This document attempts to explain the basic styles and patterns that are used in the bash completion. New code should try to conform to these standards so that it is as easy to maintain as existing code. Of course every rule has an exception, but it’s important to know the rules nonetheless!

This is particularly directed at people new to the bash completion codebase, who are in the process of getting their code reviewed. Before getting a review, please read over this document and make sure your code conforms to the recommendations here.

Indent step should be 4 spaces, no tabs.

Avoid "fancy" globbing in case labels, just use traditional style when possible. For example, do "--foo|--bar)" instead of "--@(foo|bar))". Rationale: the former is easier to read, often easier to grep, and doesn’t confuse editors as bad as the latter, and is concise enough.

Always use [[ ]] instead of [ ]. Rationale: the former is less error prone, more featureful, and slightly faster.

Try to wrap lines at 79 characters. Never go past this limit, unless you absolutely need to (example: a long sed regular expression, or the like). This also holds true for the documentation and the testsuite. Other files, like ChangeLog, or COPYING, are exempt from this rule.

When you need to do some code substitution in your completion script, you MUST use the $(…) construct, rather than the `…`. The former is preferable because anyone, with any keyboard layout, is able to type it. Backticks aren’t always available, without doing strange key combinations.

As a rule of thumb, do not use "complete -o filenames". Doing it makes it take effect for all completions from the affected function, which may break things if some completions from the function must not be escaped as filenames. Instead, use "compopt -o filenames" to turn on "-o filenames" behavior dynamically when returning completions that need that kind of processing (e.g. file and command names). The _filedir and _filedir_xspec helpers do this automatically whenever they return some completions.

The above is functionally a shorthand for:

if [[ ${#COMPREPLY[@]} -eq 1 && ${COMPREPLY[0]} == *= ]]; then compopt -o nospace fi

It is used to ensure that long options' name won't get a space
appended after the equal sign.  Calling compopt -o nospace makes sense
in case completion actually occurs: when only one completion is
available in COMPREPLY.

$split && return

Should be used in completions using the -s flag of _init_completion, or other similar cases where _split_longopt has been invoked, after $prev has been managed but before $cur is considered. If $cur of the form --foo=bar was split into $prev=--foo and $cur=bar and the $prev block did not process the option argument completion, it makes sense to return immediately after the $prev block because --foo obviously takes an argument and the remainder of the completion function is unlikely to provide meaningful results for the required argument. Think of this as a catch-all for unknown options requiring an argument.

Note that even when using this, options that are known to require an argument but for which we don’t have argument completion should be explicitly handled (non-completed) in the $prev handling block because --foo=bar options can often be written without the equals sign, and in that case the long option splitting does not occur.

When dealing with numeric data, take advantage of arithmetic evaluation. In essence, use (( … )) whenever it can replace [[ … ]] because the syntax is more readable; no need for $-prefixes, numeric comparison etc operators are more familiar and easier on the eye.

Array subscripts are arithmetic expressions, take advantage of that. E.g. write ${foo[bar]}, not ${foo[$bar]}, and similarly ${foo[bar+1]} vs ${foo[bar+1]} or ${foo[$bar+1]}, ${foo[--i]} vs ${foo[--i]}.

Use i, j, k for loop-local indices; n and m for lengths; some other descriptive name typically based on array name but in singular when looping over actual values. If an index or value is to be accessed later on instead of being just locally for looping, use a more descriptive and specific name for it.

The bash-completion package contains an automated test suite. Running the tests should help verifying that bash-completion works as expected. The tests are also very helpful in uncovering software regressions at an early stage.

The test suite is written in Python, using pytest and pexpect.

For the Python part, all of it is formatted using Black, and we also run Flake8 on it.

Installing dependencies should be easy using your local package manager or pip. Python 3.4 or newer is required, and the rest of the Python package dependencies are specified in the test/requirements.txt file. If using pip, this file can be fed directly to it, e.g. like:

pip install -r test/requirements.txt

sudo apt-get install python3-pytest python3-pexpect

sudo yum install python3-pytest python3-pexpect

Tests are in the t/ subdirectory, with t/test_\*.py being completion tests, and t/unit/test_unit_\*.py unit tests.

Tests are run by calling pytest on the desired test directories or individual files, for example in the project root directory:

pytest test/t

See test/docker/docker-script.sh for how and what we run and test in CI.