fish-shell/doc_src/cmds/trap.rst

.. _cmd-trap:

trap - perform an action when the shell receives a signal
=========================================================

Synopsis
--------

.. synopsis::

    trap [OPTIONS] [[ARG] REASON ... ]

Description
-----------

.. only:: builder_man

          NOTE: This page documents the fish builtin ``trap``.
          To see the documentation on any non-fish versions, use ``command man trap``.

``trap`` is a wrapper around the fish event delivery framework. It exists for backwards compatibility with POSIX shells. For other uses, it is recommended to define an :ref:`event handler <event>`.

The following parameters are available:

*ARG*
    Command to be executed on signal delivery.

*REASON*
    Name of the event to trap. For example, a signal like ``INT`` or ``SIGINT``, or the special symbol ``EXIT``.

**-l** or **--list-signals**
    Prints a list of signal names.

**-p** or **--print**
    Prints all defined signal handlers.

**-h** or **--help**
    Displays help about using this command.

If *ARG* and *REASON* are both specified, *ARG* is the command to be executed when the event specified by *REASON* occurs (e.g., the signal is delivered).

If *ARG* is absent (and there is a single *REASON*) or ``-``, each specified signal is reset to its original disposition (the value it had upon entrance to the shell).  If *ARG* is the null string the signal specified by each *REASON* is ignored by the shell and by the commands it invokes.

If *ARG* is not present and **-p** has been supplied, then the trap commands associated with each *REASON* are displayed. If no arguments are supplied or if only **-p** is given, ``trap`` prints the list of commands associated with each signal.

Signal names are case insensitive and the ``SIG`` prefix is optional. Trapping a signal will prevent fish from exiting in response to that signal.

The exit status is 1 if any *REASON* is invalid; otherwise trap returns 0.

Example
-------


::

    trap "status --print-stack-trace" SIGUSR1
    # Prints a stack trace each time the SIGUSR1 signal is sent to the shell.
docs: Add labels to all commands This allows us to use :ref: references, which don't require hardcoding it as html [ci skip] 2019-03-31 09:05:09 +00:00			`.. _cmd-trap:`

Switch command docs from \section to reStructuredText 2018-12-17 01:39:33 +00:00			`trap - perform an action when the shell receives a signal`
Fix command section separator line lengths 2019-01-03 04:10:47 +00:00			`=========================================================`
Switch command docs from \section to reStructuredText 2018-12-17 01:39:33 +00:00
Migrate the 'synopsis' sections to .rst format 2018-12-18 01:58:24 +00:00			`Synopsis`
			`--------`
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docs synopsis: add HTML highlighing and automate manpage markup Recent synopsis changes move from literal code blocks to [RST line blocks]. This does not translate well to HTML: it's not rendered in monospace, so aligment is lost. Additionally, we don't get syntax highlighting in HTML, which adds differences to our code samples which are highlighted. We hard-wrap synopsis lines (like code blocks). To align continuation lines in manpages we need [backslashes in weird places]. Combined with the *, , and `` markup, it's a bit hard to get the alignment right. Fix these by moving synopsis sources back to code blocks and compute HTML syntax highlighting and manpage markup with a custom Sphinx extension. The new Pygments lexer can tokenize a synopsis and assign the various highlighting roles, which closely matches fish's syntax highlighing: - command/keyword (dark blue) - parameter (light blue) - operator like and/or/not/&&/\|\| (cyan) - grammar metacharacter (black) For manpage output, we don't project the fish syntax highlighting but follow the markup convention in GNU's man(1): bold text type exactly as shown. italic text replace with appropriate argument. To make it easy to separate these two automatically, formalize that (italic) placeholders must be uppercase; while all lowercase text is interpreted literally (so rendered bold). This makes manpages more consistent, see string-join(1) and and(1). Implementation notes: Since we want manpage formatting but Sphinx's Pygments highlighing plugin does not support manpage output, add our custom "synopsis" directive. This directive parses differently when manpage output is specified. This means that the HTML and manpage build processes must not share a cache, because the parsed doctrees are cached. Work around this by using separate cache locations for build targets "sphinx-docs" (which creates HTML) and "sphinx-manpages". A better solution would be to only override Sphinx's ManualPageBuilder but that would take a bit more code (ideally we could override ManualPageWriter but Sphinx 4.3.2 doesn't really support that). --- Alternative solution: stick with line blocks but use roles like :command: or :option: (or custom ones). While this would make it possible to produce HTML that is consistent with code blocks (by adding a bit of CSS), the source would look uglier and is harder to maintain. (Let's say we want to add custom formatting to the [\|] metacharacters in HTML. This is much easier with the proposed patch.) --- [RST line blocks]: https://docutils.sourceforge.io/docs/ref/rst/restructuredtext.html#line-blocks [backslashes in weird places]: https://github.com/fish-shell/fish-shell/pull/8626#discussion_r782837750 2022-01-09 14:09:46 +00:00			`.. synopsis::`

docs synopsis: make all placeholder arguments uppercase man(1) uses lowercase placeholders but we usually don't. Additionally, the new synopsis autoformatting only recognizes placeholders if they are uppercase. Use uppercase for all placeholders. 2022-01-17 04:29:49 +00:00			`trap [OPTIONS] [[ARG] REASON ... ]`
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Convert \\subsection sections into rst format 2018-12-19 02:44:30 +00:00			`Description`
Fix command section separator line lengths 2019-01-03 04:10:47 +00:00			`-----------`
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Reference more non-fish shell builtins that have relevant differences When writing scripts for other shells, it can be confusing and annoying that our `man` function shadows other manual pages, for example `exec(1p)` from [Linux man-pages]. I almost never want to see the fish variant for such contended cases (which obviuosly don't include fish-specific commands like `string`, only widely-known shell builtins). For the contented cases like `exec`, the POSIX documentation is more substantial and useful, since it describes a (sub)set of languages widely used for scripting. Because of this I think we should stop overriding the system's man pages. Nowadays we offer `exec -h` as intuitive way to show the documentation for the fish-specific command (note that `help` is not a good replacement because it uses a web browser). Looking through the contended commands, it seems like for most of them, the fish version is not substantially different from the system version. A notable exception is `read` but I don't think it's a very important one. So I think we should can sacrifice a bit of the native fish-scripting experience in exchange for playing nicer with other shells. I think the latter is more important because scripting is not our focus, the way I see it. So maybe put our manpath at the end. In lieu of that, let's at least have `exec.rst` reference the system variant. [Linux man-pages]: https://www.kernel.org/doc/man-pages/ Closes #10376 2024-03-16 15:45:00 +00:00			`.. only:: builder_man`

			NOTE: This page documents the fish builtin ``trap``.
			To see the documentation on any non-fish versions, use ``command man trap``.

docs: Fix remaining references Fixes #5775. 2019-03-31 09:32:40 +00:00			``trap`` is a wrapper around the fish event delivery framework. It exists for backwards compatibility with POSIX shells. For other uses, it is recommended to define an :ref:`event handler <event>`.
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			`The following parameters are available:`

docs: standardise on definition lists for options Harmonizes the option listing including formatting in a similar manner to the synopsis of each entry. 2022-03-11 15:56:20 +00:00			`ARG`
			`Command to be executed on signal delivery.`
Copy doc_src to sphinx_doc_src and add a TOC 2018-12-16 21:08:41 +00:00
docs: standardise on definition lists for options Harmonizes the option listing including formatting in a similar manner to the synopsis of each entry. 2022-03-11 15:56:20 +00:00			`REASON`
			Name of the event to trap. For example, a signal like ``INT`` or ``SIGINT``, or the special symbol ``EXIT``.
Copy doc_src to sphinx_doc_src and add a TOC 2018-12-16 21:08:41 +00:00
docs: standardise on definition lists for options Harmonizes the option listing including formatting in a similar manner to the synopsis of each entry. 2022-03-11 15:56:20 +00:00			`-l or --list-signals`
			`Prints a list of signal names.`
Copy doc_src to sphinx_doc_src and add a TOC 2018-12-16 21:08:41 +00:00
docs: standardise on definition lists for options Harmonizes the option listing including formatting in a similar manner to the synopsis of each entry. 2022-03-11 15:56:20 +00:00			`-p or --print`
			`Prints all defined signal handlers.`
Copy doc_src to sphinx_doc_src and add a TOC 2018-12-16 21:08:41 +00:00
docs: omnibus cleanup Includes harmonizing the display of options and arguments, standardising terminology, using the envvar directive more broadly, adding help options to all commands that support them, simplifying some language, and tidying up multiple formatting issues. string documentation is not changed. 2022-03-11 16:18:41 +00:00			`-h or --help`
			`Displays help about using this command.`
Copy doc_src to sphinx_doc_src and add a TOC 2018-12-16 21:08:41 +00:00
docs: omnibus cleanup Includes harmonizing the display of options and arguments, standardising terminology, using the envvar directive more broadly, adding help options to all commands that support them, simplifying some language, and tidying up multiple formatting issues. string documentation is not changed. 2022-03-11 16:18:41 +00:00			`If ARG and REASON are both specified, ARG is the command to be executed when the event specified by REASON occurs (e.g., the signal is delivered).`
Copy doc_src to sphinx_doc_src and add a TOC 2018-12-16 21:08:41 +00:00
docs: omnibus cleanup Includes harmonizing the display of options and arguments, standardising terminology, using the envvar directive more broadly, adding help options to all commands that support them, simplifying some language, and tidying up multiple formatting issues. string documentation is not changed. 2022-03-11 16:18:41 +00:00			If ARG is absent (and there is a single REASON) or ``-``, each specified signal is reset to its original disposition (the value it had upon entrance to the shell). If ARG is the null string the signal specified by each REASON is ignored by the shell and by the commands it invokes.

			If ARG is not present and -p has been supplied, then the trap commands associated with each REASON are displayed. If no arguments are supplied or if only -p is given, ``trap`` prints the list of commands associated with each signal.
Copy doc_src to sphinx_doc_src and add a TOC 2018-12-16 21:08:41 +00:00
Allow trapping SIGINT and SIGTERM in scripts This teaches `--on-signal SIGINT` (and by extension `trap cmd SIGINT`) to work properly in scripts, not just interactively. Note any such function will suppress the default behavior of exiting. Do this for SIGTERM as well. 2022-05-29 00:19:38 +00:00			Signal names are case insensitive and the ``SIG`` prefix is optional. Trapping a signal will prevent fish from exiting in response to that signal.
Copy doc_src to sphinx_doc_src and add a TOC 2018-12-16 21:08:41 +00:00
docs: omnibus cleanup Includes harmonizing the display of options and arguments, standardising terminology, using the envvar directive more broadly, adding help options to all commands that support them, simplifying some language, and tidying up multiple formatting issues. string documentation is not changed. 2022-03-11 16:18:41 +00:00			`The exit status is 1 if any REASON is invalid; otherwise trap returns 0.`
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Convert \\subsection sections into rst format 2018-12-19 02:44:30 +00:00			`Example`
Fix command section separator line lengths 2019-01-03 04:10:47 +00:00			`-------`
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Switch \fish sections to rst format 2018-12-19 03:14:04 +00:00

			`::`

			`trap "status --print-stack-trace" SIGUSR1`
			`# Prints a stack trace each time the SIGUSR1 signal is sent to the shell.`