The trigger command will create or replace a trigger.

A trigger is a saved incremental query over a watched root. When files change that match the query expression, Watchman will spawn a process and pass information about the changed files to it.

Triggered processes are spawned by the Watchman server process that runs in the background; they do not have access to your terminal and their output is redirected (by default) to the Watchman log file.

Watchman waits for the filesystem to settle before processing any triggers, batching the list of changed files together before invoking the registered command. You can adjust the settle period via the .watchmanconfig file.

Note that deleted files are counted as changed files and are passed the command in exactly the same way as changed-but-existing files.

Watchman will only run a single instance of the trigger process at a time. That avoids fork-bomb style behavior in cases where your trigger also modifies files. When the process terminates, watchman will re-evaluate the trigger criteria based on the clock at the time the process was last spawned; if a file list is generated watchman will spawn a new child with the files that changed in the meantime.

Unless no-save-state is in use, triggers are saved and re-established across a Watchman process restart. If you had triggeres saved prior to upgrading to Watchman 2.9.7, those triggers will be forgotten as you upgrade past version 2.9.7; you will need to re-register them.

There are two syntaxes for registering triggers; a simple syntax that allows very simple trigger configuration with some reasonable defaults, and a second extended syntax which is available since Watchman version 2.9.7.

The simple syntax is implemented in terms of the extended syntax and is preserved for backwards compatibility with older clients.

Extended syntax

Since 2.9.7.

You may use the extended JSON trigger definition syntax detailed below. It provides more control over how the triggered commands are invoked than was possible in earlier versions.

JSON:

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["trigger", "/path/to/dir", <triggerobj>]

Where triggerobj is a trigger configuration object with the fields defined below.

Here’s an example trigger specified via the CLI that will cause make to be run whenever assets or sources are changed:

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$ watchman -j <<-EOT
["trigger", "/path/to/root", {
  "name": "assets",
  "expression": ["pcre", "\.(js|css|c|cpp)$"],
  "command": ["make"]
}]
EOT

The possible trigger object properties are:

  • name defines the name of the trigger. You may use this name to remove the trigger later. Registering a different trigger with the same name as an existing trigger will implicitly delete the old trigger and then register the new one, causing the trigger expression to be evaluated for the whole tree.

  • command specifies the command to invoke. It must be an array of string values; this will form the argv array of the trigger process. When the trigger is spawned, the $PATH of the Watchman process will be used to locate the command. If you have changed your $PATH since the Watchman process was started, Watchman won’t be able to see your new $PATH. If you are registering trigger that runs something from an unusual or non-default location, it is recommended that you specify the full path to that command. If you are registering a trigger script that can be found in the watched root, just specify the path relative to the root.

  • append_files is an optional boolean parameter; if enabled, the command array will have the set of matching file names appended when the trigger is invoked. System limits such as sysconf(_SC_ARG_MAX) and/or RLIMIT_STACK set an upper bound on the size of the parameters and environment that are passed to a spawned process. Watchman will try to ensure that the command is runnable by keeping the number of file name arguments below the system limits. If the full set cannot be passed to the process, Watchman will pass as many as it thinks will fit and omit the rest. When this argument list truncation occurs, Watchman will export WATCHMAN_FILES_OVERFLOW=true into the environment so that the child process can determine that this has happened. Watchman cannot break the arguments apart and run multiple processes for each argument batch; for that functionality, use xargs(1) for the command and set the stdin property to NAME_PER_LINE.

  • expression accepts a query expression. The expression is applied to the list of changed files to generate the set of files that are relevant to this trigger. If no files match, the command will not be invoked. Omitting the expression will match all changed files.

  • stdin specifies how stdin should be configured for the command invocation. You may set the value of this property to one of the following:

    • the string value /dev/null - sets stdin to read from /dev/null. This is the default and will be used if you omit the stdin property.

    • an array value will be interpreted as a list of field names. When the command is invoked, Watchman will generate an array of JSON objects that contain those field names on stdin. For example, if stdin is set to ["name", "size"], stdin will be a JSON array containing the list of changed files, represented as objects with the name and size properties: [{"name": "filename.txt", "size": 123}]. The list of valid fields is the same as the same as that documented in the query command. Just as with the query command, if the field list is comprised of a single field then the JSON will be an array of those field values. For instance, if you set stdin to ["name"] the JSON will be of the form ["filename.txt"] instead of [{"name": "filename.txt"}].

    • the string value NAME_PER_LINE will cause Watchman to generate a list of file names on stdin, one name per line. No quoting will be applied to the names, and they may contain spaces.

  • stdout and stderr control the output and error streams. If omitted, the corresponding stream will be inherited from the Watchman process, which typically means that the command output/error stream will show up in the Watchman log file. If specified, the value must be a string:

    • >path/to/file - causes output to redirected to the specifie file. The path is relative to the watched root, and will be truncated prior to being written to, if it exists, or created if it does not exist.

    • >>path/to/file - causes output to redirected to the specifie file. The path is relative to the watched root. If the file already exists then it will be appended to. The file will be created if it does not exist.

  • max_files_stdin specifies a limit on the number of files reported on stdin when stdin is set to hold the set of matched files. If the number of files that matched exceeds this limit, the input will be truncated to match this limit and WATCHMAN_FILES_OVERFLOW=true will also be exported into the environment. The default, if omitted, is no limit.

  • chdir can be used to specify the working directory that should be set prior to spawning the process. The default is to set the working directory to the watched root. The value of this property is a string that will be interpreted relative to the watched root. Note that changing the working dir does not cause the file names from the query result to be re-written: they will always be relative to the watched root. The path to the root can be found in the $WATCHMAN_ROOT environmental variable.

Simple syntax

The simple syntax is easier to execute from the CLI than the JSON based extended syntax, but doesn’t allow all of the trigger options to be set. In only supports the Simple Pattern Syntax for queries.

From the command line:

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$ watchman -- trigger /path/to/dir triggername [patterns] -- [cmd]

Note that the first -- is to distinguish watchman CLI switches from the second --, which delimits patterns from the trigger command. This is only needed when using the CLI, not when using the JSON protocol.

JSON:

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["trigger", "/path/to/dir", "triggername", <patterns>, "--", <cmd>]

For example:

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$ watchman -- trigger ~/www jsfiles '*.js' -- ls -l

Note the single quotes around the *.js; if you omit them, your shell will expand it to a list of file names and register those in the trigger. While this would work, any *.js files that you add after registering the trigger will not cause the trigger to run.

or in JSON:

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["trigger", "/home/wez/www", "jsfiles", "*.js", "--", "ls", "-l"]

The simple syntax is interpreted as a trigger object with the following settings:

  • name is set to the triggername
  • command is set to the <cmd> list
  • expression is generated from the <patterns> list using the rules laid out in Simple Pattern Syntax
  • append_files is set to true
  • stdin is set to ["name", "exists", "new", "size", "mode"]
  • stdout and stderr will be set to output to the Watchman log file
  • max_files_stdin will be left unset

For this simple example, if ~/www/scripts/foo.js is changed, watchman will chdir to ~/www then invoke ls -l scripts/foo.js. Note that the output will show up in the Watchman log file, not in your terminal.

Environment for trigger commands

Since Watchman version 2.9.7, the following environment variables are set for all trigger commands, even those registered using the simple trigger syntax:

  • WATCHMAN_FILES_OVERFLOW is set to true if the number of files exceeds either the max_files_stdin limit or the system argument size limit.
  • WATCHMAN_CLOCK is set to the current clock at the time of the trigger invocation
  • WATCHMAN_SINCE is set to the clock value of the prior trigger invocation, or unset if this is the first trigger invocation.
  • WATCHMAN_ROOT is set to the path to the watched root
  • WATCHMAN_TRIGGER is set to the name of the trigger
  • WATCHMAN_SOCK is set to the path to the Watchman socket, so that you can figure out how to connect back to Watchman.

Relative roots

Since 3.4.

Watchman supports optionally evaluating triggers with respect to a path within a watched root. This is used with the relative_root parameter:

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["trigger", "/path/to/watched/root", {
  "name": "relative-assets",
  "expression": ["pcre", "\.(js|css|c|cpp)$"],
  "command": ["make"],
  "relative_root": "project1"
}]

Setting a relative root results in the following modifications to triggers:

  • Queries are evaluated with respect to the relative root. See File Queries for more.
  • The current directory for triggered processes is set to the relative root, unless it is changed with chdir. If chdir is a relative path then it will be evaluated with respect to the relative root. So, for the example trigger above, if chdir is "subdir2", the current directory for triggered make invocations is /path/to/watched/root/project1/subdir2.
  • In the environment, WATCHMAN_ROOT is still set to the actual root.
  • WATCHMAN_RELATIVE_ROOT is set to the full path of the relative root.

Relative roots behave similarly to a separate Watchman watch on the subdirectory, without any of the system overhead that that imposes. This is useful for large repositories, where your script or tool is only interested in a particular directory inside the repository.