Provided by: fswatch_1.14.0+repack-13.1_amd64 
NAME
fswatch — Ask for notification when the contents of the specified files or directory hierarchies are
modified.
This man page is kept for reference but it is not to be considered an authoritative or complete source of
information. Please, consult the official Info documentation shipped with .
SYNOPSIS
fswatch (option)* path+
DESCRIPTION
The fswatch command receives notifications when the contents of the specified files or directories are
modified. fswatch implements six kind of monitors:
- A monitor based on the File System Events API of Apple OS X.
- A monitor based on kqueue, an event notification interface introduces in FreeBSD 4.1 and
supported on most *BSD systems (including OS X).
- A monitor based on the File Events Notification API of the Solaris kernel and its derivatives.
- A monitor based on inotify, a Linux kernel subsystem that reports file system changes to
applications.
- A monitor based on the ReadDirectoryChangesW Microsoft Windows API.
- A monitor which periodically stats the file system, saves file modification times in memory and
manually calculates changes.
fswatch writes a record for each event it receives containing:
- The timestamp when the event was received (optionally).
- The path affected by the current event.
- A space-separated list of event types (see “EVENT TYPES” ).
fswatch enters an infinite loop and never returns. When it receives a SIGABRT, SIGINT or SIGTERM signal,
fswatch closes the notification streams and exits gracefully returning 0 to the calling process.
The following options are available:
-0, --print0
Use the ASCII NUL character (\0) as line separator. Since file names can potentially contain any
character but NUL, this option assures that the output of fswatch can be safely parsed using NUL
as delimiter, such as using xargs -0 and the shell builtin read -d ''.
-1, --one-event
Exit fswatch after the first set of events is received.
--batch-marker marker
Print a marker at the end of every batch. An optional marker marker can be specified to override
its default value `NoOp'.
--event name
Filter event with the specified name. This option can be used multiple times, one for each event
name that must be included in the output.
-e, --exclude regexp
Exclude paths matching regexp. Multiple exclude filters can be specified using this option
multiple times. See “FILTERING PATHS” for further information.
-E, --extended
Use extended regular expressions.
-f, --format-time format
Print the event time using the specified format. Supported formats are specified by strftime(3).
-h, --help
Show the help message.
-i, --include regexp
Include paths matching regexp. Multiple include filters can be specified using this option
multiple times. See “FILTERING PATHS” for further information.
-I, --insensitive
Use case insensitive regular expressions.
-l, --latency latency
Set the latency in seconds. The latency is a double value greater than 0.1. Smaller values are
currently not allowed in order not to compromise the performance of the system. The default
latency is 1 second.
-L, --follow-links
Follow symbolic links.
-M, --list-monitors
List the available monitors.
-m, --monitor name
Uses the monitor specified by name. The list of currently available monitors can be obtained
using the -h option.
-n, --numeric
Print the numeric value of the event flag, instead of the array of symbolic names. The numeric
value of the event flags are system-specific and may vary across different versions of OS X. As
a consequence, the use of numeric values is discouraged.
-o, --one-per-batch
Print a single message with the number of change events.
-r, --recursive
Watch subdirectories recursively. This option may not be supported on all systems.
-t, --timestamp
Print the event timestamp.
-u, --utf-time
Print the event time in UTC format. When this option is not specified, the time is printed using
the system local time, as defined by localtime (3).
-v, --verbose
Print verbose output.
--version
Print the version of fswatch and exits.
-x, --event-flags
Prints the event flags.
--event-flag-separator separator
Print event flags using the specified separator.
MONITORS
fswatch acts as a front-end to system-specific monitors. Currently, the available monitors are:
- The FSEvents monitor, a monitor based on the File System Events API of Apple OS X.
- The kqueue monitor, a monitor based on kqueue, an event notification interface introduced in
FreeBSD 4.1 and supported on most *BSD systems (including OS X).
- The inotify monitor, a Linux kernel subsystem that reports file system changes to applications.
- The poll monitor, a monitor which periodically stats the file system, saves file modification
times in memory and manually calculates file system changes, which can work on any operating
system where stat(2) can be used.
Each monitor has its own strengths, weakness and peculiarities. Although fswatch strives to provide a
uniform experience no matter which monitor is used, it is still important for users to know which monitor
they are using and to be aware of existing bugs, limitations, corner cases or pathological behaviour.
The FSEvents Monitor
The FSEvents monitor, available only on Apple OS X, has no known limitations and scales very well with
the number of files being observed. In fact, I observed no performance degradation when testing fswatch
observing changes on a filesystem of 500 GB over long periods of time. On OS X, this is the default
monitor.
The kqueue Monitor
The kqueue monitor, available on any *BSD system featuring kqueue, requires a file descriptor to be
opened for every file being watched. As a result, this monitor scales badly with the number of files
being observed and may begin to misbehave as soon as the fswatch process runs out of file descriptors.
In this case, fswatch dumps one error on standard error for every file that cannot be opened. Beware
that on some systems the maximum number of file descriptors that can be opened by a process is set to a
very low value (values as low as 256 are not uncommon), even if the operating system may allow a much
larger value.
If you are running out of file descriptors when using this monitor and you cannot reduce the number of
observed items, either:
- Consider raising the number of maximum open file descriptors (check your OS documentation).
- Consider using another monitor.
The inotify Monitor
The inotify monitor, available on Linux since kernel 2.6.13, may suffer a queue overflow if events are
generated faster than they are read from the queue. In any case, the application is guaranteed to
receive an overflow notification which can be handled to gracefully recover. fswatch currently throws an
exception if a queue overflow occurs. Future versions will handle the overflow by emitting proper
notifications. However, the odds of observing a queue overflow on a default configured mainstream
GNU/Linux distribution is very low.
The inotify API sends events for the direct child elements of a watched directory and it scales pretty
well with the number of watched items. For this reason, depending on the number of files to watch, it
may sometimes be preferable to watch a common parent directory and filter received events rather than
adding a huge number of file watches.
The Poll Monitor
The poll monitor was added as a fallback mechanisms in the cases where no other monitor could be used,
including:
- Operating system without any sort of file events API.
- Situations where the limitations of the available monitors cannot be overcome (i.e.: observing a
number of files greater than the available file descriptors on a system using the kqueue
monitor).
The poll monitor, available on any platform, only relies on available CPU and memory to perform its task
(besides the stat(2) function). The performance of this monitor degrades linearly with the number of
files being watched. The authors' experience indicates that fswatch requires approximately 150 MB or RAM
memory to observe a hierarchy of 500.000 files with a minimum path length of 32 characters. A common
bottleneck of the poll monitor is disk access, since stat()-ing a great number of files may take a huge
amount of time. In this case, the latency should be set to a sufficiently large value in order to reduce
the performance degradation that may result from frequent disk access.
How to Choose a Monitor
fswatch already chooses the "best" monitor for your platform if you do not specify any. However, a
specific monitor may be better suited to specific use cases. Please, read the “MONITORS” section to get
a description of all the available monitors and their limitations.
Usage recommendations are as follows:
- On OS X, use only the FSEvents monitor (which is the default behaviour).
- On Linux, use the inotify monitor (which is the default behaviour).
- If the number of files to observe is sufficiently small, use the kqueue monitor. Beware that on
some systems the maximum number of file descriptors that can be opened by a process is set to a
very low value (values as low as 256 are not uncommon), even if the operating system may allow a
much larger value. In this case, check your OS documentation to raise this limit on either a per
process or a system-wide basis.
- If feasible, watch directories instead of watching files. Properly crafting the receiving side
of the events to deal with directories may sensibly reduce the monitor resource consumption.
- If none of the above applies, use the poll monitor. The authors' experience indicates that
fswatch requires approximately 150 MB or RAM memory to observe a hierarchy of 500.000 files with
a minimum path length of 32 characters. A common bottleneck of the poll monitor is disk access,
since stat()-ing a great number of files may take a huge amount of time. In this case, the
latency should be set to a sufficiently large value in order to reduce the performance
degradation that may result from frequent disk access.
FILTERING PATHS
Received events can be filtered by path using regular expressions. Regular expressions can be used to
include or exclude matching paths. The user can specify multiple filter expression in any order and the
first matching expression wins.
Other options govern how regular expressions are interpreted:
- Regular expressions can be extended if option -E is specified.
- Regular expressions can be case insensitive if option -I is specified.
EXAMPLES
Basic Usage
fswatch syntax is the following:
$ fswatch [options] [paths] ...
fswatch will then output change events to standard output. By default, only the affected file name is
printed. However, many options are available to format the event record, including:
- The possibility of adding the event timestamp.
- The possibility of adding the event mask in both textual and numerical form.
The following command listens for changes in the current directory and events are delivered every 5
seconds:
$ fswatch -l 5 .
The following command listens for changes in the current user home directory and /var/log:
$ fswatch ~ /var/log
Piping fswatch Output to Another Process
Very often you wish to not only receive an event, but react to it. The simplest way to do it is piping
fswatch output to another process. Since in Unix and Unix-like file system file names may potentially
contain any character but NUL (\0) and the path separator (/), fswatch has a specific mode of operation
when its output must be piped to another process. When the [-0] option is used, fswatch will use the NUL
character as record separator, thus allowing any other character to appear in a path. This is important
because many commands and shell builtins (such as read) split words and lines by default using the
characters in $IFS, which by default contains characters which may be present (although rarely) in a file
name, resulting in a wrong event path being received and processed.
Probably the simplest way to pipe fswatch to another program in order to respond to an event is using
xargs:
$ fswatch -0 [opts] [paths] | xargs -0 -n 1 -I {} [command]
- fswatch -0 will split records using the NUL character.
- xargs -0 will split records using the NUL character. This is required to correctly match
impedance with fswatch.
- xargs -n 1 will invoke command every record. If you want to do it every x records, then use
xargs -n x.
- xargs -I {} will substitute occurrences of {} in command with the parsed argument. If the
command you are running does not need the event path name, just delete this option. If you
prefer using another replacement string, substitute {} with yours.
Bubbling Events
An often requested feature is being able to receive a single event "per batch", instead of receiving
multiple events. This use case is implemented by the [-o, --one-per-batch] option which tells fswatch to
dump a record containing the number of received events, without any other detail:
$ fswatch -or /path/to/watch
1
10
[...]
This is useful if, for example, you want to respond to change events in a way which is (or can easily be)
path-independent (because you are not receiving any event detail) and you prefer to "bubble" events
together to reduce the overhead of the command being executed. A typical case is a directory
synchronisation job whenever some files change.
Receiving a Single Event
Another requested feature is the possibility of receiving a single event and exit. This is most useful
when existing scripts processing events include the restart logic of fswatch This use case is implemented
by the [-1, --one-event] option:
$ fswatch -1 /path/to/watch
/path/to/watch
Compatibility With fswatch 0.x
The previous major version of fswatch (v. 0.x) allowed users to run a command whenever a set of changes
was detected with the following syntax:
$ fswatch path program
Starting with fswatch v. 1.x this behaviour is no longer supported. The rationale behind this decision
includes:
- The old version only allows watching one path.
- The command to execute was passed as last argument, alongside the path to watch, making it
difficult to extend the program functionality to add multiple path support
- The old version forks and executes /bin/bash, which is neither portable, nor guaranteed to
succeed, nor desirable by users of other shells.
- No information about the change events is passed to the forked process.
To solve the aforementioned issues and keep fswatch consistent with common UNIX practices, the behaviour
has changed and fswatch now prints event records to the standard output that users can process further by
piping the output of fswatch to other programs.
To fully support the old use, the [-o, --one-per-batch] option was added in v. 1.3.3. When specified,
fswatch will only dump 1 event to standard output which can be used to trigger another program:
$ fswatch -o path | xargs -n1 program
In this case, program will receive the number of change events as first argument. If no argument should
be passed to program, then the following command could be used:
$ fswatch -o path | xargs -n1 -I{} program
Although we encourage you to embrace the new fswatch behaviour and update your scripts, we provide a
little wrapper called fswatch-run which is installed alongside fswatch which lets you use the legacy
syntax:
$ fswatch-run path [paths] program
Under the hood, fswatch-run simply calls fswatch -o piping its output to xargs.
fswatch-run is a symbolic link to a shell-specific wrapper. Currently, ZSH and Bash scripts are
provided. If no suitable shells are found in the target system, the fswatch-run symbolic link is not
created.
EXIT STATUS
fswatch may exit with one of the following exit statuses:
FSW_OK 0
FSW_ERR_UNKNOWN_ERROR (1 << 0)
FSW_ERR_SESSION_UNKNOWN (1 << 1)
FSW_ERR_MONITOR_ALREADY_EXISTS (1 << 2)
FSW_ERR_MEMORY (1 << 3)
FSW_ERR_UNKNOWN_MONITOR_TYPE (1 << 4)
FSW_ERR_CALLBACK_NOT_SET (1 << 5)
FSW_ERR_PATHS_NOT_SET (1 << 6)
FSW_ERR_UNKNOWN_MONITOR (1 << 7)
FSW_ERR_MISSING_CONTEXT (1 << 8)
FSW_ERR_INVALID_PATH (1 << 9)
FSW_ERR_INVALID_CALLBACK (1 << 10)
FSW_ERR_INVALID_LATENCY (1 << 11)
FSW_ERR_INVALID_REGEX (1 << 12)
FSW_ERR_MONITOR_ALREADY_RUNNING (1 << 13)
FSW_ERR_STALE_MONITOR_THREAD (1 << 14)
FSW_ERR_THREAD_FAULT (1 << 15)
FSW_ERR_UNSUPPORTED_OPERATION (1 << 16)
FSW_ERR_UNKNOWN_VALUE (1 << 17)
DIAGNOSTICS
fswatch exits 0 on success, and >0 if an error occurs.
COMPATIBILITY
fswatch can be built on any system supporting at least one of the available monitors.
BUGS
See https://github.com/emcrisostomo/fswatch/issues for open issues or to create a new one.
Bugs can also be submitted to enrico.m.crisostomo@gmail.com.
pc linux-gnu September 15, 2021 fswatch(7)