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fswatch(7) Miscellaneous Information Manual fswatch(7) 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 au- thoritative or complete source of information. Please, consult the offi- cial Info documentation shipped with fswatch. SYNOPSIS fswatch (option)* path+ DESCRIPTION The fswatch command receives notifications when the contents of the speci- fied files or directories are modified. fswatch implements seven kind of monitors: - A monitor based on the File System Events API of Apple macOS. - A monitor based on kqueue, an event notification interface intro- duces 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 fanotify, a Linux kernel subsystem that can re- port file system changes with optional process attribution. - A monitor based on the ReadDirectoryChangesW Microsoft Windows API. Native MinGW-w64 builds are the recommended Windows target; Cygwin builds are kept for compatibility when the Cygwin path conversion API is available. - A monitor which periodically stats the file system, saves file mod- ification 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" ). By default, fswatch enters an infinite loop and never returns. When it re- ceives a SIGABRT, SIGINT or SIGTERM signal, fswatch closes the notification streams and exits gracefully returning 0 to the calling process. (Unless running in [-1, --one-event] mode, in which case it sets its return code according to the signal it received, as usual.) 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 mul- tiple 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). --format format Use the specified event record format. The format string supports path, flag, timestamp, correlation, and monitor-specific metadata directives including fanotify process attribution. -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 avail- able 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, --utc-time Print the event time in UTC format. When this option is not speci- fied, 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 notifica- tion 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 fanotify monitor, a Linux kernel subsystem that can report file system changes with optional process attribution. - The poll monitor, a monitor which periodically stats the file sys- tem, saves file modification times in memory and manually calcu- lates 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 limita- tions 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 descrip- tors. 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 han- dle 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 di- rectory 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 fanotify Monitor The fanotify monitor, available on Linux when the build system detects the required fanotify headers and symbols, uses file-handle based fanotify events. It is not the default Linux monitor; select it explicitly with: $ fswatch -m fanotify_monitor path ... The fanotify monitor is useful when Linux-specific process attribution is required. Custom format directive %K prints the identifier kind, such as `pid' or `tid', and prints the reported process or thread identifier. The monitor property accepts `pid' or `tid'. The fanotify.report-pidfd prop- erty requests pidfd reporting when supported by the build headers and ker- nel; the pidfd is exposed through the library event metadata only while the callback is executing. The fanotify.unlimited-queue and fanotify.unlimited-marks properties re- quest FAN_UNLIMITED_QUEUE and FAN_UNLIMITED_MARKS, respectively. These properties are opt-in because they require matching build headers and the capability expected by the running kernel, typically CAP_SYS_ADMIN, and fanotify initialization fails if the process lacks it. Fanotify support depends on kernel, C library, filesystem, and permission support for the requested mode. Some filesystems do not support file han- dles, and some fanotify modes require additional privileges. Use the ino- tify monitor when broad Linux compatibility is more important than process attribution. Fanotify directory marks are not equivalent to macOS FSEvents recursive tree watches. With ordinary directory marks, FAN_EVENT_ON_CHILD reports events for immediate children only, not grandchildren or deeper descen- dants. Recursive fanotify monitoring therefore still marks subdirectories explicitly. Avoiding recursive mark expansion requires fanotify mount or filesystem marks, which have broader scope and different permission con- straints. 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 perfor- mance of this monitor degrades linearly with the number of files being watched. The authors' experience indicates that fswatch requires approxi- mately 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 mon- itor 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 suffi- ciently 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 be- haviour). - On Linux, use the inotify monitor (which is the default behaviour). - On Linux, use the fanotify monitor only when its Linux-specific process attribution or fanotify semantics are required. - 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 op- erating 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 mon- itor 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 perfor- mance 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 filters are processed this way: - If a path matches an including filter, the path is accepted no mat- ter any other filter. - If a path matches an excluding filter, the path is rejected. - If a path matches no filters, the path is accepted. Said another way: - All paths are accepted @emph{by default}, unless an exclusion fil- ter says otherwise. - Inclusion filters may override any other exclusion filter. - The order in the definition of filters in the command line has no effect. 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 speci- fied. 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 cre- ation, 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 avail- able 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 objectas owner has changed. AttributeModified An objectas attribute has changed. MovedFrom The object has moved from this location to a new lo- cation 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 numer- ical 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 direc- tory 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 sepa- rator, thus allowing any other character to appear in a path. This is im- portant because many commands and shell builtins (such as read) split words and lines by default using the characters in $IFS, which by default con- tains characters which may be present (although rarely) in a file name, re- sulting 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 re- quired 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 an- other 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 re- ceiving any event detail) and you prefer to "bubble" events together to re- duce the overhead of the command being executed. A typical case is a di- rectory 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 in- clude 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 com- mand 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 function- ality 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 out- put 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 ar- gument. 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) 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. portbld freebsd15.0 July 30, 2026 fswatch(7)
NAME | SYNOPSIS | DESCRIPTION | MONITORS | FILTERING PATHS | EVENT TYPES | EXAMPLES | Compatibility With fswatch 0.x | EXIT STATUS | DIAGNOSTICS | COMPATIBILITY | BUGS
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