zsh-completions/zsh-completions-howto.org

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Intro

The official documentation for writing zsh completion functions is difficult to understand, and doesn't give many examples. At the time of writing this document I was able to find a few other tutorials on the web, however those tutorials only explain a small portion of the capabilities of the completion system. This document aims to cover areas not explained elsewhere, with examples, so that you can learn how to write more advanced completion functions. I do not go into all the details, but will give enough information and examples to get you up and running. If you need more details you can look it up for yourself in the official documentation.

Please make any scripts that you create publically available for others (e.g. by forking this repo and making a pull request).

Getting started

Telling zsh which function to use for completing a command

Completion functions for commands are stored in files with names beginning with an underscore _, and these files should be placed in a directory listed in the $fpath variable. You can add a directory to $fpath by adding a line like this to your ~/.zshrc file:

fpath=(~/newdir $fpath)

The first line of a completion function file can look something like this:

#compdef foobar

This tells zsh that the file contains code for completing the foobar command. This is the format that you will use most often for the first line, but you can also use the same file for completing several different functions if you want. See here for more details.

You can also use the compdef command directly (e.g. in your ~/.zshrc file) to tell zsh which function to use for completing a command like this:

> compdef _function foobar

or to use the same completions for several commands:

> compdef _function foobar goocar hoodar

or if you want to supply arguments:

> compdef '_function arg1 arg2' foobar

See here for more details.

Completing generic gnu commands

Many gnu commands have a standardized way of listing option descriptions (when the help option is used). For these commands you can use the _gnu_generic function for automatically creating completions, like this:

> compdef _gnu_generic foobar

or to use _gnu_generic with several different commands:

> compdef _gnu_generic foobar goocar hoodar

This line can be placed in your ~/.zshrc file.

Copying completions from another command

If you want a command, say cmd1, to have the same completions as another, say cmd2, which has already had completions defined for it, you can do this:

> compdef cmd1=cmd2

This can be useful for example if you have created an alias for a command to help you remember it.

Writing your own completion functions

A good way to get started is to look at some already defined completion functions. On my linux installation these are found in /usr/share/zsh/functions/Completion/Unix and /usr/share/zsh/functions/Completion/Linux and a few other subdirs.

You will notice that the _arguments function is used a lot in these files. This is a utility function that makes it easy to write simple completion functions. The _arguments function is a wrapper around the compadd builtin function. The compadd builtin is the core function used to add completion words to the command line, and control its behaviour. However, most of the time you will not need to use compadd, since there are many utility functions such as _arguments and _describe which are easier to use.

Utility functions

Here is a list of some of the utility functions that may be of use. The full list of utility functions, with full explanations, is available here. Examples of how to use these functions are given in the next section.

main utility functions for overall completion

_alternative Loop over tag labels and perform actions based on matching tag label.
_arguments Used to specify how to complete individual command line options for a command with unix style options.
_describe Used for creating simple completions consisting of single words with descriptions (but no actions). Easier to use than _arguments
_gnu_generic Can be used to complete options for commands that understand the `help' option.
_regex_arguments Creates a function for matching commandline arguments with regular expressions, and then performing actions/completions.

functions for performing complex completions

_values Used for completing arbitrary keywords (values) and their arguments, or comma separated lists of such combinations.
_combination Used to complete combinations of values, for example pairs of hostnames and usernames.
_multi_parts Used for completing multiple parts of words separately where each part is separated by some char, e.g. for completing partial filepaths: /u/i/sy -> /usr/include/sys
_sep_parts Like _multi_parts but allows different separators at different parts of the completion.

functions for completing specific types of objects

_path_files Used to complete filepaths. Take several options to control behaviour.
_files Calls _path_files with all options except -g and -/. These options depend on file-patterns style setting.
_net_interfaces Used for completing network interface names
_users Used for completing user names
_groups Used for completing group names
_options Used for completing the names of shell options.
_parameters Used for completing the names of shell parameters/variables (can restrict to those matching a pattern).

functions for handling cached completions

If you have a very large number of completions you can save them in a cache file so that the completions load quickly.

_cache_invalid indicates whether the completions cache corresponding to a given cache identifier needs rebuilding
_retrieve_cache retrieves completion information from a cache file
_store_cache store completions corresponding to a given cache identifier in a cache file

other functions

_message Used for displaying help messages in places where no completions can be generated.
_regex_words Can be used to generate arguments for the _regex_arguments command. This is easier than writing the arguments manually.
_guard Can be used in the ACTION of specifications for _arguments and similar functions to check the word being completed.

Actions

Many of the utility functions such as _arguments, _regex_arguments, _alternative and _values may include an action at the end of an option/argument specification. This action indicates how to complete the corresponding argument. The actions can take one of the following forms:

( ) Argument is required but no matches are generated for it.
(ITEM1 ITEM2) List of possible matches
((ITEM1\:DESC1 ITEM2\:DESC2)) List of possible matches, with descriptions.
->STRING Set $state to STRING and continue ($state can be checked in a case statement after the utility function call)
FUNCTION Name of a function to call for generating matches or performing some other action, e.g. _files or _message
{EVAL-STRING} Evaluate string as shell code to generate matches.
=ACTION Inserts a dummy word into completion command line without changing the point at which completion takes place.

Not all action types are available for all utility functions that use them. For example the ->STRING type is not available in the _regex_arguments or _alternative functions.

Examples

Here the non-option argument

_arguments '--help[show help]' '-?[show help]' '1:First arg:_files'

Writing completion functions using _describe

The _describe function can be used for simple completions where the order and position of the options/arguments is not important. You just need to create an array parameter to hold the options & their descriptions, and then pass the parameter name as an argument to _describe. The following example creates completion candidates -c and -d, with the descriptions.

local -a options 
options=('-c:description for -c opt' '-d:description for -d opt')
_describe 'values' options

You can use several different lists separated by a double hyphen e.g. like this:

local -a options arguments 
options=('-c:description for -c opt' '-d:description for -d opt')
arguments=('e:description for e arg' 'f:description for f arg')
_describe 'values' options -- arguments

See the official documentation for more info.

Writing completion functions using _alternative

Writing completion functions using _arguments

The _arguments function makes it easy to create completion functions. As arguments it takes special strings specifying the options & arguments to the function being completed, e.g. like this:

_arguments '--help[show help]' '-?[show help]' '1:First arg:_files'

This example completes the options help & -? when trying to complete a hyphen, which will both be listed together with the same description in this case. The first non-option argument is completed using the _files function which completes file/directories.

There are a couple of tutorials on how to use _arguments here and here, so I won't cover any more here. Also have a look at the many completion functions listed here many of which use _arguments. The full documentation for _arguments is available here.

Writing completion functions using _regex_arguments and _regex_words

Patterns

Testing & debugging

To reload a completion function:

> unfunction _func
> autoload -U _func

The following functions can be called to obtain useful information. If the default keybindings don't work you can try pressing Alt+x and then enter the command name.

Function Default keybinding Description
_complete_help Ctrl+x h displays information about context names, tags, and completion functions used when completing at the current cursor position
_complete_help Alt+2 Ctrl+x h as above but displays even more information
_complete_debug Ctrl+x ? performs ordinary completion, but captures in a temporary file a trace of the shell commands executed by the completion system

Gotchas

Take care to use the correct type of quotes around specifications to _arguments or _regex_arguments: use double quotes if there is a parameter that needs to be expanded in the specification, single quotes otherwise.

Putting it all together

Other resources

Here is a nicely formatted short tutorial showing basic usage of the _arguments function, and here is a slightly more advanced tutorial using the _arguments function. Here is the zshcompsys man page.