Writing your own completions

To specify a completion, use the complete command. complete takes as a parameter the name of the command to specify a completion for. For example, to add a completion for the program myprog, one would start the completion command with complete -c myprog ...

To provide a list of possible completions for myprog, use the -a switch. If myprog accepts the arguments start and stop, this can be specified as complete -c myprog -a 'start stop'. The argument to the -a switch is always a single string. At completion time, it will be tokenized on spaces and tabs, and variable expansion, command substitution and other forms of parameter expansion will take place.

fish has a special syntax to support specifying switches accepted by a command. The switches -s, -l and -o are used to specify a short switch (single character, such as -l), a gnu style long switch (such as --color) and an old-style long switch (like -shuffle), respectively. If the command 'myprog' has an option '-o' which can also be written as --output, and which can take an additional value of either 'yes' or 'no', this can be specified by writing:

complete -c myprog -s o -l output -a "yes no"

There are also special switches for specifying that a switch requires an argument, to disable filename completion, to create completions that are only available in some combinations, etc.. For a complete description of the various switches accepted by the complete command, see the documentation for the complete builtin, or write complete --help inside the fish shell.

As a more comprehensive example, here's a commented excerpt of the completions for systemd's timedatectl:

# All subcommands that timedatectl knows - this is useful for later.
set -l commands status set-time set-timezone list-timezones set-local-rtc set-ntp

# Disable file completions for the entire command
# because it does not take files anywhere
# Note that this can be undone by using "-F".
# File completions also need to be disabled
# if you want to have more control over what files are offered
# (e.g. just directories, or just files ending in ".mp3").
complete -c timedatectl -f

# This line offers the subcommands
# -"status",
# -"set-timezone",
# -"set-time"
# -"list-timezones"
# if no subcommand has been given so far.
# The `-n`/`--condition` option takes script as a string, which it executes.
# If it returns true, the completion is offered.
# Here the condition is the `__fish_seen_subcommands_from` helper function.
# If returns true if any of the given commands is used on the commandline,
# as determined by a simple heuristic.
# For more complex uses, you can write your own function.
# See e.g. the git completions for an example.
complete -c timedatectl -n "not __fish_seen_subcommand_from $commands" \
    -a "status set-time set-timezone list-timezones"

# If the "set-timezone" subcommand is used,
# offer the output of `timedatectl list-timezones` as completions.
# Each line of output is used as a separate candidate,
# and anything after a tab is taken as the description.
# It's often useful to transform command output with `string` into that form.
complete -c timedatectl -n "__fish_seen_subcommand_from set-timezone" \
    -a "(timedatectl list-timezones)"

# Completion candidates can also be described via `-d`,
# which is useful if the description is constant.
# Try to keep these short, because that means the user gets to see more at once.
complete -c timedatectl -n "not __fish_seen_subcommand_from $commands" \
    -a "set-local-rtc" -d "Maintain RTC in local time"

# We can also limit options to certain subcommands by using conditions.
complete -c timedatectl -n "__fish_seen_subcommand_from set-local-rtc" \
    -l adjust-system-clock -d 'Synchronize system clock from the RTC'

# These are simple options that can be used everywhere.
complete -c timedatectl -s h -l help -d 'Print a short help text and exit'
complete -c timedatectl -l version -d 'Print a short version string and exit'
complete -c timedatectl -l no-pager -d 'Do not pipe output into a pager'

For examples of how to write your own complex completions, study the completions in /usr/share/fish/completions. (The exact path depends on your chosen installation prefix and may be slightly different)

Useful functions for writing completions

fish ships with several functions that are very useful when writing command specific completions. Most of these functions name begins with the string __fish_. Such functions are internal to fish and their name and interface may change in future fish versions. Still, some of them may be very useful when writing completions. A few of these functions are described here. Be aware that they may be removed or changed in future versions of fish.

Functions beginning with the string __fish_print_ print a newline separated list of strings. For example, __fish_print_filesystems prints a list of all known file systems. Functions beginning with __fish_complete_ print out a newline separated list of completions with descriptions. The description is separated from the completion by a tab character.

  • __fish_complete_directories STRING DESCRIPTION performs path completion on STRING, allowing only directories, and giving them the description DESCRIPTION.

  • __fish_complete_path STRING DESCRIPTION performs path completion on STRING, giving them the description DESCRIPTION.

  • __fish_complete_groups prints a list of all user groups with the groups members as description.

  • __fish_complete_pids prints a list of all processes IDs with the command name as description.

  • __fish_complete_suffix SUFFIX performs file completion but sorts files ending in SUFFIX first. This is useful in conjunction with complete --keep-order.

  • __fish_complete_users prints a list of all users with their full name as description.

  • __fish_print_filesystems prints a list of all known file systems. Currently, this is a static list, and not dependent on what file systems the host operating system actually understands.

  • __fish_print_hostnames prints a list of all known hostnames. This functions searches the fstab for nfs servers, ssh for known hosts and checks the /etc/hosts file.

  • __fish_print_interfaces prints a list of all known network interfaces.

  • __fish_print_packages prints a list of all installed packages. This function currently handles Debian, rpm and Gentoo packages.

Where to put completions

Completions can be defined on the commandline or in a configuration file, but they can also be automatically loaded. Fish automatically searches through any directories in the list variable $fish_complete_path, and any completions defined are automatically loaded when needed. A completion file must have a filename consisting of the name of the command to complete and the suffix .fish.

By default, Fish searches the following for completions, using the first available file that it finds:

  • A directory for end-users to keep their own completions, usually ~/.config/fish/completions (controlled by the XDG_CONFIG_HOME environment variable);

  • A directory for systems administrators to install completions for all users on the system, usually /etc/fish/completions;

  • A directory for third-party software vendors to ship their own completions for their software, usually /usr/share/fish/vendor_completions.d;

  • The completions shipped with fish, usually installed in /usr/share/fish/completions; and

  • Completions automatically generated from the operating system's manual, usually stored in ~/.local/share/fish/generated_completions.

These paths are controlled by parameters set at build, install, or run time, and may vary from the defaults listed above.

This wide search may be confusing. If you are unsure, your completions probably belong in ~/.config/fish/completions.

If you have written new completions for a common Unix command, please consider sharing your work by submitting it via the instructions in Further help and development.

If you are developing another program and would like to ship completions with your program, install them to the "vendor" completions directory. As this path may vary from system to system, the pkgconfig framework should be used to discover this path with the output of pkg-config --variable completionsdir fish.