complgen allows you to generate completion scripts for all major shells from a single, concise EBNF-like grammar. It compiles the grammar down to a standalone bash/fish/zsh shell script that can be distributed on its own. As a separate use case, it can also produce completions directly on stdout, which is meant to be used in interactive shells (see below).

complgen takes flags to complete from grammar files. Ideally, the grammar files are meant to be developed and versioned along with the completed command line tool to avoid version mismatches. There's nothing stopping you however from writing the grammar file yourself, optionally tailoring it for your most-frequent use cases, sort of like shell aliases on steroids.

There are two ways to use complgen:

This mode is most useful if you're a CLI tool author and want to ship shell completions in your installation package.

$ complgen aot --bash-script grep.bash usage/small.usage
$ bash
$$ source grep.bash
$$ grep --color <TAB>
always auto never

Note: ZSH also supports automatic loading of completion scripts. It is enough to place the generated script at one of directories listed in the $fpath variable.

This mode is useful if you're command line user and want to improve the CLI experience on your machine by either implementing a missing autocompletion for a specific CLI tool, or override the default one with a one better tailored for your needs and usage patterns. Or you simply want to iterate quickly on a .usage file before you compile it to a shell script.

$ complgen jit usage/small.usage bash -- --color
always
auto
never

The just-in-time mode is intended to be further integrated with shells so that it provides completions directly from grammars, bypassing compilation and sourceing completion shell script files.

Note that it is assummed the .usage file stem is the same as the completed command name, so to complete grep command, its grammar needs to land in grep.usage.

Note: This assumes you have bash-completion OS-level package installed and it's been sourced! It often boils down to apt install bash-completion; source /etc/bash_completion or brew install bash-completion; source /opt/homebrew/etc/profile.d/bash_completion.sh, depending on your OS. Without this package, scripts generated by complgen are not able to correctly process command lines containing characters like =, :, @, or any other from $COMP_WORDBREAKS.

See also https://github.com/git/git/commit/da48616f1df51ff43acc64cdf8966f7b72142a11

Assumming your .usage files are stored in the ~/.config/complgen directory, add this to your ~/.bashrc:

for path in ~/.config/complgen/*.usage; do
    stem=$(basename "$path" .usage)
    eval "
_complgen_jit_$stem () {
    local words cword
    _get_comp_words_by_ref -n \"\$COMP_WORDBREAKS\" words cword
    local prefix="\${words[\$cword]}"
    local bash_code=\"\$(complgen jit \"{usage_files_dir}/${stem}.usage\" bash --comp-wordbreaks=\"\$COMP_WORDBREAKS\" --prefix=\"\$prefix\" -- \"\${words[@]:1:\$cword-1}\")\"
    eval \"\$bash_code\"
    return 0
}
"
    complete -o nospace -F _complgen_jit_$stem "$stem"
    unset stem
done

Assumming your .usage files are stored in the ~/.config/complgen directory, add this to your ~/.config/fish/config.fish:

function _complgen_jit
    set --local COMP_LINE (commandline --cut-at-cursor)
    set --local COMP_WORDS
    echo $COMP_LINE | read --tokenize --array COMP_WORDS
    if string match --quiet --regex '.*\s$' $COMP_LINE
        set COMP_CWORD (math (count $COMP_WORDS)   1)
    else
        set COMP_CWORD (count $COMP_WORDS)
    end
    set --local usage_file_path $argv[1]
    set --local prefix $COMP_WORDS[$COMP_CWORD]
    set --local last (math $COMP_CWORD - 1)
    if test $last -lt 2
        set words
    else
        set words $COMP_WORDS[2..$last]
    end
    complgen jit $usage_file_path fish --prefix="$prefix" -- $words | source -
    __complgen_jit "$prefix"
end

for path in ~/.config/complgen/*.usage
    set --local stem (basename $path .usage)
    complete --command $stem --no-files --arguments "(_complgen_jit ~/.config/complgen/$basename.usage)"
end

Assumming your .usage files are stored in the ~/.config/complgen directory, add this to your ~/.zshrc:

_complgen_jit () {
    local stem=$1
    local -a w=("${(@)words[2,$CURRENT-1]}")
    local zsh_code=$(complgen jit ~/.config/complgen/${stem}.usage zsh --prefix="$PREFIX" -- "${w[@]}")
    eval $zsh_code
    return 0
}

for f in $HOME/.config/complgen/*.usage(N); do
    local stem=$f:t:r
    compdef "_complgen_jit $stem" $stem
done

cargo install --git https://github.com/adaszko/complgen complgen

See the examples subdirectory for simple examples and usage subdirectory for more involved ones.

Try piping through the scrape subcommand to quickly generate grammar skeleton that can be tweaked further, e.g.:

$ grep --help | complgen scrape
 | (-E | --extended-regexp) "PATTERNS are extended regular expressions"
 | (-F | --fixed-strings) "PATTERNS are strings"
 | (-G | --basic-regexp) "PATTERNS are basic regular expressions"
[...]

The grammar is based on compleat's one.

A grammar is a series of lines terminated by a semicolon (;). Each line either represents a single variant of invoking the completed command or is a nonterminal definition.

• a b matches a followed by b.
• a b | c matches either a b or c (IOW: sequence binds stronger than alternative).
• [a] matches zero or one occurrences of a.
• a... matches one or more occurrences of a
• [a]... matches zero or more occurrences of a.

Use parentheses to group patterns:

• a (b | c) matches a followed by either b or c.
• (a | b) ... matches a or b followed by any number of additional a or b.

There's a couple of predefined nonterminals that are handled specially by complgen:

The reason there's no predefined <FILE> nonterminal is that it would work only for files from the current directory which is too specific to be generally useful.

These nonterminals can still be defined in the grammar in the usual way (<PATH> ::= ...), in which case their predefined meaning gets overriden.

If a literal is immediately followed with a quoted string, it's going to appear as a hint to the user at completion time. E.g. the grammar:

grep --extended-regexp "PATTERNS are extended regular expressions" | --exclude  (skip files that match GLOB)

results in something like this under fish (and zsh):

fish> grep --ex<TAB>
--exclude  (skip files that match GLOB)  --extended-regexp  (PATTERNS are extended regular expressions)

Note that bash does not support showing descriptions.

It is possible to use entire shell commands as a source of completions:

cargo {{{ rustup toolchain list | cut -d' ' -f1 | sed 's/^/ /' }}};

The stdout of the pipeline above will be automatically filtered by the shell based on the prefix entered so far.

Sometimes, it's more efficient to take into account the entered prefix in the shell command itself. For all three shells (bash, fish, zsh), it's available in the $1 variable:

cargo {{{ rustup toolchain list | cut -d' ' -f1 | grep "^$1" | sed 's/^/ /' }}};

Note that in general, it's best to leave the filtering up to the executing shell since it may be configured to perform some non-standard filtering. zsh for example is capable of expanding /u/l/b to /usr/local/bin.

Externals commands are also assumed to produce descriptions similar to those described in the section above. Their expected stdout format is a sequence of lines of the form

COMPLETION\tDESCRIPTION

For fish and zsh, the DESCRIPTION part will be presented to the user. Under bash, only the COMPLETION part will be visible. All external commands nonetheless need to take care as to not produce superfluous \t characters that may confuse the resulting shell scripts.

In order to make use of shell-specific completion functions, complgen supports a mechanism that allows for picking a specific nonterminal expansion based on the target shell. To use an example: all shells are able to complete a user on the system, although each has a different function for it. We unify their interface under the nonterminal <USER> using few nonterminal@shell definitions:

cmd <USER>;
<USER@bash> ::= {{{ compgen -A user "$1" | sort | uniq }}}; # bash produces duplicates for some reason
<USER@fish> ::= {{{ __fish_complete_users "$1" }}};
<USER@zsh> ::= {{{ _users }}};

It's possible to match not only entire words, but also within words themselves, using the same grammar syntax as for matching entire words. In that sense, it all fractally works on subwords too (there are limitations on {{{ ... }}} usage though). The most common application of that general mechanism is to handle equal sign arguments (--option=ARGUMENT):

grep --color=(always | never | auto);

Note however that equal sign arguments aren't some special case within complgen — the same mechanism works for more complicated things, e.g.:

strace -e <EXPR>;
<EXPR> ::= [<qualifier>=][!]<value>[,<value>]...;
<qualifier> ::= trace | read | write | fault;
<value> ::= %file | file | all;

The above grammar was pulled straight out of strace man page.

This feature is currently work-in-progress, here's where it's supported already:

If you do git <TAB> in most shells you're presented with a list of git subcommands. Even though git accepts a bunch of global options (--help, --version, etc.), they don't show up there (sic!). That's a special mechanism intended for reducing clutter. Under complgen, the same effect is achieved via a construction called fallbacks, which are represented in the grammar as the double bar operator (||):

mygit (<SUBCOMMAND> || <OPTION>);
<SUBCOMMAND> ::= fetch | add | commit | push;
<OPTION> ::= --help | --version;

With the grammar above, git <TAB> will offer to complete only subcommands. For git --<TAB> OTOH, complgen will offer to complete options.

|| has the lowest priority of all operators, so the grammar above might have been written without any use of <NONTERMINALS>. They're there only for readability sake.

There are few general rules governing whether to append a space to a completion:

• A space is appended if the completion corresponds to an entire literal from the .usage file, e.g. for the grammar cmd --help; and the command line cmd <TAB>, it completes to cmd --help<SPACE>.

• A trailing space isn't appended if the literal is a part of a subword and the entire subword hasn't been completed yet, e.g. for the grammar cmd --color=(auto|always); and the command line cmd --col<TAB>, it completes to cmd --color= (no trailing space).

There are exceptions:

• Under Fish, if your completion contains one of the special characters, fish won't insert the trailing space. See also Not adding space after dot at completion time · Issue #6928.

• {{{ ... }}} is only allowed at tail positions, where it doesn't lead to matching against an arbitrary output of an external command:

  • OK: cmd {{{ echo foo }}} {{{ echo bar }}};

  • ERROR: cmd ({{{ echo foo }}} | {{{ echo bar }}});

    We'd have to match against the output of {{{ echo foo }}} at compilation time to determine which branch to take, which is impossible to do in general as echo foo might as well have been an arbitrary shell command.

  • OK: cmd (foo | {{{ echo bar }}});

  • ERROR: cmd ({{{ echo foo }}} || {{{ echo bar }}});

    Reason: Same as for | above.

  • OK: cmd (foo || {{{ echo bar }}});

  • OK: cmd [{{{ echo foo }}}] foo;

    The entire commands always gets tokenized into shell words as a first step, so it's possible to tell where the output of {{{ echo foo }}} ends. Note that two foo completions are produced here, then they're deduplicated, and finally, only one foo is offered to the user.

  • OK: cmd {{{ echo foo }}}... foo baz;

• Within subwords, {{{ ... }}} is only allowed at tail position, where it doesn't lead to matching against an arbitrary output of an external command:

  • ERROR: {{{ git tag }}}..{{{ git tag }}}

    Impossible to guess at compilation time where the output of the first {{{ git tag }} ends and our .. begins.

  • OK: --option={{{ echo foo }}}

  • ERROR: {{{ echo foo }}}{{{ echo bar }}}

    Impossible to guess at compilation time where the output of the first {{{ echo foo }} ends the second {{{ echo bar }}} begins.

• The limitations above also apply to predefined nonterminals (<PATH>, <DIRECTORY>, etc.) since they're internally implemented as external commands.

• Bash requires bash-completion OS package to be installed because completion scripts produced by complgen, call shell functions from that package at completion time. This is necessary to work around Bash's default behavior of breaking shell words on any character present in the $COMP_WORDBREAKS environment variable.

• Non-regular grammars aren't completed 100% precisely. For instance, in case of find(1), complgen will still suggest ) even in cases when all ( have already been properly closed before the cursor.

Best way is to watch GitHub releases.