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.. _cmd-trap:
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trap - perform an action when the shell receives a signal
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=========================================================
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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
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.. synopsis ::
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trap [OPTIONS] [[ARG] REASON ... ]
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Description
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-----------
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`` 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:
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*ARG*
Command to be executed on signal delivery.
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*REASON*
Name of the event to trap. For example, a signal like `` INT `` or `` SIGINT `` , or the special symbol `` EXIT `` .
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**-l** or **--list-signals**
Prints a list of signal names.
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**-p** or **--print**
Prints all defined signal handlers.
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**-h** or **--help**
Displays help about using this command.
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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).
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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.
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Signal names are case insensitive and the `` SIG `` prefix is optional. Trapping a signal will prevent fish from exiting in response to that signal.
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The exit status is 1 if any *REASON* is invalid; otherwise trap returns 0.
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Example
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-------
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::
trap "status --print-stack-trace" SIGUSR1
# Prints a stack trace each time the SIGUSR1 signal is sent to the shell.
