fswatch - Man Page

Ask for notification when the contents of the specified files or directory hierarchies are modified.

Examples (TL;DR)

Run a Bash command on file creation, update or deletion: fswatch path/to/file | xargs -n 1 bash_command
Watch one or more files and/or directories: fswatch path/to/file path/to/directory path/to/another_directory/**/*.js | xargs -n 1 bash_command
Print the absolute paths of the changed files: fswatch path/to/directory | xargs -n 1 -I {} echo {}
Filter by event type: fswatch --event Updated|Deleted|Created path/to/directory | xargs -n 1 bash_command

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 macOS.
-: A monitor based on kqueue, an event notification interface introduces in FreeBSD 4.1 and supported on most *BSD systems (including macOS).
-: 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.
--no-defer: Sets the kFSEventStreamCreateFlagNoDefer flag on macOS FSEvents monitor, which makes the monitor more responsive. This flag is more appropriate for sessions, while the default behaviour is more appropriate for background, daemon or batch processing apps.
-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 macOS. 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 macOS.
-: The kqueue monitor, a monitor based on kqueue, an event notification interface introduced in FreeBSD 4.1 and supported on most *BSD systems (including macOS).
-: 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 macOS, 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 macOS, 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 macOS, 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.

Event Types

Event flags identify the kind of change a file system object has undergone. Many of them directly map to common file system operations (such as creation, deletion, update, etc.), others are less common (such as attribute modification), and others are monitor and platform specific.

Currently, fswatch maps monitor-specific event flags to 'global' event flags acting as a sort of 'greatest common denominator' of all the available monitor flags. The following global event flags are available::

NoOp: Idle event, optionally issued when no changes were detected.
PlatformSpecific: This event maps a platform-specific event that has no corresponding flag.
Created: The object has been created.
Updated: The object has been updated. The kind of update is monitor-dependent.
Removed: The object has been removed.
Renamed: The object has been renamed.
OwnerModified: The object’s owner has changed.
AttributeModified: An object’s attribute has changed.
MovedFrom: The object has moved from this location to a new location of the same file system.
MovedTo: The object has moved from another location in the same file system into this location.
IsFile: The object is a regular file.
IsDir: The object is a directory.
IsSymLink: The object is a symbolic link.
Link: The object link count has changed.
Overflow: The monitor has overflowed.

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

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)