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.. _cmd-switch:
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switch - conditionally execute a block of commands
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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 ::
switch VALUE; [case [GLOB ...]; [COMMANDS ...]; ...] end
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Description
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-----------
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`` switch `` performs one of several blocks of commands, depending on whether a specified value equals one of several globbed values. `` case `` is used together with the `` switch `` statement in order to determine which block should be executed.
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Each `` case `` command is given one or more parameters. The first `` case `` command with a parameter that matches the string specified in the switch command will be evaluated. `` case `` parameters may contain globs. These need to be escaped or quoted in order to avoid regular glob expansion using filenames.
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Note that fish does not fall through on case statements. Only the first matching case is executed.
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Note that :doc: `break <break>` cannot be used to exit a case/switch block early like in other languages. It can only be used in loops.
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Note that command substitutions in a case statement will be evaluated even if its body is not taken. All substitutions, including command substitutions, must be performed before the value can be compared against the parameter.
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Example
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-------
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If the variable :envvar: `animal` contains the name of an animal, the following code would attempt to classify it:
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::
switch $animal
case cat
echo evil
case wolf dog human moose dolphin whale
echo mammal
case duck goose albatross
echo bird
case shark trout stingray
echo fish
case '*'
echo I have no idea what a $animal is
end
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If the above code was run with `` $animal `` set to `` whale `` , the output
would be `` mammal `` .
