What's the use of complete command?

complete is a bash builtin function. So there is not a binary on the system. It handles how commands will be completed when pressing tab.

Example: if you type:

user@host:~$ pidof <tab><tab>

...a list is appearing with all possible values for this command. In this case it means all running processes. See the output of the complete function:

user@host:~$ complete | grep pidof
complete -F _pgrep pidof

This means that the function _pgrep (-F) is executed when tabbing the command pidof. The definition of this function is in /etc/bash_completion.d/procps.

Another example: if you type:

user@host:~$ cd /usr/<tab><tab>
bin/     games/   include/ lib/     lib32/   local/   sbin/    share/   src/

...you see the list of folders you can cd to under /usr/. Which function is executed? greping the complete function (as above) tells us it's the funtction _cd in /etc/bash_completion.

Do it yourself: You have a program/script called /bin/myprog and you want that if you execute it as follows

user@host:~$ myprog /home/user/<tab><tab>

...it should only list folders, not files. So extend your bash completion with the following command:

user@host:~$ complete -F _cd myprog

That's it. You can also write own functions to complete custom things, for example complete only specific files or numbers or lists of static values...

complete is a bash command used to perform the auto-complete action when the user hit the TAB key in a terminal.

Calling just complete will list all the functions registered for auto-completion of commands or services options.

From the bash man pages:

complete: complete [-abcdefgjksuv] [-pr] [-DE] [-o option] [-A action] [-G globpat]
          [-W wordlist]  [-F function] [-C command] [-X filterpat] [-P prefix]
          [-S suffix] [name ...]
    Specify how arguments are to be completed by Readline.

    For each NAME, specify how arguments are to be completed.  If no options
    are supplied, existing completion specifications are printed in a way that
    allows them to be reused as input.

    Options:
      -p    print existing completion specifications in a reusable format
      -r    remove a completion specification for each NAME, or, if no
        NAMEs are supplied, all completion specifications
      -D    apply the completions and actions as the default for commands
        without any specific completion defined
      -E    apply the completions and actions to "empty" commands --
        completion attempted on a blank line

    When completion is attempted, the actions are applied in the order the
    uppercase-letter options are listed above.  The -D option takes
    precedence over -E.

    Exit Status:
    Returns success unless an invalid option is supplied or an error occurs.

Check /usr/share/bash-completion/bash_completion to see the default completions that come with bash.

Visit http://www.linuxjournal.com/content/more-using-bash-complete-command for a full tutorial about this command.