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In bash, it's easy enough to set up customized completion of command arguments using the complete built-in. For example, if, 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, e.g., 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? E.g., if I'm sitting at:

anthony@Zia:~$ f

how do customize completion so pressing tab would always complete to foo?

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

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Have you considered simply aliasing loc to localc? I suggest alternatives because after quite some time digging and searching I have not found a way to customize bash completion this way. It may not be possible. –  jw013 Aug 5 '14 at 20:47
@jw013 Yeah, I looked for a while and couldn't find anything either. I'm half expecting someone to suggest switching to zsh, too. At least if zsh does it, I can rest comfortably knowing the next version of bash will as well. 😀 –  derobert Aug 5 '14 at 20:51
you have to press TAB twice and it will complete commands. –  Floyd Oct 10 '14 at 15:30
Won't that break all the other completions though? I mean, even if it is possible, you would no longer be able to get locate, locale, lockfile or any of the other expansions of loc. Perhaps a better approach would be to map a different key to that specific completion. –  terdon Oct 28 '14 at 18:00
can you give an example of such context (that would lead to the desired effect to be loc<TAB>->localc )? –  damienfrancois Nov 4 '14 at 14:29

4 Answers 4

up vote 1 down vote accepted

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).

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 ) )
        ## compgen -c for default "command" complete 
        _cmds=( $(PATH=$_path compgen -o bashdefault -o default -c ${_aa[0]}) )  

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

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

    ## emulate completion list of matches
    if [[ ${#_cmds[*]} -gt 1 ]]; then
        for (( _ii=0 ; _ii<${#_cmds[*]} ; _ii++ )); do
            [[ -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"
    ## exactly one match
    if [[ ${#_cmds[*]} -eq 1 ]]; then
        printf -v READLINE_LINE "%s " "${_aa[@]}"
        : # nop

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:

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...

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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.

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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

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Those files have shell script in them, some fairly complex if I remember correctly. Also, they only complete arguments once you've already typed the command... They don't complete the command. –  derobert Sep 25 '14 at 3:41
I understand what you mean. When you can take a look at this doc: debian-administration.org/article/316/… –  unixmiah Sep 25 '14 at 3:53

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.

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Hi, Mathew, welcome to Unix & Linux. I think the OP's question is a little more complicated than the answer you're suggesting. I presume mplayer is already in their $PATH and they want something like mp<tab> to produce mplayer, instead of all the binaries that begin with mp (e.g., mpage mpcd mpartition mplayer etc.) –  drs Oct 15 '14 at 14:48

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