How to customize Bash command completion?

Question

In bash, it's easy enough to set up customized completion of command arguments using the complete built-in. For example, for a hypothetical command with a synopsis of

foo --a | --b | --c

you could do

complete -W '--a --b --c' foo

You can also customize the completion you get when you press Tab at an empty prompt using complete -E, for example complete -E -W 'foo bar'. Then, pressing tab at the empty prompt would suggest only foo and bar.

How do I customize command completion at a non-empty prompt? For example, if I write f, how do I customize the completion to make it complete to foo?

(The actual case I'd like is locTAB → localc. And my brother, who prompted me to ask this, wants it with mplayer.)

Answer 1

Completion of the command (along with other things) is handled via bash readline completion. This operates at a slightly lower level than the usual "programmable completion" (which is invoked only when the command is identified, and the two special cases you identified above).

Update: the new release of bash-5.0 (Jan 2019) adds complete -I for exactly this problem.

The relevant readline commands are:

complete (TAB)
       Attempt to perform completion on the text  before  point.   Bash
       attempts completion treating the text as a variable (if the text
       begins with $), username (if the text begins with  ~),  hostname
       (if  the  text begins with @), or command (including aliases and
       functions) in turn.  If none of these produces a match, filename
       completion is attempted.

complete-command (M-!)
       Attempt  completion  on  the text before point, treating it as a
       command name.  Command completion attempts  to  match  the  text
       against   aliases,   reserved   words,  shell  functions,  shell
       builtins, and finally executable filenames, in that order.

In a similar way to the more common complete -F, some of this can be handed over to a function by using bind -x.

function _complete0 () {
    local -a _cmds
    local -A _seen
    local _path=$PATH _ii _xx _cc _cmd _short
    local _aa=( ${READLINE_LINE} )

    if [[ -f ~/.complete.d/"${_aa[0]}" && -x  ~/.complete.d/"${_aa[0]}" ]]; then
        ## user-provided hook
        _cmds=( $( ~/.complete.d/"${_aa[0]}" ) )
    elif [[ -x  ~/.complete.d/DEFAULT ]]; then
        _cmds=( $( ~/.complete.d/DEFAULT ) )
    else 
        ## compgen -c for default "command" complete 
        _cmds=( $(PATH=$_path compgen -o bashdefault -o default -c ${_aa[0]}) )  
    fi

    ## remove duplicates, cache shortest name
    _short="${_cmds[0]}"
    _cc=${#_cmds[*]} # NB removing indexes inside loop
    for (( _ii=0 ; _ii<$_cc ; _ii++ )); do
        _cmd=${_cmds[$_ii]}
        [[ -n "${_seen[$_cmd]}" ]] && unset _cmds[$_ii]
        _seen[$_cmd]+=1
        (( ${#_short} > ${#_cmd} )) && _short="$_cmd"
    done
    _cmds=( "${_cmds[@]}" )  ## recompute contiguous index

    ## find common prefix
    declare -a _prefix=()
    for (( _xx=0; _xx<${#_short}; _xx++ )); do
        _prev=${_cmds[0]}
        for (( _ii=0 ; _ii<${#_cmds[*]} ; _ii++ )); do
            _cmd=${_cmds[$_ii]}
             [[ "${_cmd:$_xx:1}" != "${_prev:$_xx:1}" ]] && break
            _prev=$_cmd
        done
        [[ $_ii -eq ${#_cmds[*]} ]] && _prefix[$_xx]="${_cmd:$_xx:1}"
    done
    printf -v _short "%s" "${_prefix[@]}"  # flatten 

    ## emulate completion list of matches
    if [[ ${#_cmds[*]} -gt 1 ]]; then
        for (( _ii=0 ; _ii<${#_cmds[*]} ; _ii++ )); do
            _cmd=${_cmds[$_ii]}
            [[ -n "${_seen[$_cmds]}" ]] && printf "%-12s " "$_cmd" 
        done | sort | fmt -w $((COLUMNS-8)) | column -tx
        # fill in shortest match (prefix)
        printf -v READLINE_LINE "%s" "$_short"
        READLINE_POINT=${#READLINE_LINE}  
    fi
    ## exactly one match
    if [[ ${#_cmds[*]} -eq 1 ]]; then
        _aa[0]="${_cmds[0]}"
        printf -v READLINE_LINE "%s " "${_aa[@]}"
        READLINE_POINT=${#READLINE_LINE}  
    else
        : # nop
    fi
}

bind -x '"\C-i":_complete0'

This enables your own per-command or prefix string hooks in ~/.complete.d/. E.g. if you create an executable ~/.complete.d/loc with:

#!/bin/bash
echo localc

This will do (roughly) what you expect.

The function above goes to some lengths to emulate the normal bash command completion behaviour, though it is imperfect (particularly the dubious sort | fmt | column carry-on to display a list of matches).

However, a non-trivial issue with this it can only use a function to replace the binding to the main complete function (invoked with TAB by default).

This approach would work well with a different key-binding used for just custom command completion, but it simply does not implement the full completion logic after that (e.g. later words in the command line). Doing so would require parsing the command line, dealing with cursor position, and other tricky things that probably should not be considered in a shell script...

Answer 2

I don't know if I unterstood your need for this...
This would imply that your bash only knows one command beginning with f.
A basic idea of completion is: if it's ambiguous, print the possiblities.
So you could set your PATH to a directory only containing this one command and disable all bash builtins to get this work.

Anyhow, I can give you also a kind of workaround:

alias _='true &&'
complete -W foo _

So if you type _ <Tab> it will complete to _ foo which executes foo.

But nethertheless the alias f='foo' would be much easier.

Answer 3

Simple answer for you would be to

$ cd into /etc/bash_completion.d
$ ls

just the basic outputs

autoconf       gpg2               ntpdate           shadow
automake       gzip               open-iscsi        smartctl
bash-builtins  iconv              openssl           sqlite3
bind-utils     iftop              perl              ssh
brctl          ifupdown           pkg-config        strace
bzip2          info               pm-utils          subscription-manager
chkconfig      ipmitool           postfix           tar
configure      iproute2           procps            tcpdump
coreutils      iptables           python            util-linux
cpio           lsof               quota-tools       wireless-tools
crontab        lvm                redefine_filedir  xmllint
cryptsetup     lzma               rfkill            xmlwf
dd             make               rpm               xz
dhclient       man                rsync             yum.bash
e2fsprogs      mdadm              scl.bash          yum-utils.bash
findutils      module-init-tools  service
getent         net-tools          sh

just add your desired program to auto complete to bash completion

Answer 4

Run the below command to find where mplayer binary is installed:

which mplayer

OR use the path to the mplayer binary if you aleady know it, in the below command:

ln -s /path/to/mplayer /bin/mplayer

Ideally anything you type is searched in all directories specified in the $PATH variable.