A Programmable Completion Example

The most common way to obtain additional completion functionality beyond the default actions complete and compgen provide is to use a shell function and bind it to a particular command using complete -F.

The following function provides completions for the cd builtin. It is a reasonably good example of what shell functions must do when used for completion. This function uses the word passsed as $2 to determine the directory name to complete. You can also use the COMP_WORDS array variable; the current word is indexed by the COMP_CWORD variable.

The function relies on the complete and compgen builtins to do much of the work, adding only the things that the Bash cd does beyond accepting basic directory names: tilde expansion (see Tilde Expansion), searching directories in $CDPATH, which is described above (see Bourne Shell Builtins), and basic support for the cdable_vars shell option (see The Shopt Builtin). _comp_cd modifies the value of IFS so that it contains only a newline to accommodate file names containing spaces and tabs – compgen prints the possible completions it generates one per line.

Possible completions go into the COMPREPLY array variable, one completion per array element. The programmable completion system retrieves the completions from there when the function returns.

# A completion function for the cd builtin
# based on the cd completion function from the bash_completion package
_comp_cd()
{
    local IFS=$' \t\n'    # normalize IFS
    local cur _skipdot _cdpath
    local i j k

    # Tilde expansion, with side effect of expanding tilde to full pathname
    case "$2" in
    \~*)    eval cur="$2" ;;
    *)      cur=$2 ;;
    esac

    # no cdpath or absolute pathname -- straight directory completion
    if [[ -z "${CDPATH:-}" ]] || [[ "$cur" == @(./*|../*|/*) ]]; then
        # compgen prints paths one per line; could also use while loop
        IFS=$'\n'
        COMPREPLY=( $(compgen -d -- "$cur") )
        IFS=$' \t\n'
    # CDPATH+directories in the current directory if not in CDPATH
    else
        IFS=$'\n'
        _skipdot=false
        # preprocess CDPATH to convert null directory names to .
        _cdpath=${CDPATH/#:/.:}
        _cdpath=${_cdpath//::/:.:}
        _cdpath=${_cdpath/%:/:.}
        for i in ${_cdpath//:/$'\n'}; do
            if [[ $i -ef . ]]; then _skipdot=true; fi
            k="${#COMPREPLY[@]}"
            for j in $( compgen -d -- "$i/$cur" ); do
                COMPREPLY[k++]=${j#$i/}        # cut off directory
            done
        done
        $_skipdot || COMPREPLY+=( $(compgen -d -- "$cur") )
        IFS=$' \t\n'
    fi

    # variable names if appropriate shell option set and no completions
    if shopt -q cdable_vars && [[ ${#COMPREPLY[@]} -eq 0 ]]; then
        COMPREPLY=( $(compgen -v -- "$cur") )
    fi

    return 0
}

We install the completion function using the -F option to complete:

# Tell readline to quote appropriate and append slashes to directories;
# use the bash default completion for other arguments
complete -o filenames -o nospace -o bashdefault -F _comp_cd cd

Since we’d like Bash and Readline to take care of some of the other details for us, we use several other options to tell Bash and Readline what to do. The -o filenames option tells Readline that the possible completions should be treated as filenames, and quoted appropriately. That option will also cause Readline to append a slash to filenames it can determine are directories (which is why we might want to extend _comp_cd to append a slash if we’re using directories found via CDPATH: Readline can’t tell those completions are directories). The -o nospace option tells Readline to not append a space character to the directory name, in case we want to append to it. The -o bashdefault option brings in the rest of the "Bash default" completions – possible completion that Bash adds to the default Readline set. These include things like command name completion, variable completion for words beginning with ‘{’, completions containing pathname expansion patterns (see Filename Expansion), and so on.

Once installed using complete, _comp_cd will be called every time we attempt word completion for a cd command.

Many more examples – an extensive collection of completions for most of the common GNU, Unix, and Linux commands – are available as part of the bash_completion project. This is installed by default on many GNU/Linux distributions. Originally written by Ian Macdonald, the project now lives at http://bash-completion.alioth.debian.org/. There are ports for other systems such as Solaris and Mac OS X.

An older version of the bash_completion package is distributed with bash in the examples/complete subdirectory.