the I/O will operate on UTF-8 encoded Unicode characters, not bytes. Similarly for the
":encoding" pragma: in that case pretty much any characters can be read.
readdir DIRHANDLE
Returns the next directory entry for a directory opened by "opendir". If used in list context,
returns all the rest of the entries in the directory. If there are no more entries, returns an
undefined value in scalar context or a null list in list context.
If you're planning to filetest the return values out of a "readdir", you'd better prepend the
directory in question. Otherwise, because we didn't "chdir" there, it would have been testing
the wrong file.
opendir(DIR, $some_dir) ⎪⎪ die "can't opendir $some_dir: $!";
@dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
closedir DIR;
readline EXPR
Reads from the filehandle whose typeglob is contained in EXPR. In scalar context, each call
reads and returns the next line, until end-of-file is reached, whereupon the subsequent call
returns undef. In list context, reads until end-of-file is reached and returns a list of lines.
Note that the notion of "line" used here is however you may have defined it with $/ or
$INPUT_RECORD_SEPARATOR). See "$/" in perlvar.
When $/ is set to "undef", when readline() is in scalar context (i.e. file slurp mode), and when
an empty file is read, it returns '' the first time, followed by "undef" subsequently.
This is the internal function implementing the "<EXPR>" operator, but you can use it directly.
The "<EXPR>" operator is discussed in more detail in "I/O Operators" in perlop.
$line = <STDIN>;
$line = readline(*STDIN); # same thing
If readline encounters an operating system error, $! will be set with the corresponding error
message. It can be helpful to check $! when you are reading from filehandles you don't trust,
such as a tty or a socket. The following example uses the operator form of "readline", and takes
the necessary steps to ensure that "readline" was successful.
for (;;) {
undef $!;
unless (defined( $line = <> )) {
die $! if $!;
last; # reached EOF
}
# ...
}
readlink EXPR
readlink
Returns the value of a symbolic link, if symbolic links are implemented. If not, gives a fatal
error. If there is some system error, returns the undefined value and sets $! (errno). If EXPR
is omitted, uses $_.
readpipe EXPR
EXPR is executed as a system command. The collected standard output of the command is returned.
In scalar context, it comes back as a single (potentially multi-line) string. In list context,
returns a list of lines (however you've defined lines with $/ or $INPUT_RECORD_SEPARATOR). This
is the internal function implementing the "qx/EXPR/" operator, but you can use it directly. The
"qx/EXPR/" operator is discussed in more detail in "I/O Operators" in perlop.
recv SOCKET,SCALAR,LENGTH,FLAGS
Receives a message on a socket. Attempts to receive LENGTH characters of data into variable
SCALAR from the specified SOCKET filehandle. SCALAR will be grown or shrunk to the length
actually read. Takes the same flags as the system call of the same name. Returns the address of
the sender if SOCKET's protocol supports this; returns an empty string otherwise. If there's an
error, returns the undefined value. This call is actually implemented in terms of recvfrom(2)
system call. See "UDP: Message Passing" in perlipc for examples.
Note the characters: depending on the status of the socket, either (8-bit) bytes or characters
are received. By default all sockets operate on bytes, but for example if the socket has been
changed using binmode() to operate with the ":utf8" I/O layer (see the "open" pragma, open), the
I/O will operate on UTF-8 encoded Unicode characters, not bytes. Similarly for the ":encoding"
pragma: in that case pretty much any characters can be read.
redo LABEL
redo The "redo" command restarts the loop block without evaluating the conditional again. The
"continue" block, if any, is not executed. If the LABEL is omitted, the command refers to the
innermost enclosing loop. This command is normally used by programs that want to lie to
themselves about what was just input:
# a simpleminded Pascal comment stripper
# (warning: assumes no { or } in strings)
LINE: while (<STDIN>) {
while (s⎪({.*}.*){.*}⎪$1 ⎪) {}
s⎪{.*}⎪ ⎪;
if (s⎪{.*⎪ ⎪) {
$front = $_;
while (<STDIN>) {
if (/}/) { # end of comment?
s⎪^⎪$front\{⎪;
redo LINE;
}
}
}
print;
}
"redo" cannot be used to retry a block which returns a value such as "eval {}", "sub {}" or "do
{}", and should not be used to exit a grep() or map() operation.
Note that a block by itself is semantically identical to a loop that executes once. Thus "redo"
inside such a block will effectively turn it into a looping construct.
See also "continue" for an illustration of how "last", "next", and "redo" work.
ref EXPR
ref Returns a true value if EXPR is a reference, false otherwise. If EXPR is not specified, $_ will
be used. The value returned depends on the type of thing the reference is a reference to.
Builtin types include:
SCALAR
ARRAY
HASH
CODE
REF
GLOB
LVALUE
If the referenced object has been blessed into a package, then that package name is returned
instead. You can think of "ref" as a "typeof" operator.
if (ref($r) eq "HASH") {
print "r is a reference to a hash.\n";
}
unless (ref($r)) {
print "r is not a reference at all.\n";
}
if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
print "r is a reference to something that isa hash.\n";
}
See also perlref.
rename OLDNAME,NEWNAME
Changes the name of a file; an existing file NEWNAME will be clobbered. Returns true for
success, false otherwise.
Behavior of this function varies wildly depending on your system implementation. For example, it
will usually not work across file system boundaries, even though the system mv command sometimes
compensates for this. Other restrictions include whether it works on directories, open files, or
pre-existing files. Check perlport and either the rename(2) manpage or equivalent system
documentation for details.
require VERSION
require EXPR
require Demands a version of Perl specified by VERSION, or demands some semantics specified by EXPR or by
$_ if EXPR is not supplied.
VERSION may be either a numeric argument such as 5.006, which will be compared to $], or a
literal of the form v5.6.1, which will be compared to $^V (aka $PERL_VERSION). A fatal error is
produced at run time if VERSION is greater than the version of the current Perl interpreter.
Compare with "use", which can do a similar check at compile time.
Specifying VERSION as a literal of the form v5.6.1 should generally be avoided, because it leads
to misleading error messages under earlier versions of Perl which do not support this syntax.
The equivalent numeric version should be used instead.
require v5.6.1; # run time version check
require 5.6.1; # ditto
require 5.006_001; # ditto; preferred for backwards compatibility
Otherwise, demands that a library file be included if it hasn't already been included. The file
is included via the do-FILE mechanism, which is essentially just a variety of "eval". Has
semantics similar to the following subroutine:
sub require {
my($filename) = @_;
return 1 if $INC{$filename};
my($realfilename,$result);
ITER: {
foreach $prefix (@INC) {
$realfilename = "$prefix/$filename";
if (-f $realfilename) {
$INC{$filename} = $realfilename;
$result = do $realfilename;
last ITER;
}
}
die "Can't find $filename in \@INC";
}
delete $INC{$filename} if $@ ⎪⎪ !$result;
die $@ if $@;
die "$filename did not return true value" unless $result;
return $result;
}
Note that the file will not be included twice under the same specified name. The file must
return true as the last statement to indicate successful execution of any initialization code, so
it's customary to end such a file with "1;" unless you're sure it'll return true otherwise. But
it's better just to put the "1;", in case you add more statements.
If EXPR is a bareword, the require assumes a ".pm" extension and replaces "::" with "/" in the
filename for you, to make it easy to load standard modules. This form of loading of modules does
not risk altering your namespace.
In other words, if you try this:
require Foo::Bar; # a splendid bareword
The require function will actually look for the "Foo/Bar.pm" file in the directories specified in
the @INC array.
But if you try this:
$class = 'Foo::Bar';
require $class; # $class is not a bareword
#or
require "Foo::Bar"; # not a bareword because of the ""
The require function will look for the "Foo::Bar" file in the @INC array and will complain about
not finding "Foo::Bar" there. In this case you can do:
eval "require $class";
Now that you understand how "require" looks for files in the case of a bareword argument, there
is a little extra functionality going on behind the scenes. Before "require" looks for a ".pm"
extension, it will first look for a filename with a ".pmc" extension. A file with this extension
is assumed to be Perl bytecode generated by B::Bytecode. If this file is found, and it's
modification time is newer than a coinciding ".pm" non-compiled file, it will be loaded in place
of that non-compiled file ending in a ".pm" extension.
You can also insert hooks into the import facility, by putting directly Perl code into the @INC
array. There are three forms of hooks: subroutine references, array references and blessed
objects.
Subroutine references are the simplest case. When the inclusion system walks through @INC and
encounters a subroutine, this subroutine gets called with two parameters, the first being a
reference to itself, and the second the name of the file to be included (e.g. "Foo/Bar.pm"). The
subroutine should return "undef" or a filehandle, from which the file to include will be read.
If "undef" is returned, "require" will look at the remaining elements of @INC.
If the hook is an array reference, its first element must be a subroutine reference. This
subroutine is called as above, but the first parameter is the array reference. This enables to
pass indirectly some arguments to the subroutine.
In other words, you can write:
push @INC, \&my_sub;
sub my_sub {
my ($coderef, $filename) = @_; # $coderef is \&my_sub
...
}
or:
push @INC, [ \&my_sub, $x, $y, ... ];
sub my_sub {
my ($arrayref, $filename) = @_;
# Retrieve $x, $y, ...
my @parameters = @$arrayref[1..$#$arrayref];
...
}
If the hook is an object, it must provide an INC method, that will be called as above, the first
parameter being the object itself. (Note that you must fully qualify the sub's name, as it is
always forced into package "main".) Here is a typical code layout:
# In Foo.pm
package Foo;
sub new { ... }
sub Foo::INC {
my ($self, $filename) = @_;
...
}
# In the main program
push @INC, new Foo(...);
Note that these hooks are also permitted to set the %INC entry corresponding to the files they
have loaded. See "%INC" in perlvar.
For a yet-more-powerful import facility, see "use" and perlmod.
reset EXPR
reset Generally used in a "continue" block at the end of a loop to clear variables and reset "??"
searches so that they work again. The expression is interpreted as a list of single characters
(hyphens allowed for ranges). All variables and arrays beginning with one of those letters are
reset to their pristine state. If the expression is omitted, one-match searches ("?pattern?")
are reset to match again. Resets only variables or searches in the current package. Always
returns 1. Examples:
reset 'X'; # reset all X variables
reset 'a-z'; # reset lower case variables
reset; # just reset ?one-time? searches
Resetting "A-Z" is not recommended because you'll wipe out your @ARGV and @INC arrays and your
%ENV hash. Resets only package variables--lexical variables are unaffected, but they clean
themselves up on scope exit anyway, so you'll probably want to use them instead. See "my".
return EXPR
return Returns from a subroutine, "eval", or "do FILE" with the value given in EXPR. Evaluation of EXPR
may be in list, scalar, or void context, depending on how the return value will be used, and the
context may vary from one execution to the next (see "wantarray"). If no EXPR is given, returns
an empty list in list context, the undefined value in scalar context, and (of course) nothing at
all in a void context.
(Note that in the absence of an explicit "return", a subroutine, eval, or do FILE will
automatically return the value of the last expression evaluated.)
reverse LIST
In list context, returns a list value consisting of the elements of LIST in the opposite order.
In scalar context, concatenates the elements of LIST and returns a string value with all
characters in the opposite order.
print reverse <>; # line tac, last line first
undef $/; # for efficiency of <>
print scalar reverse <>; # character tac, last line tsrif
This operator is also handy for inverting a hash, although there are some caveats. If a value is
duplicated in the original hash, only one of those can be represented as a key in the inverted
hash. Also, this has to unwind one hash and build a whole new one, which may take some time on a
large hash, such as from a DBM file.
%by_name = reverse %by_address; # Invert the hash
rewinddir DIRHANDLE
Sets the current position to the beginning of the directory for the "readdir" routine on
DIRHANDLE.
rindex STR,SUBSTR,POSITION
rindex STR,SUBSTR
Works just like index() except that it returns the position of the LAST occurrence of SUBSTR in
STR. If POSITION is specified, returns the last occurrence at or before that position.
rmdir FILENAME
rmdir Deletes the directory specified by FILENAME if that directory is empty. If it succeeds it
returns true, otherwise it returns false and sets $! (errno). If FILENAME is omitted, uses $_.
s/// The substitution operator. See perlop.
scalar EXPR
Forces EXPR to be interpreted in scalar context and returns the value of EXPR.
@counts = ( scalar @a, scalar @b, scalar @c );
There is no equivalent operator to force an expression to be interpolated in list context because
in practice, this is never needed. If you really wanted to do so, however, you could use the
construction "@{[ (some expression) ]}", but usually a simple "(some expression)" suffices.
Because "scalar" is unary operator, if you accidentally use for EXPR a parenthesized list, this
behaves as a scalar comma expression, evaluating all but the last element in void context and
returning the final element evaluated in scalar context. This is seldom what you want.
The following single statement:
print uc(scalar(&foo,$bar)),$baz;
is the moral equivalent of these two:
&foo;
print(uc($bar),$baz);
See perlop for more details on unary operators and the comma operator.
seek FILEHANDLE,POSITION,WHENCE
Sets FILEHANDLE's position, just like the "fseek" call of "stdio". FILEHANDLE may be an
expression whose value gives the name of the filehandle. The values for WHENCE are 0 to set the
new position in bytes to POSITION, 1 to set it to the current position plus POSITION, and 2 to
set it to EOF plus POSITION (typically negative). For WHENCE you may use the constants
"SEEK_SET", "SEEK_CUR", and "SEEK_END" (start of the file, current position, end of the file)
from the Fcntl module. Returns 1 upon success, 0 otherwise.
Note the in bytes: even if the filehandle has been set to operate on characters (for example by
using the ":utf8" open layer), tell() will return byte offsets, not character offsets (because
implementing that would render seek() and tell() rather slow).
If you want to position file for "sysread" or "syswrite", don't use "seek"--buffering makes its
effect on the file's system position unpredictable and non-portable. Use "sysseek" instead.
Due to the rules and rigors of ANSI C, on some systems you have to do a seek whenever you switch
between reading and writing. Amongst other things, this may have the effect of calling stdio's
clearerr(3). A WHENCE of 1 ("SEEK_CUR") is useful for not moving the file position:
seek(TEST,0,1);
This is also useful for applications emulating "tail -f". Once you hit EOF on your read, and
then sleep for a while, you might have to stick in a seek() to reset things. The "seek" doesn't
change the current position, but it does clear the end-of-file condition on the handle, so that
the next "<FILE>" makes Perl try again to read something. We hope.
If that doesn't work (some IO implementations are particularly cantankerous), then you may need
something more like this:
for (;;) {
for ($curpos = tell(FILE); $_ = <FILE>;
$curpos = tell(FILE)) {
# search for some stuff and put it into files
}
sleep($for_a_while);
seek(FILE, $curpos, 0);
}
seekdir DIRHANDLE,POS
Sets the current position for the "readdir" routine on DIRHANDLE. POS must be a value returned
by "telldir". Has the same caveats about possible directory compaction as the corresponding
system library routine.
select FILEHANDLE
select Returns the currently selected filehandle. Sets the current default filehandle for output, if
FILEHANDLE is supplied. This has two effects: first, a "write" or a "print" without a filehandle
will default to this FILEHANDLE. Second, references to variables related to output will refer to
this output channel. For example, if you have to set the top of form format for more than one
output channel, you might do the following:
select(REPORT1);
$^ = 'report1_top';
select(REPORT2);
$^ = 'report2_top';
FILEHANDLE may be an expression whose value gives the name of the actual filehandle. Thus:
$oldfh = select(STDERR); $⎪ = 1; select($oldfh);
Some programmers may prefer to think of filehandles as objects with methods, preferring to write
the last example as:
use IO::Handle;
STDERR->autoflush(1);
select RBITS,WBITS,EBITS,TIMEOUT
This calls the select(2) system call with the bit masks specified, which can be constructed using
"fileno" and "vec", along these lines:
$rin = $win = $ein = '';
vec($rin,fileno(STDIN),1) = 1;
vec($win,fileno(STDOUT),1) = 1;
$ein = $rin ⎪ $win;
If you want to select on many filehandles you might wish to write a subroutine:
sub fhbits {
my(@fhlist) = split(' ',$_[0]);
my($bits);
for (@fhlist) {
vec($bits,fileno($_),1) = 1;
}
$bits;
}
$rin = fhbits('STDIN TTY SOCK');
The usual idiom is:
($nfound,$timeleft) =
select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
or to block until something becomes ready just do this
$nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
Most systems do not bother to return anything useful in $timeleft, so calling select() in scalar
context just returns $nfound.
Any of the bit masks can also be undef. The timeout, if specified, is in seconds, which may be
fractional. Note: not all implementations are capable of returning the $timeleft. If not, they
always return $timeleft equal to the supplied $timeout.
You can effect a sleep of 250 milliseconds this way:
select(undef, undef, undef, 0.25);
Note that whether "select" gets restarted after signals (say, SIGALRM) is
implementation-dependent.
WARNING: One should not attempt to mix buffered I/O (like "read" or <FH>) with "select", except
as permitted by POSIX, and even then only on POSIX systems. You have to use "sysread" instead.
semctl ID,SEMNUM,CMD,ARG
Calls the System V IPC function "semctl". You'll probably have to say
use IPC::SysV;
first to get the correct constant definitions. If CMD is IPC_STAT or GETALL, then ARG must be a
variable which will hold the returned semid_ds structure or semaphore value array. Returns like
"ioctl": the undefined value for error, ""0 but true"" for zero, or the actual return value
otherwise. The ARG must consist of a vector of native short integers, which may be created with
"pack("s!",(0)x$nsem)". See also "SysV IPC" in perlipc, "IPC::SysV", "IPC::Semaphore"
documentation.
semget KEY,NSEMS,FLAGS
Calls the System V IPC function semget. Returns the semaphore id, or the undefined value if
there is an error. See also "SysV IPC" in perlipc, "IPC::SysV", "IPC::SysV::Semaphore"
documentation.
semop KEY,OPSTRING
Calls the System V IPC function semop to perform semaphore operations such as signalling and
waiting. OPSTRING must be a packed array of semop structures. Each semop structure can be
generated with "pack("s!3", $semnum, $semop, $semflag)". The number of semaphore operations is
implied by the length of OPSTRING. Returns true if successful, or false if there is an error.
As an example, the following code waits on semaphore $semnum of semaphore id $semid:
$semop = pack("s!3", $semnum, -1, 0);
die "Semaphore trouble: $!\n" unless semop($semid, $semop);
To signal the semaphore, replace "-1" with 1. See also "SysV IPC" in perlipc, "IPC::SysV", and
"IPC::SysV::Semaphore" documentation.
send SOCKET,MSG,FLAGS,TO
send SOCKET,MSG,FLAGS
Sends a message on a socket. Attempts to send the scalar MSG to the SOCKET filehandle. Takes
the same flags as the system call of the same name. On unconnected sockets you must specify a
destination to send TO, in which case it does a C "sendto". Returns the number of characters
sent, or the undefined value if there is an error. The C system call sendmsg(2) is currently
unimplemented. See "UDP: Message Passing" in perlipc for examples.
Note the characters: depending on the status of the socket, either (8-bit) bytes or characters
are sent. By default all sockets operate on bytes, but for example if the socket has been
changed using binmode() to operate with the ":utf8" I/O layer (see "open", or the "open" pragma,
open), the I/O will operate on UTF-8 encoded Unicode characters, not bytes. Similarly for the
":encoding" pragma: in that case pretty much any characters can be sent.
setpgrp PID,PGRP
Sets the current process group for the specified PID, 0 for the current process. Will produce a
fatal error if used on a machine that doesn't implement POSIX setpgid(2) or BSD setpgrp(2). If
the arguments are omitted, it defaults to "0,0". Note that the BSD 4.2 version of "setpgrp" does
not accept any arguments, so only "setpgrp(0,0)" is portable. See also "POSIX::setsid()".
setpriority WHICH,WHO,PRIORITY
Sets the current priority for a process, a process group, or a user. (See setpriority(2).) Will
produce a fatal error if used on a machine that doesn't implement setpriority(2).
setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
Sets the socket option requested. Returns undefined if there is an error. OPTVAL may be
specified as "undef" if you don't want to pass an argument.
shift ARRAY
shift Shifts the first value of the array off and returns it, shortening the array by 1 and moving
everything down. If there are no elements in the array, returns the undefined value. If ARRAY
is omitted, shifts the @_ array within the lexical scope of subroutines and formats, and the
@ARGV array at file scopes or within the lexical scopes established by the "eval ''", "BEGIN {}",
"INIT {}", "CHECK {}", and "END {}" constructs.
See also "unshift", "push", and "pop". "shift" and "unshift" do the same thing to the left end
of an array that "pop" and "push" do to the right end.
shmctl ID,CMD,ARG
Calls the System V IPC function shmctl. You'll probably have to say
use IPC::SysV;
first to get the correct constant definitions. If CMD is "IPC_STAT", then ARG must be a variable
which will hold the returned "shmid_ds" structure. Returns like ioctl: the undefined value for
error, "0 but true" for zero, or the actual return value otherwise. See also "SysV IPC" in
perlipc and "IPC::SysV" documentation.
shmget KEY,SIZE,FLAGS
Calls the System V IPC function shmget. Returns the shared memory segment id, or the undefined
value if there is an error. See also "SysV IPC" in perlipc and "IPC::SysV" documentation.
shmread ID,VAR,POS,SIZE
shmwrite ID,STRING,POS,SIZE
Reads or writes the System V shared memory segment ID starting at position POS for size SIZE by
attaching to it, copying in/out, and detaching from it. When reading, VAR must be a variable
that will hold the data read. When writing, if STRING is too long, only SIZE bytes are used; if
STRING is too short, nulls are written to fill out SIZE bytes. Return true if successful, or
false if there is an error. shmread() taints the variable. See also "SysV IPC" in perlipc,
"IPC::SysV" documentation, and the "IPC::Shareable" module from CPAN.
shutdown SOCKET,HOW
Shuts down a socket connection in the manner indicated by HOW, which has the same interpretation
as in the system call of the same name.
shutdown(SOCKET, 0); # I/we have stopped reading data
shutdown(SOCKET, 1); # I/we have stopped writing data
shutdown(SOCKET, 2); # I/we have stopped using this socket
This is useful with sockets when you want to tell the other side you're done writing but not done
reading, or vice versa. It's also a more insistent form of close because it also disables the
file descriptor in any forked copies in other processes.
sin EXPR
sin Returns the sine of EXPR (expressed in radians). If EXPR is omitted, returns sine of $_.
For the inverse sine operation, you may use the "Math::Trig::asin" function, or use this
relation:
sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
sleep EXPR
sleep Causes the script to sleep for EXPR seconds, or forever if no EXPR. May be interrupted if the
process receives a signal such as "SIGALRM". Returns the number of seconds actually slept. You
probably cannot mix "alarm" and "sleep" calls, because "sleep" is often implemented using
"alarm".
On some older systems, it may sleep up to a full second less than what you requested, depending
on how it counts seconds. Most modern systems always sleep the full amount. They may appear to
sleep longer than that, however, because your process might not be scheduled right away in a busy
multitasking system.
For delays of finer granularity than one second, you may use Perl's "syscall" interface to access
setitimer(2) if your system supports it, or else see "select" above. The Time::HiRes module
(from CPAN, and starting from Perl 5.8 part of the standard distribution) may also help.
See also the POSIX module's "pause" function.
socket SOCKET,DOMAIN,TYPE,PROTOCOL
Opens a socket of the specified kind and attaches it to filehandle SOCKET. DOMAIN, TYPE, and
PROTOCOL are specified the same as for the system call of the same name. You should "use Socket"
first to get the proper definitions imported. See the examples in "Sockets: Client/Server
Communication" in perlipc.
On systems that support a close-on-exec flag on files, the flag will be set for the newly opened
file descriptor, as determined by the value of $^F. See "$^F" in perlvar.
socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
Creates an unnamed pair of sockets in the specified domain, of the specified type. DOMAIN, TYPE,
and PROTOCOL are specified the same as for the system call of the same name. If unimplemented,
yields a fatal error. Returns true if successful.
On systems that support a close-on-exec flag on files, the flag will be set for the newly opened
file descriptors, as determined by the value of $^F. See "$^F" in perlvar.
Some systems defined "pipe" in terms of "socketpair", in which a call to "pipe(Rdr, Wtr)" is
essentially:
use Socket;
socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
shutdown(Rdr, 1); # no more writing for reader
shutdown(Wtr, 0); # no more reading for writer
See perlipc for an example of socketpair use. Perl 5.8 and later will emulate socketpair using
IP sockets to localhost if your system implements sockets but not socketpair.
sort SUBNAME LIST
sort BLOCK LIST
sort LIST
In list context, this sorts the LIST and returns the sorted list value. In scalar context, the
behaviour of "sort()" is undefined.
If SUBNAME or BLOCK is omitted, "sort"s in standard string comparison order. If SUBNAME is
specified, it gives the name of a subroutine that returns an integer less than, equal to, or
greater than 0, depending on how the elements of the list are to be ordered. (The "<=>" and
"cmp" operators are extremely useful in such routines.) SUBNAME may be a scalar variable name
(unsubscripted), in which case the value provides the name of (or a reference to) the actual
subroutine to use. In place of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
subroutine.
If the subroutine's prototype is "($$)", the elements to be compared are passed by reference in
@_, as for a normal subroutine. This is slower than unprototyped subroutines, where the elements
to be compared are passed into the subroutine as the package global variables $a and $b (see
example below). Note that in the latter case, it is usually counter-productive to declare $a and
$b as lexicals.
In either case, the subroutine may not be recursive. The values to be compared are always passed
by reference, so don't modify them.
You also cannot exit out of the sort block or subroutine using any of the loop control operators
described in perlsyn or with "goto".
When "use locale" is in effect, "sort LIST" sorts LIST according to the current collation locale.
See perllocale.
Perl 5.6 and earlier used a quicksort algorithm to implement sort. That algorithm was not
stable, and could go quadratic. (A stable sort preserves the input order of elements that
compare equal. Although quicksort's run time is O(NlogN) when averaged over all arrays of length
N, the time can be O(N**2), quadratic behavior, for some inputs.) In 5.7, the quicksort
implementation was replaced with a stable mergesort algorithm whose worst case behavior is
O(NlogN). But benchmarks indicated that for some inputs, on some platforms, the original
quicksort was faster. 5.8 has a sort pragma for limited control of the sort. Its rather blunt
control of the underlying algorithm may not persist into future perls, but the ability to
characterize the input or output in implementation independent ways quite probably will. See
sort.
Examples:
# sort lexically
@articles = sort @files;
# same thing, but with explicit sort routine
@articles = sort {$a cmp $b} @files;
# now case-insensitively
@articles = sort {uc($a) cmp uc($b)} @files;
# same thing in reversed order
@articles = sort {$b cmp $a} @files;
# sort numerically ascending
@articles = sort {$a <=> $b} @files;
# sort numerically descending
@articles = sort {$b <=> $a} @files;
# this sorts the %age hash by value instead of key
# using an in-line function
@eldest = sort { $age{$b} <=> $age{$a} } keys %age;
# sort using explicit subroutine name
sub byage {
$age{$a} <=> $age{$b}; # presuming numeric
}
@sortedclass = sort byage @class;
sub backwards { $b cmp $a }
@harry = qw(dog cat x Cain Abel);
@george = qw(gone chased yz Punished Axed);
print sort @harry;
# prints AbelCaincatdogx
print sort backwards @harry;
# prints xdogcatCainAbel
print sort @george, 'to', @harry;
# prints AbelAxedCainPunishedcatchaseddoggonetoxyz
# inefficiently sort by descending numeric compare using
# the first integer after the first = sign, or the
# whole record case-insensitively otherwise
@new = sort {
($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
⎪⎪
uc($a) cmp uc($b)
} @old;
# same thing, but much more efficiently;
# we'll build auxiliary indices instead
# for speed
@nums = @caps = ();
for (@old) {
push @nums, /=(\d+)/;
push @caps, uc($_);
}
@new = @old[ sort {
$nums[$b] <=> $nums[$a]
⎪⎪
$caps[$a] cmp $caps[$b]
} 0..$#old
];
# same thing, but without any temps
@new = map { $_->[0] }
sort { $b->[1] <=> $a->[1]
⎪⎪
$a->[2] cmp $b->[2]
} map { [$_, /=(\d+)/, uc($_)] } @old;
# using a prototype allows you to use any comparison subroutine
# as a sort subroutine (including other package's subroutines)
package other;
sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
package main;
@new = sort other::backwards @old;
# guarantee stability, regardless of algorithm
use sort 'stable';
@new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
# force use of mergesort (not portable outside Perl 5.8)
use sort '_mergesort'; # note discouraging _
@new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
If you're using strict, you must not declare $a and $b as lexicals. They are package globals.
That means if you're in the "main" package and type
@articles = sort {$b <=> $a} @files;
then $a and $b are $main::a and $main::b (or $::a and $::b), but if you're in the "FooPack"
package, it's the same as typing
@articles = sort {$FooPack::b <=> $FooPack::a} @files;
The comparison function is required to behave. If it returns inconsistent results (sometimes
saying $x[1] is less than $x[2] and sometimes saying the opposite, for example) the results are
not well-defined.
Because "<=>" returns "undef" when either operand is "NaN" (not-a-number), and because "sort"
will trigger a fatal error unless the result of a comparison is defined, when sorting with a
comparison function like "$a <=> $b", be careful about lists that might contain a "NaN". The
following example takes advantage of the fact that "NaN != NaN" to eliminate any "NaN"s from the
input.
@result = sort { $a <=> $b } grep { $_ == $_ } @input;
splice ARRAY,OFFSET,LENGTH,LIST
splice ARRAY,OFFSET,LENGTH
splice ARRAY,OFFSET
splice ARRAY
Removes the elements designated by OFFSET and LENGTH from an array, and replaces them with the
elements of LIST, if any. In list context, returns the elements removed from the array. In
scalar context, returns the last element removed, or "undef" if no elements are removed. The
array grows or shrinks as necessary. If OFFSET is negative then it starts that far from the end
of the array. If LENGTH is omitted, removes everything from OFFSET onward. If LENGTH is
negative, removes the elements from OFFSET onward except for -LENGTH elements at the end of the
array. If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is past the end of
the array, perl issues a warning, and splices at the end of the array.
The following equivalences hold (assuming "$[ == 0 and $#a >= $i" )
push(@a,$x,$y) splice(@a,@a,0,$x,$y)
pop(@a) splice(@a,-1)
shift(@a) splice(@a,0,1)
unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
$a[$i] = $y splice(@a,$i,1,$y)
Example, assuming array lengths are passed before arrays:
sub aeq { # compare two list values
my(@a) = splice(@_,0,shift);
my(@b) = splice(@_,0,shift);
return 0 unless @a == @b; # same len?
while (@a) {
return 0 if pop(@a) ne pop(@b);
}
return 1;
}
if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
split /PATTERN/,EXPR,LIMIT
split /PATTERN/,EXPR
split /PATTERN/
split Splits a string into a list of strings and returns that list. By default, empty leading fields
are preserved, and empty trailing ones are deleted.
In scalar context, returns the number of fields found and splits into the @_ array. Use of split
in scalar context is deprecated, however, because it clobbers your subroutine arguments.
If EXPR is omitted, splits the $_ string. If PATTERN is also omitted, splits on whitespace
(after skipping any leading whitespace). Anything matching PATTERN is taken to be a delimiter
separating the fields. (Note that the delimiter may be longer than one character.)
If LIMIT is specified and positive, it represents the maximum number of fields the EXPR will be
split into, though the actual number of fields returned depends on the number of times PATTERN
matches within EXPR. If LIMIT is unspecified or zero, trailing null fields are stripped (which
potential users of "pop" would do well to remember). If LIMIT is negative, it is treated as if
an arbitrarily large LIMIT had been specified. Note that splitting an EXPR that evaluates to the
empty string always returns the empty list, regardless of the LIMIT specified.
A pattern matching the null string (not to be confused with a null pattern "//", which is just
one member of the set of patterns matching a null string) will split the value of EXPR into
separate characters at each point it matches that way. For example:
print join(':', split(/ */, 'hi there'));
produces the output 'h:i:t:h:e:r:e'.
Using the empty pattern "//" specifically matches the null string, and is not be confused with
the use of "//" to mean "the last successful pattern match".
Empty leading (or trailing) fields are produced when there are positive width matches at the
beginning (or end) of the string; a zero-width match at the beginning (or end) of the string does
not produce an empty field. For example:
print join(':', split(/(?=\w)/, 'hi there!'));
produces the output 'h:i :t:h:e:r:e!'.
The LIMIT parameter can be used to split a line partially
($login, $passwd, $remainder) = split(/:/, $_, 3);
When assigning to a list, if LIMIT is omitted, or zero, Perl supplies a LIMIT one larger than the
number of variables in the list, to avoid unnecessary work. For the list above LIMIT would have
been 4 by default. In time critical applications it behooves you not to split into more fields
than you really need.
If the PATTERN contains parentheses, additional list elements are created from each matching
substring in the delimiter.
split(/([,-])/, "1-10,20", 3);
produces the list value
(1, '-', 10, ',', 20)
If you had the entire header of a normal Unix email message in $header, you could split it up
into fields and their values this way:
$header =~ s/\n\s+/ /g; # fix continuation lines
%hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
The pattern "/PATTERN/" may be replaced with an expression to specify patterns that vary at
runtime. (To do runtime compilation only once, use "/$variable/o".)
As a special case, specifying a PATTERN of space (' ') will split on white space just as "split"
with no arguments does. Thus, "split(' ')" can be used to emulate awk's default behavior,
whereas "split(/ /)" will give you as many null initial fields as there are leading spaces. A
"split" on "/\s+/" is like a "split(' ')" except that any leading whitespace produces a null
first field. A "split" with no arguments really does a "split(' ', $_)" internally.
A PATTERN of "/^/" is treated as if it were "/^/m", since it isn't much use otherwise.
Example:
open(PASSWD, '/etc/passwd');
while (<PASSWD>) {
chomp;
($login, $passwd, $uid, $gid,
$gcos, $home, $shell) = split(/:/);
#...
}
As with regular pattern matching, any capturing parentheses that are not matched in a "split()"
will be set to "undef" when returned:
@fields = split /(A)⎪B/, "1A2B3";
# @fields is (1, 'A', 2, undef, 3)
sprintf FORMAT, LIST
Returns a string formatted by the usual "printf" conventions of the C library function "sprintf".
See below for more details and see sprintf(3) or printf(3) on your system for an explanation of
the general principles.
For example:
# Format number with up to 8 leading zeroes
$result = sprintf("%08d", $number);
# Round number to 3 digits after decimal point
$rounded = sprintf("%.3f", $number);
Perl does its own "sprintf" formatting--it emulates the C function "sprintf", but it doesn't use
it (except for floating-point numbers, and even then only the standard modifiers are allowed).
As a result, any non-standard extensions in your local "sprintf" are not available from Perl.
Unlike "printf", "sprintf" does not do what you probably mean when you pass it an array as your
first argument. The array is given scalar context, and instead of using the 0th element of the
array as the format, Perl will use the count of elements in the array as the format, which is
almost never useful.
Perl's "sprintf" permits the following universally-known conversions:
%% a percent sign
%c a character with the given number
%s a string
%d a signed integer, in decimal
%u an unsigned integer, in decimal
%o an unsigned integer, in octal
%x an unsigned integer, in hexadecimal
%e a floating-point number, in scientific notation
%f a floating-point number, in fixed decimal notation
%g a floating-point number, in %e or %f notation
In addition, Perl permits the following widely-supported conversions:
%X like %x, but using upper-case letters
%E like %e, but using an upper-case "E"
%G like %g, but with an upper-case "E" (if applicable)
%b an unsigned integer, in binary
%p a pointer (outputs the Perl value's address in hexadecimal)
%n special: *stores* the number of characters output so far
into the next variable in the parameter list
Finally, for backward (and we do mean "backward") compatibility, Perl permits these unnecessary
but widely-supported conversions:
%i a synonym for %d
%D a synonym for %ld
%U a synonym for %lu
%O a synonym for %lo
%F a synonym for %f
Note that the number of exponent digits in the scientific notation produced by %e, %E, %g and %G
for numbers with the modulus of the exponent less than 100 is system-dependent: it may be three
or less (zero-padded as necessary). In other words, 1.23 times ten to the 99th may be either
"1.23e99" or "1.23e099".
Between the "%" and the format letter, you may specify a number of additional attributes
controlling the interpretation of the format. In order, these are:
format parameter index
An explicit format parameter index, such as "2$". By default sprintf will format the next
unused argument in the list, but this allows you to take the arguments out of order. Eg:
printf '%2$d %1$d', 12, 34; # prints "34 12"
printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
flags
one or more of:
space prefix positive number with a space
+ prefix positive number with a plus sign
- left-justify within the field
0 use zeros, not spaces, to right-justify
# prefix non-zero octal with "0", non-zero hex with "0x",
non-zero binary with "0b"
For example:
printf '<% d>', 12; # prints "< 12>"
printf '<%+d>', 12; # prints "<+12>"
printf '<%6s>', 12; # prints "< 12>"
printf '<%-6s>', 12; # prints "<12 >"
printf '<%06s>', 12; # prints "<000012>"
printf '<%#x>', 12; # prints "<0xc>"
vector flag
The vector flag "v", optionally specifying the join string to use. This flag tells perl to
interpret the supplied string as a vector of integers, one for each character in the string,
separated by a given string (a dot "." by default). This can be useful for displaying ordinal
values of characters in arbitrary strings:
printf "version is v%vd\n", $^V; # Perl's version
Put an asterisk "*" before the "v" to override the string to use to separate the numbers:
printf "address is %*vX\n", ":", $addr; # IPv6 address
printf "bits are %0*v8b\n", " ", $bits; # random bitstring
You can also explicitly specify the argument number to use for the join string using eg
"*2$v":
printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":"; # 3 IPv6 addresses
(minimum) width
Arguments are usually formatted to be only as wide as required to display the given value.
You can override the width by putting a number here, or get the width from the next argument
(with "*") or from a specified argument (with eg "*2$"):
printf '<%s>', "a"; # prints "<a>"
printf '<%6s>', "a"; # prints "< a>"
printf '<%*s>', 6, "a"; # prints "< a>"
printf '<%*2$s>', "a", 6; # prints "< a>"
printf '<%2s>', "long"; # prints "<long>" (does not truncate)
If a field width obtained through "*" is negative, it has the same effect as the "-" flag:
left-justification.
precision, or maximum width
You can specify a precision (for numeric conversions) or a maximum width (for string
conversions) by specifying a "." followed by a number. For floating point formats, with the
exception of 'g' and 'G', this specifies the number of decimal places to show (the default
being 6), eg:
# these examples are subject to system-specific variation
printf '<%f>', 1; # prints "<1.000000>"
printf '<%.1f>', 1; # prints "<1.0>"
printf '<%.0f>', 1; # prints "<1>"
printf '<%e>', 10; # prints "<1.000000e+01>"
printf '<%.1e>', 10; # prints "<1.0e+01>"
For 'g' and 'G', this specifies the maximum number of digits to show, including prior to the
decimal point as well as after it, eg:
# these examples are subject to system-specific variation
printf '<%g>', 1; # prints "<1>"
printf '<%.10g>', 1; # prints "<1>"
printf '<%g>', 100; # prints "<100>"
printf '<%.1g>', 100; # prints "<1e+02>"
printf '<%.2g>', 100.01; # prints "<1e+02>"
printf '<%.5g>', 100.01; # prints "<100.01>"
printf '<%.4g>', 100.01; # prints "<100>"
For integer conversions, specifying a precision implies that the output of the number itself
should be zero-padded to this width:
printf '<%.6x>', 1; # prints "<000001>"
printf '<%#.6x>', 1; # prints "<0x000001>"
printf '<%-10.6x>', 1; # prints "<000001 >"
For string conversions, specifying a precision truncates the string to fit in the specified
width:
printf '<%.5s>', "truncated"; # prints "<trunc>"
printf '<%10.5s>', "truncated"; # prints "< trunc>"
You can also get the precision from the next argument using ".*":
printf '<%.6x>', 1; # prints "<000001>"
printf '<%.*x>', 6, 1; # prints "<000001>"
You cannot currently get the precision from a specified number, but it is intended that this
will be possible in the future using eg ".*2$":
printf '<%.*2$x>', 1, 6; # INVALID, but in future will print "<000001>"
size
For numeric conversions, you can specify the size to interpret the number as using "l", "h",
"V", "q", "L", or "ll". For integer conversions ("d u o x X b i D U O"), numbers are usually
assumed to be whatever the default integer size is on your platform (usually 32 or 64 bits),
but you can override this to use instead one of the standard C types, as supported by the
compiler used to build Perl:
l interpret integer as C type "long" or "unsigned long"
h interpret integer as C type "short" or "unsigned short"
q, L or ll interpret integer as C type "long long", "unsigned long long".
or "quads" (typically 64-bit integers)
The last will produce errors if Perl does not understand "quads" in your installation. (This
requires that either the platform natively supports quads or Perl was specifically compiled
to support quads.) You can find out whether your Perl supports quads via Config:
use Config;
($Config{use64bitint} eq 'define' ⎪⎪ $Config{longsize} >= 8) &&
print "quads\n";
For floating point conversions ("e f g E F G"), numbers are usually assumed to be the default
floating point size on your platform (double or long double), but you can force 'long double'
with "q", "L", or "ll" if your platform supports them. You can find out whether your Perl
supports long doubles via Config:
use Config;
$Config{d_longdbl} eq 'define' && print "long doubles\n";
You can find out whether Perl considers 'long double' to be the default floating point size
to use on your platform via Config:
use Config;
($Config{uselongdouble} eq 'define') &&
print "long doubles by default\n";
It can also be the case that long doubles and doubles are the same thing:
use Config;
($Config{doublesize} == $Config{longdblsize}) &&
print "doubles are long doubles\n";
The size specifier "V" has no effect for Perl code, but it is supported for compatibility
with XS code; it means 'use the standard size for a Perl integer (or floating-point number)',
which is already the default for Perl code.
order of arguments
Normally, sprintf takes the next unused argument as the value to format for each format
specification. If the format specification uses "*" to require additional arguments, these
are consumed from the argument list in the order in which they appear in the format
specification before the value to format. Where an argument is specified using an explicit
index, this does not affect the normal order for the arguments (even when the explicitly
specified index would have been the next argument in any case).
So:
printf '<%*.*s>', $a, $b, $c;
would use $a for the width, $b for the precision and $c as the value to format, while:
print '<%*1$.*s>', $a, $b;
would use $a for the width and the precision, and $b as the value to format.
Here are some more examples - beware that when using an explicit index, the "$" may need to
be escaped:
printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
If "use locale" is in effect, the character used for the decimal point in formatted real numbers
is affected by the LC_NUMERIC locale. See perllocale.
sqrt EXPR
sqrt Return the square root of EXPR. If EXPR is omitted, returns square root of $_. Only works on
non-negative operands, unless you've loaded the standard Math::Complex module.
use Math::Complex;
print sqrt(-2); # prints 1.4142135623731i
srand EXPR
srand Sets the random number seed for the "rand" operator.
The point of the function is to "seed" the "rand" function so that "rand" can produce a different
sequence each time you run your program.
If srand() is not called explicitly, it is called implicitly at the first use of the "rand"
operator. However, this was not the case in versions of Perl before 5.004, so if your script
will run under older Perl versions, it should call "srand".
Most programs won't even call srand() at all, except those that need a cryptographically-strong
starting point rather than the generally acceptable default, which is based on time of day,
process ID, and memory allocation, or the /dev/urandom device, if available.
You can call srand($seed) with the same $seed to reproduce the same sequence from rand(), but
this is usually reserved for generating predictable results for testing or debugging. Otherwise,
don't call srand() more than once in your program.
Do not call srand() (i.e. without an argument) more than once in a script. The internal state of
the random number generator should contain more entropy than can be provided by any seed, so
calling srand() again actually loses randomness.
Most implementations of "srand" take an integer and will silently truncate decimal numbers. This
means "srand(42)" will usually produce the same results as "srand(42.1)". To be safe, always
pass "srand" an integer.
In versions of Perl prior to 5.004 the default seed was just the current "time". This isn't a
particularly good seed, so many old programs supply their own seed value (often "time ^ $$" or
"time ^ ($$ + ($$ << 15))"), but that isn't necessary any more.
Note that you need something much more random than the default seed for cryptographic purposes.
Checksumming the compressed output of one or more rapidly changing operating system status
programs is the usual method. For example:
srand (time ^ $$ ^ unpack "%L*", `ps axww ⎪ gzip`);
If you're particularly concerned with this, see the "Math::TrulyRandom" module in CPAN.
Frequently called programs (like CGI scripts) that simply use
time ^ $$
for a seed can fall prey to the mathematical property that
a^b == (a+1)^(b+1)
one-third of the time. So don't do that.
stat FILEHANDLE
stat EXPR
stat Returns a 13-element list giving the status info for a file, either the file opened via
FILEHANDLE, or named by EXPR. If EXPR is omitted, it stats $_. Returns a null list if the stat
fails. Typically used as follows:
($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
$atime,$mtime,$ctime,$blksize,$blocks)
= stat($filename);
Not all fields are supported on all filesystem types. Here are the meaning of the fields:
0 dev device number of filesystem
1 ino inode number
2 mode file mode (type and permissions)
3 nlink number of (hard) links to the file
4 uid numeric user ID of file's owner
5 gid numeric group ID of file's owner
6 rdev the device identifier (special files only)
7 size total size of file, in bytes
8 atime last access time in seconds since the epoch
9 mtime last modify time in seconds since the epoch
10 ctime inode change time in seconds since the epoch (*)
11 blksize preferred block size for file system I/O
12 blocks actual number of blocks allocated
(The epoch was at 00:00 January 1, 1970 GMT.)
(*) The ctime field is non-portable, in particular you cannot expect it to be a "creation time",
see "Files and Filesystems" in perlport for details.
If stat is passed the special filehandle consisting of an underline, no stat is done, but the
current contents of the stat structure from the last stat or filetest are returned. Example:
if (-x $file && (($d) = stat(_)) && $d < 0) {
print "$file is executable NFS file\n";
}
(This works on machines only for which the device number is negative under NFS.)
Because the mode contains both the file type and its permissions, you should mask off the file
type portion and (s)printf using a "%o" if you want to see the real permissions.
$mode = (stat($filename))[2];
printf "Permissions are %04o\n", $mode & 07777;
In scalar context, "stat" returns a boolean value indicating success or failure, and, if
successful, sets the information associated with the special filehandle "_".
The File::stat module provides a convenient, by-name access mechanism:
use File::stat;
$sb = stat($filename);
printf "File is %s, size is %s, perm %04o, mtime %s\n",
$filename, $sb->size, $sb->mode & 07777,
scalar localtime $sb->mtime;
You can import symbolic mode constants ("S_IF*") and functions ("S_IS*") from the Fcntl module:
use Fcntl ':mode';
$mode = (stat($filename))[2];
$user_rwx = ($mode & S_IRWXU) >> 6;
$group_read = ($mode & S_IRGRP) >> 3;
$other_execute = $mode & S_IXOTH;
printf "Permissions are %04o\n", S_IMODE($mode), "\n";
$is_setuid = $mode & S_ISUID;
$is_setgid = S_ISDIR($mode);
You could write the last two using the "-u" and "-d" operators. The commonly available S_IF*
constants are
# Permissions: read, write, execute, for user, group, others.
S_IRWXU S_IRUSR S_IWUSR S_IXUSR
S_IRWXG S_IRGRP S_IWGRP S_IXGRP
S_IRWXO S_IROTH S_IWOTH S_IXOTH
# Setuid/Setgid/Stickiness/SaveText.
# Note that the exact meaning of these is system dependent.
S_ISUID S_ISGID S_ISVTX S_ISTXT
# File types. Not necessarily all are available on your system.
S_IFREG S_IFDIR S_IFLNK S_IFBLK S_ISCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
# The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
S_IREAD S_IWRITE S_IEXEC
and the S_IF* functions are
S_IMODE($mode) the part of $mode containing the permission bits
and the setuid/setgid/sticky bits
S_IFMT($mode) the part of $mode containing the file type
which can be bit-anded with e.g. S_IFREG
or with the following functions
# The operators -f, -d, -l, -b, -c, -p, and -s.
S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
# No direct -X operator counterpart, but for the first one
# the -g operator is often equivalent. The ENFMT stands for
# record flocking enforcement, a platform-dependent feature.
S_ISENFMT($mode) S_ISWHT($mode)
See your native chmod(2) and stat(2) documentation for more details about the S_* constants.
To get status info for a symbolic link instead of the target file behind the link, use the
"lstat" function, see "stat".
study SCALAR
study Takes extra time to study SCALAR ($_ if unspecified) in anticipation of doing many pattern
matches on the string before it is next modified. This may or may not save time, depending on
the nature and number of patterns you are searching on, and on the distribution of character
frequencies in the string to be searched--you probably want to compare run times with and without
it to see which runs faster. Those loops which scan for many short constant strings (including
the constant parts of more complex patterns) will benefit most. You may have only one "study"
active at a time--if you study a different scalar the first is "unstudied". (The way "study"
works is this: a linked list of every character in the string to be searched is made, so we know,
for example, where all the 'k' characters are. From each search string, the rarest character is
selected, based on some static frequency tables constructed from some C programs and English
text. Only those places that contain this "rarest" character are examined.)
For example, here is a loop that inserts index producing entries before any line containing a
certain pattern:
while (<>) {
study;
print ".IX foo\n" if /\bfoo\b/;
print ".IX bar\n" if /\bbar\b/;
print ".IX blurfl\n" if /\bblurfl\b/;
# ...
print;
}
In searching for "/\bfoo\b/", only those locations in $_ that contain "f" will be looked at,
because "f" is rarer than "o". In general, this is a big win except in pathological cases. The
only question is whether it saves you more time than it took to build the linked list in the
first place.
Note that if you have to look for strings that you don't know till runtime, you can build an
entire loop as a string and "eval" that to avoid recompiling all your patterns all the time.
Together with undefining $/ to input entire files as one record, this can be very fast, often
faster than specialized programs like fgrep(1). The following scans a list of files (@files) for
a list of words (@words), and prints out the names of those files that contain a match:
$search = 'while (<>) { study;';
foreach $word (@words) {
$search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
}
$search .= "}";
@ARGV = @files;
undef $/;
eval $search; # this screams
$/ = "\n"; # put back to normal input delimiter
foreach $file (sort keys(%seen)) {
print $file, "\n";
}
sub NAME BLOCK
sub NAME (PROTO) BLOCK
sub NAME : ATTRS BLOCK
sub NAME (PROTO) : ATTRS BLOCK
This is subroutine definition, not a real function per se. Without a BLOCK it's just a forward
declaration. Without a NAME, it's an anonymous function declaration, and does actually return a
value: the CODE ref of the closure you just created.
See perlsub and perlref for details about subroutines and references, and attributes and
Attribute::Handlers for more information about attributes.
substr EXPR,OFFSET,LENGTH,REPLACEMENT
substr EXPR,OFFSET,LENGTH
substr EXPR,OFFSET
Extracts a substring out of EXPR and returns it. First character is at offset 0, or whatever
you've set $[ to (but don't do that). If OFFSET is negative (or more precisely, less than $[),
starts that far from the end of the string. If LENGTH is omitted, returns everything to the end
of the string. If LENGTH is negative, leaves that many characters off the end of the string.
You can use the substr() function as an lvalue, in which case EXPR must itself be an lvalue. If
you assign something shorter than LENGTH, the string will shrink, and if you assign something
longer than LENGTH, the string will grow to accommodate it. To keep the string the same length
you may need to pad or chop your value using "sprintf".
If OFFSET and LENGTH specify a substring that is partly outside the string, only the part within
the string is returned. If the substring is beyond either end of the string, substr() returns
the undefined value and produces a warning. When used as an lvalue, specifying a substring that
is entirely outside the string is a fatal error. Here's an example showing the behavior for
boundary cases:
my $name = 'fred';
substr($name, 4) = 'dy'; # $name is now 'freddy'
my $null = substr $name, 6, 2; # returns '' (no warning)
my $oops = substr $name, 7; # returns undef, with warning
substr($name, 7) = 'gap'; # fatal error
An alternative to using substr() as an lvalue is to specify the replacement string as the 4th
argument. This allows you to replace parts of the EXPR and return what was there before in one
operation, just as you can with splice().
If the lvalue returned by substr is used after the EXPR is changed in any way, the behaviour may
not be as expected and is subject to change. This caveat includes code such as
"print(substr($foo,$a,$b)=$bar)" or "(substr($foo,$a,$b)=$bar)=$fud" (where $foo is changed via
the substring assignment, and then the substr is used again), or where a substr() is aliased via
a "foreach" loop or passed as a parameter or a reference to it is taken and then the alias,
parameter, or deref'd reference either is used after the original EXPR has been changed or is
assigned to and then used a second time.
symlink OLDFILE,NEWFILE
Creates a new filename symbolically linked to the old filename. Returns 1 for success, 0
otherwise. On systems that don't support symbolic links, produces a fatal error at run time. To
check for that, use eval:
$symlink_exists = eval { symlink("",""); 1 };
syscall NUMBER, LIST
Calls the system call specified as the first element of the list, passing the remaining elements
as arguments to the system call. If unimplemented, produces a fatal error. The arguments are
interpreted as follows: if a given argument is numeric, the argument is passed as an int. If
not, the pointer to the string value is passed. You are responsible to make sure a string is
pre-extended long enough to receive any result that might be written into a string. You can't
use a string literal (or other read-only string) as an argument to "syscall" because Perl has to
assume that any string pointer might be written through. If your integer arguments are not
literals and have never been interpreted in a numeric context, you may need to add 0 to them to
force them to look like numbers. This emulates the "syswrite" function (or vice versa):
require 'syscall.ph'; # may need to run h2ph
$s = "hi there\n";
syscall(&SYS_write, fileno(STDOUT), $s, length $s);
Note that Perl supports passing of up to only 14 arguments to your system call, which in practice
should usually suffice.
Syscall returns whatever value returned by the system call it calls. If the system call fails,
"syscall" returns "-1" and sets $! (errno). Note that some system calls can legitimately return
"-1". The proper way to handle such calls is to assign "$!=0;" before the call and check the
value of $! if syscall returns "-1".
There's a problem with "syscall(&SYS_pipe)": it returns the file number of the read end of the
pipe it creates. There is no way to retrieve the file number of the other end. You can avoid
this problem by using "pipe" instead.
sysopen FILEHANDLE,FILENAME,MODE
sysopen FILEHANDLE,FILENAME,MODE,PERMS
Opens the file whose filename is given by FILENAME, and associates it with FILEHANDLE. If
FILEHANDLE is an expression, its value is used as the name of the real filehandle wanted. This
function calls the underlying operating system's "open" function with the parameters FILENAME,
MODE, PERMS.
The possible values and flag bits of the MODE parameter are system-dependent; they are available
via the standard module "Fcntl". See the documentation of your operating system's "open" to see
which values and flag bits are available. You may combine several flags using the "⎪"-operator.
Some of the most common values are "O_RDONLY" for opening the file in read-only mode, "O_WRONLY"
for opening the file in write-only mode, and "O_RDWR" for opening the file in read-write mode,
and.
For historical reasons, some values work on almost every system supported by perl: zero means
read-only, one means write-only, and two means read/write. We know that these values do not work
under OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to use them in new code.
If the file named by FILENAME does not exist and the "open" call creates it (typically because
MODE includes the "O_CREAT" flag), then the value of PERMS specifies the permissions of the newly
created file. If you omit the PERMS argument to "sysopen", Perl uses the octal value 0666.
These permission values need to be in octal, and are modified by your process's current "umask".
In many systems the "O_EXCL" flag is available for opening files in exclusive mode. This is not
locking: exclusiveness means here that if the file already exists, sysopen() fails. The "O_EXCL"
wins "O_TRUNC".
Sometimes you may want to truncate an already-existing file: "O_TRUNC".
You should seldom if ever use 0644 as argument to "sysopen", because that takes away the user's
option to have a more permissive umask. Better to omit it. See the perlfunc(1) entry on "umask"
for more on this.
Note that "sysopen" depends on the fdopen() C library function. On many UNIX systems, fdopen()
is known to fail when file descriptors exceed a certain value, typically 255. If you need more
file descriptors than that, consider rebuilding Perl to use the "sfio" library, or perhaps using
the POSIX::open() function.
See perlopentut for a kinder, gentler explanation of opening files.
sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
sysread FILEHANDLE,SCALAR,LENGTH
Attempts to read LENGTH bytes of data into variable SCALAR from the specified FILEHANDLE, using
the system call read(2). It bypasses buffered IO, so mixing this with other kinds of reads,
"print", "write", "seek", "tell", or "eof" can cause confusion because the perlio or stdio layers
usually buffers data. Returns the number of bytes actually read, 0 at end of file, or undef if
there was an error (in the latter case $! is also set). SCALAR will be grown or shrunk so that
the last byte actually read is the last byte of the scalar after the read.
An OFFSET may be specified to place the read data at some place in the string other than the
beginning. A negative OFFSET specifies placement at that many characters counting backwards from
the end of the string. A positive OFFSET greater than the length of SCALAR results in the string
being padded to the required size with "\0" bytes before the result of the read is appended.
There is no syseof() function, which is ok, since eof() doesn't work very well on device files
(like ttys) anyway. Use sysread() and check for a return value for 0 to decide whether you're
done.
Note that if the filehandle has been marked as ":utf8" Unicode characters are read instead of
bytes (the LENGTH, OFFSET, and the return value of sysread() are in Unicode characters). The
":encoding(...)" layer implicitly introduces the ":utf8" layer. See "binmode", "open", and the
"open" pragma, open.
sysseek FILEHANDLE,POSITION,WHENCE
Sets FILEHANDLE's system position in bytes using the system call lseek(2). FILEHANDLE may be an
expression whose value gives the name of the filehandle. The values for WHENCE are 0 to set the
new position to POSITION, 1 to set the it to the current position plus POSITION, and 2 to set it
to EOF plus POSITION (typically negative).
Note the in bytes: even if the filehandle has been set to operate on characters (for example by
using the ":utf8" I/O layer), tell() will return byte offsets, not character offsets (because
implementing that would render sysseek() very slow).
sysseek() bypasses normal buffered IO, so mixing this with reads (other than "sysread", for
example >< or read()) "print", "write", "seek", "tell", or "eof" may cause confusion.
For WHENCE, you may also use the constants "SEEK_SET", "SEEK_CUR", and "SEEK_END" (start of the
file, current position, end of the file) from the Fcntl module. Use of the constants is also
more portable than relying on 0, 1, and 2. For example to define a "systell" function:
use Fcntl 'SEEK_CUR';
sub systell { sysseek($_[0], 0, SEEK_CUR) }
Returns the new position, or the undefined value on failure. A position of zero is returned as
the string "0 but true"; thus "sysseek" returns true on success and false on failure, yet you can
still easily determine the new position.
system LIST
system PROGRAM LIST
Does exactly the same thing as "exec LIST", except that a fork is done first, and the parent
process waits for the child process to complete. Note that argument processing varies depending
on the number of arguments. If there is more than one argument in LIST, or if LIST is an array
with more than one value, starts the program given by the first element of the list with
arguments given by the rest of the list. If there is only one scalar argument, the argument is
checked for shell metacharacters, and if there are any, the entire argument is passed to the
system's command shell for parsing (this is "/bin/sh -c" on Unix platforms, but varies on other
platforms). If there are no shell metacharacters in the argument, it is split into words and
passed directly to "execvp", which is more efficient.
Beginning with v5.6.0, Perl will attempt to flush all files opened for output before any
operation that may do a fork, but this may not be supported on some platforms (see perlport). To
be safe, you may need to set $⎪ ($AUTOFLUSH in English) or call the "autoflush()" method of
"IO::Handle" on any open handles.
The return value is the exit status of the program as returned by the "wait" call. To get the
actual exit value shift right by eight (see below). See also "exec". This is not what you want
to use to capture the output from a command, for that you should use merely backticks or "qx//",
as described in "`STRING`" in perlop. Return value of -1 indicates a failure to start the
program (inspect $! for the reason).
Like "exec", "system" allows you to lie to a program about its name if you use the "system
PROGRAM LIST" syntax. Again, see "exec".
Because "system" and backticks block "SIGINT" and "SIGQUIT", killing the program they're running
doesn't actually interrupt your program.
@args = ("command", "arg1", "arg2");
system(@args) == 0
or die "system @args failed: $?"
You can check all the failure possibilities by inspecting $? like this:
if ($? == -1) {
print "failed to execute: $!\n";
}
elsif ($? & 127) {
printf "child died with signal %d, %s coredump\n",
($? & 127), ($? & 128) ? 'with' : 'without';
}
else {
printf "child exited with value %d\n", $? >> 8;
}
or more portably by using the W*() calls of the POSIX extension; see perlport for more
information.
When the arguments get executed via the system shell, results and return codes will be subject to
its quirks and capabilities. See "`STRING`" in perlop and "exec" for details.
syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
syswrite FILEHANDLE,SCALAR,LENGTH
syswrite FILEHANDLE,SCALAR
Attempts to write LENGTH bytes of data from variable SCALAR to the specified FILEHANDLE, using
the system call write(2). If LENGTH is not specified, writes whole SCALAR. It bypasses buffered
IO, so mixing this with reads (other than sysread()), "print", "write", "seek", "tell", or "eof"
may cause confusion because the perlio and stdio layers usually buffers data. Returns the number
of bytes actually written, or "undef" if there was an error (in this case the errno variable $!
is also set). If the LENGTH is greater than the available data in the SCALAR after the OFFSET,
only as much data as is available will be written.
An OFFSET may be specified to write the data from some part of the string other than the
beginning. A negative OFFSET specifies writing that many characters counting backwards from the
end of the string. In the case the SCALAR is empty you can use OFFSET but only zero offset.
Note that if the filehandle has been marked as ":utf8", Unicode characters are written instead of
bytes (the LENGTH, OFFSET, and the return value of syswrite() are in UTF-8 encoded Unicode
characters). The ":encoding(...)" layer implicitly introduces the ":utf8" layer. See "binmode",
"open", and the "open" pragma, open.
tell FILEHANDLE
tell Returns the current position in bytes for FILEHANDLE, or -1 on error. FILEHANDLE may be an
expression whose value gives the name of the actual filehandle. If FILEHANDLE is omitted,
assumes the file last read.
Note the in bytes: even if the filehandle has been set to operate on characters (for example by
using the ":utf8" open layer), tell() will return byte offsets, not character offsets (because
that would render seek() and tell() rather slow).
The return value of tell() for the standard streams like the STDIN depends on the operating
system: it may return -1 or something else. tell() on pipes, fifos, and sockets usually returns
-1.
There is no "systell" function. Use "sysseek(FH, 0, 1)" for that.
Do not use tell() on a filehandle that has been opened using sysopen(), use sysseek() for that as
described above. Why? Because sysopen() creates unbuffered, "raw", filehandles, while open()
creates buffered filehandles. sysseek() make sense only on the first kind, tell() only makes
sense on the second kind.
telldir DIRHANDLE
Returns the current position of the "readdir" routines on DIRHANDLE. Value may be given to
"seekdir" to access a particular location in a directory. Has the same caveats about possible
directory compaction as the corresponding system library routine.
tie VARIABLE,CLASSNAME,LIST
This function binds a variable to a package class that will provide the implementation for the
variable. VARIABLE is the name of the variable to be enchanted. CLASSNAME is the name of a
class implementing objects of correct type. Any additional arguments are passed to the "new"
method of the class (meaning "TIESCALAR", "TIEHANDLE", "TIEARRAY", or "TIEHASH"). Typically
these are arguments such as might be passed to the "dbm_open()" function of C. The object
returned by the "new" method is also returned by the "tie" function, which would be useful if you
want to access other methods in CLASSNAME.
Note that functions such as "keys" and "values" may return huge lists when used on large objects,
like DBM files. You may prefer to use the "each" function to iterate over such. Example:
# print out history file offsets
use NDBM_File;
tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
while (($key,$val) = each %HIST) {
print $key, ' = ', unpack('L',$val), "\n";
}
untie(%HIST);
A class implementing a hash should have the following methods:
TIEHASH classname, LIST
FETCH this, key
STORE this, key, value
DELETE this, key
CLEAR this
EXISTS this, key
FIRSTKEY this
NEXTKEY this, lastkey
DESTROY this
UNTIE this
A class implementing an ordinary array should have the following methods:
TIEARRAY classname, LIST
FETCH this, key
STORE this, key, value
FETCHSIZE this
STORESIZE this, count
CLEAR this
PUSH this, LIST
POP this
SHIFT this
UNSHIFT this, LIST
SPLICE this, offset, length, LIST
EXTEND this, count
DESTROY this
UNTIE this
A class implementing a file handle should have the following methods:
TIEHANDLE classname, LIST
READ this, scalar, length, offset
READLINE this
GETC this
WRITE this, scalar, length, offset
PRINT this, LIST
PRINTF this, format, LIST
BINMODE this
EOF this
FILENO this
SEEK this, position, whence
TELL this
OPEN this, mode, LIST
CLOSE this
DESTROY this
UNTIE this
A class implementing a scalar should have the following methods:
TIESCALAR classname, LIST
FETCH this,
STORE this, value
DESTROY this
UNTIE this
Not all methods indicated above need be implemented. See perltie, Tie::Hash, Tie::Array,
Tie::Scalar, and Tie::Handle.
Unlike "dbmopen", the "tie" function will not use or require a module for you--you need to do
that explicitly yourself. See DB_File or the Config module for interesting "tie"
implementations.
For further details see perltie, "tied VARIABLE".
tied VARIABLE
Returns a reference to the object underlying VARIABLE (the same value that was originally
returned by the "tie" call that bound the variable to a package.) Returns the undefined value if
VARIABLE isn't tied to a package.
time Returns the number of non-leap seconds since whatever time the system considers to be the epoch
(that's 00:00:00, January 1, 1904 for Mac OS, and 00:00:00 UTC, January 1, 1970 for most other
systems). Suitable for feeding to "gmtime" and "localtime".
For measuring time in better granularity than one second, you may use either the Time::HiRes
module (from CPAN, and starting from Perl 5.8 part of the standard distribution), or if you have
gettimeofday(2), you may be able to use the "syscall" interface of Perl. See perlfaq8 for
details.
times Returns a four-element list giving the user and system times, in seconds, for this process and
the children of this process.
($user,$system,$cuser,$csystem) = times;
In scalar context, "times" returns $user.
tr/// The transliteration operator. Same as "y///". See perlop.
truncate FILEHANDLE,LENGTH
truncate EXPR,LENGTH
Truncates the file opened on FILEHANDLE, or named by EXPR, to the specified length. Produces a
fatal error if truncate isn't implemented on your system. Returns true if successful, the
undefined value otherwise.
The behavior is undefined if LENGTH is greater than the length of the file.
uc EXPR
uc Returns an uppercased version of EXPR. This is the internal function implementing the "\U"
escape in double-quoted strings. Respects current LC_CTYPE locale if "use locale" in force. See
perllocale and perlunicode for more details about locale and Unicode support. It does not
attempt to do titlecase mapping on initial letters. See "ucfirst" for that.
If EXPR is omitted, uses $_.
ucfirst EXPR
ucfirst Returns the value of EXPR with the first character in uppercase (titlecase in Unicode). This is
the internal function implementing the "\u" escape in double-quoted strings. Respects current
LC_CTYPE locale if "use locale" in force. See perllocale and perlunicode for more details about
locale and Unicode support.
If EXPR is omitted, uses $_.
umask EXPR
umask Sets the umask for the process to EXPR and returns the previous value. If EXPR is omitted,
merely returns the current umask.
The Unix permission "rwxr-x---" is represented as three sets of three bits, or three octal
digits: 0750 (the leading 0 indicates octal and isn't one of the digits). The "umask" value is
such a number representing disabled permissions bits. The permission (or "mode") values you pass
"mkdir" or "sysopen" are modified by your umask, so even if you tell "sysopen" to create a file
with permissions 0777, if your umask is 0022 then the file will actually be created with
permissions 0755. If your "umask" were 0027 (group can't write; others can't read, write, or
execute), then passing "sysopen" 0666 would create a file with mode 0640 ("0666 &~ 027" is 0640).
Here's some advice: supply a creation mode of 0666 for regular files (in "sysopen") and one of
0777 for directories (in "mkdir") and executable files. This gives users the freedom of choice:
if they want protected files, they might choose process umasks of 022, 027, or even the
particularly antisocial mask of 077. Programs should rarely if ever make policy decisions better
left to the user. The exception to this is when writing files that should be kept private: mail
files, web browser cookies, .rhosts files, and so on.
If umask(2) is not implemented on your system and you are trying to restrict access for yourself
(i.e., (EXPR & 0700) > 0), produces a fatal error at run time. If umask(2) is not implemented
and you are not trying to restrict access for yourself, returns "undef".
Remember that a umask is a number, usually given in octal; it is not a string of octal digits.
See also "oct", if all you have is a string.
undef EXPR
undef Undefines the value of EXPR, which must be an lvalue. Use only on a scalar value, an array
(using "@"), a hash (using "%"), a subroutine (using "&"), or a typeglob (using "*"). (Saying
"undef $hash{$key}" will probably not do what you expect on most predefined variables or DBM list
values, so don't do that; see delete.) Always returns the undefined value. You can omit the
EXPR, in which case nothing is undefined, but you still get an undefined value that you could,
for instance, return from a subroutine, assign to a variable or pass as a parameter. Examples:
undef $foo;
undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
undef @ary;
undef %hash;
undef &mysub;
undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
select undef, undef, undef, 0.25;
($a, $b, undef, $c) = &foo; # Ignore third value returned
Note that this is a unary operator, not a list operator.
unlink LIST
unlink Deletes a list of files. Returns the number of files successfully deleted.
$cnt = unlink 'a', 'b', 'c';
unlink @goners;
unlink <*.bak>;
Note: "unlink" will not delete directories unless you are superuser and the -U flag is supplied
to Perl. Even if these conditions are met, be warned that unlinking a directory can inflict
damage on your filesystem. Use "rmdir" instead.
If LIST is omitted, uses $_.
unpack TEMPLATE,EXPR
"unpack" does the reverse of "pack": it takes a string and expands it out into a list of values.
(In scalar context, it returns merely the first value produced.)
The string is broken into chunks described by the TEMPLATE. Each chunk is converted separately
to a value. Typically, either the string is a result of "pack", or the bytes of the string
represent a C structure of some kind.
The TEMPLATE has the same format as in the "pack" function. Here's a subroutine that does
substring:
sub substr {
my($what,$where,$howmuch) = @_;
unpack("x$where a$howmuch", $what);
}
and then there's
sub ordinal { unpack("c",$_[0]); } # same as ord()
In addition to fields allowed in pack(), you may prefix a field with a %<number> to indicate that
you want a <number>-bit checksum of the items instead of the items themselves. Default is a
16-bit checksum. Checksum is calculated by summing numeric values of expanded values (for string
fields the sum of "ord($char)" is taken, for bit fields the sum of zeroes and ones).
For example, the following computes the same number as the System V sum program:
$checksum = do {
local $/; # slurp!
unpack("%32C*",<>) % 65535;
};
The following efficiently counts the number of set bits in a bit vector:
$setbits = unpack("%32b*", $selectmask);
The "p" and "P" formats should be used with care. Since Perl has no way of checking whether the
value passed to "unpack()" corresponds to a valid memory location, passing a pointer value that's
not known to be valid is likely to have disastrous consequences.
If there are more pack codes or if the repeat count of a field or a group is larger than what the
remainder of the input string allows, the result is not well defined: in some cases, the repeat
count is decreased, or "unpack()" will produce null strings or zeroes, or terminate with an
error. If the input string is longer than one described by the TEMPLATE, the rest is ignored.
See "pack" for more examples and notes.
untie VARIABLE
Breaks the binding between a variable and a package. (See "tie".) Has no effect if the variable
is not tied.
unshift ARRAY,LIST
Does the opposite of a "shift". Or the opposite of a "push", depending on how you look at it.
Prepends list to the front of the array, and returns the new number of elements in the array.
unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
Note the LIST is prepended whole, not one element at a time, so the prepended elements stay in
the same order. Use "reverse" to do the reverse.
use Module VERSION LIST
use Module VERSION
use Module LIST
use Module
use VERSION
Imports some semantics into the current package from the named module, generally by aliasing
certain subroutine or variable names into your package. It is exactly equivalent to
BEGIN { require Module; import Module LIST; }
except that Module must be a bareword.
VERSION may be either a numeric argument such as 5.006, which will be compared to $], or a
literal of the form v5.6.1, which will be compared to $^V (aka $PERL_VERSION. A fatal error is
produced if VERSION is greater than the version of the current Perl interpreter; Perl will not
attempt to parse the rest of the file. Compare with "require", which can do a similar check at
run time.
Specifying VERSION as a literal of the form v5.6.1 should generally be avoided, because it leads
to misleading error messages under earlier versions of Perl which do not support this syntax.
The equivalent numeric version should be used instead.
use v5.6.1; # compile time version check
use 5.6.1; # ditto
use 5.006_001; # ditto; preferred for backwards compatibility
This is often useful if you need to check the current Perl version before "use"ing library
modules that have changed in incompatible ways from older versions of Perl. (We try not to do
this more than we have to.)
The "BEGIN" forces the "require" and "import" to happen at compile time. The "require" makes
sure the module is loaded into memory if it hasn't been yet. The "import" is not a builtin--it's
just an ordinary static method call into the "Module" package to tell the module to import the
list of features back into the current package. The module can implement its "import" method any
way it likes, though most modules just choose to derive their "import" method via inheritance
from the "Exporter" class that is defined in the "Exporter" module. See Exporter. If no
"import" method can be found then the call is skipped.
If you do not want to call the package's "import" method (for instance, to stop your namespace
from being altered), explicitly supply the empty list:
use Module ();
That is exactly equivalent to
BEGIN { require Module }
If the VERSION argument is present between Module and LIST, then the "use" will call the VERSION
method in class Module with the given version as an argument. The default VERSION method,
inherited from the UNIVERSAL class, croaks if the given version is larger than the value of the
variable $Module::VERSION.
Again, there is a distinction between omitting LIST ("import" called with no arguments) and an
explicit empty LIST "()" ("import" not called). Note that there is no comma after VERSION!
Because this is a wide-open interface, pragmas (compiler directives) are also implemented this
way. Currently implemented pragmas are:
use constant;
use diagnostics;
use integer;
use sigtrap qw(SEGV BUS);
use strict qw(subs vars refs);
use subs qw(afunc blurfl);
use warnings qw(all);
use sort qw(stable _quicksort _mergesort);
Some of these pseudo-modules import semantics into the current block scope (like "strict" or
"integer", unlike ordinary modules, which import symbols into the current package (which are
effective through the end of the file).
There's a corresponding "no" command that unimports meanings imported by "use", i.e., it calls
"unimport Module LIST" instead of "import".
no integer;
no strict 'refs';
no warnings;
See perlmodlib for a list of standard modules and pragmas. See perlrun for the "-M" and "-m"
command-line options to perl that give "use" functionality from the command-line.
utime LIST
Changes the access and modification times on each file of a list of files. The first two
elements of the list must be the NUMERICAL access and modification times, in that order. Returns
the number of files successfully changed. The inode change time of each file is set to the
current time. For example, this code has the same effect as the Unix touch(1) command when the
files already exist.
#!/usr/bin/perl
$now = time;
utime $now, $now, @ARGV;
Note: Under NFS, touch(1) uses the time of the NFS server, not the time of the local machine.
If there is a time synchronization problem, the NFS server and local machine will have different
times.
Since perl 5.7.2, if the first two elements of the list are "undef", then the utime(2) function
in the C library will be called with a null second argument. On most systems, this will set the
file's access and modification times to the current time (i.e. equivalent to the example above.)
utime undef, undef, @ARGV;
values HASH
Returns a list consisting of all the values of the named hash. (In a scalar context, returns the
number of values.)
The values are returned in an apparently random order. The actual random order is subject to
change in future versions of perl, but it is guaranteed to be the same order as either the "keys"
or "each" function would produce on the same (unmodified) hash. Since Perl 5.8.1 the ordering is
different even between different runs of Perl for security reasons (see "Algorithmic Complexity
Attacks" in perlsec).
As a side effect, calling values() resets the HASH's internal iterator, see "each".
Note that the values are not copied, which means modifying them will modify the contents of the
hash:
for (values %hash) { s/foo/bar/g } # modifies %hash values
for (@hash{keys %hash}) { s/foo/bar/g } # same
See also "keys", "each", and "sort".
vec EXPR,OFFSET,BITS
Treats the string in EXPR as a bit vector made up of elements of width BITS, and returns the
value of the element specified by OFFSET as an unsigned integer. BITS therefore specifies the
number of bits that are reserved for each element in the bit vector. This must be a power of two
from 1 to 32 (or 64, if your platform supports that).
If BITS is 8, "elements" coincide with bytes of the input string.
If BITS is 16 or more, bytes of the input string are grouped into chunks of size BITS/8, and each
group is converted to a number as with pack()/unpack() with big-endian formats "n"/"N" (and
analogously for BITS==64). See "pack" for details.
If bits is 4 or less, the string is broken into bytes, then the bits of each byte are broken into
8/BITS groups. Bits of a byte are numbered in a little-endian-ish way, as in 0x01, 0x02, 0x04,
0x08, 0x10, 0x20, 0x40, 0x80. For example, breaking the single input byte "chr(0x36)" into two
groups gives a list "(0x6, 0x3)"; breaking it into 4 groups gives "(0x2, 0x1, 0x3, 0x0)".
"vec" may also be assigned to, in which case parentheses are needed to give the expression the
correct precedence as in
vec($image, $max_x * $x + $y, 8) = 3;
If the selected element is outside the string, the value 0 is returned. If an element off the
end of the string is written to, Perl will first extend the string with sufficiently many zero
bytes. It is an error to try to write off the beginning of the string (i.e. negative OFFSET).
The string should not contain any character with the value > 255 (which can only happen if you're
using UTF-8 encoding). If it does, it will be treated as something which is not UTF-8 encoded.
When the "vec" was assigned to, other parts of your program will also no longer consider the
string to be UTF-8 encoded. In other words, if you do have such characters in your string, vec()
will operate on the actual byte string, and not the conceptual character string.
Strings created with "vec" can also be manipulated with the logical operators "⎪", "&", "^", and
"~". These operators will assume a bit vector operation is desired when both operands are
strings. See "Bitwise String Operators" in perlop.
The following code will build up an ASCII string saying 'PerlPerlPerl'. The comments show the
string after each step. Note that this code works in the same way on big-endian or little-endian
machines.
my $foo = '';
vec($foo, 0, 32) = 0x5065726C; # 'Perl'
# $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
vec($foo, 2, 16) = 0x5065; # 'PerlPe'
vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
vec($foo, 21, 4) = 7; # 'PerlPerlPer'
# 'r' is "\x72"
vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
# 'l' is "\x6c"
To transform a bit vector into a string or list of 0's and 1's, use these:
$bits = unpack("b*", $vector);
@bits = split(//, unpack("b*", $vector));
If you know the exact length in bits, it can be used in place of the "*".
Here is an example to illustrate how the bits actually fall in place:
#!/usr/bin/perl -wl
print <<'EOT';
0 1 2 3
unpack("V",$_) 01234567890123456789012345678901
------------------------------------------------------------------
EOT
for $w (0..3) {
$width = 2**$w;
for ($shift=0; $shift < $width; ++$shift) {
for ($off=0; $off < 32/$width; ++$off) {
$str = pack("B*", "0"x32);
$bits = (1<<$shift);
vec($str, $off, $width) = $bits;
$res = unpack("b*",$str);
$val = unpack("V", $str);
write;
}
}
}
format STDOUT =
vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
$off, $width, $bits, $val, $res
.
__END__
Regardless of the machine architecture on which it is run, the above example should print the
following table:
0 1 2 3
unpack("V",$_) 01234567890123456789012345678901
------------------------------------------------------------------
vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
wait Behaves like the wait(2) system call on your system: it waits for a child process to terminate
and returns the pid of the deceased process, or "-1" if there are no child processes. The status
is returned in $?. Note that a return value of "-1" could mean that child processes are being
automatically reaped, as described in perlipc.
waitpid PID,FLAGS
Waits for a particular child process to terminate and returns the pid of the deceased process, or
"-1" if there is no such child process. On some systems, a value of 0 indicates that there are
processes still running. The status is returned in $?. If you say
use POSIX ":sys_wait_h";
#...
do {
$kid = waitpid(-1, WNOHANG);
} until $kid > 0;
then you can do a non-blocking wait for all pending zombie processes. Non-blocking wait is
available on machines supporting either the waitpid(2) or wait4(2) system calls. However,
waiting for a particular pid with FLAGS of 0 is implemented everywhere. (Perl emulates the
system call by remembering the status values of processes that have exited but have not been
harvested by the Perl script yet.)
Note that on some systems, a return value of "-1" could mean that child processes are being
automatically reaped. See perlipc for details, and for other examples.
wantarray
Returns true if the context of the currently executing subroutine is looking for a list value.
Returns false if the context is looking for a scalar. Returns the undefined value if the context
is looking for no value (void context).
return unless defined wantarray; # don't bother doing more
my @a = complex_calculation();
return wantarray ? @a : "@a";
This function should have been named wantlist() instead.
warn LIST
Produces a message on STDERR just like "die", but doesn't exit or throw an exception.
If LIST is empty and $@ already contains a value (typically from a previous eval) that value is
used after appending "\t...caught" to $@. This is useful for staying almost, but not entirely
similar to "die".
If $@ is empty then the string "Warning: Something's wrong" is used.
No message is printed if there is a $SIG{__WARN__} handler installed. It is the handler's
responsibility to deal with the message as it sees fit (like, for instance, converting it into a
"die"). Most handlers must therefore make arrangements to actually display the warnings that
they are not prepared to deal with, by calling "warn" again in the handler. Note that this is
quite safe and will not produce an endless loop, since "__WARN__" hooks are not called from
inside one.
You will find this behavior is slightly different from that of $SIG{__DIE__} handlers (which
don't suppress the error text, but can instead call "die" again to change it).
Using a "__WARN__" handler provides a powerful way to silence all warnings (even the so-called
mandatory ones). An example:
# wipe out *all* compile-time warnings
BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
my $foo = 10;
my $foo = 20; # no warning about duplicate my $foo,
# but hey, you asked for it!
# no compile-time or run-time warnings before here
$DOWARN = 1;
# run-time warnings enabled after here
warn "\$foo is alive and $foo!"; # does show up
See perlvar for details on setting %SIG entries, and for more examples. See the Carp module for
other kinds of warnings using its carp() and cluck() functions.
write FILEHANDLE
write EXPR
write Writes a formatted record (possibly multi-line) to the specified FILEHANDLE, using the format
associated with that file. By default the format for a file is the one having the same name as
the filehandle, but the format for the current output channel (see the "select" function) may be
set explicitly by assigning the name of the format to the $~ variable.
Top of form processing is handled automatically: if there is insufficient room on the current
page for the formatted record, the page is advanced by writing a form feed, a special top-of-page
format is used to format the new page header, and then the record is written. By default the
top-of-page format is the name of the filehandle with "_TOP" appended, but it may be dynamically
set to the format of your choice by assigning the name to the $^ variable while the filehandle is
selected. The number of lines remaining on the current page is in variable "$-", which can be
set to 0 to force a new page.
If FILEHANDLE is unspecified, output goes to the current default output channel, which starts out
as STDOUT but may be changed by the "select" operator. If the FILEHANDLE is an EXPR, then the
expression is evaluated and the resulting string is used to look up the name of the FILEHANDLE at
run time. For more on formats, see perlform.
Note that write is not the opposite of "read". Unfortunately.
y/// The transliteration operator. Same as "tr///". See perlop.
跋
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perl v5.8.1 2003-09-02 PERLFUNC(7)