How to make autocomplete for any bash commands

Stolen and awkwardly translated from here, with a few additions.

Simple version — static list

Suppose there is a command dothis. Let’s create a file dothis-completion.bash:

#/usr/bin/env bash
complete -W "now tomorrow never" dothis

Run . ./dothis-completion.bash, and now pressing Tab will produce:

$ dothis <tab><tab>
never     now       tomorrow

$ dothis n<tab><tab>
never now

The -W flag specifies to take a word list. -F func — call a function and use its output as the completion list. -C cmd — same, but execute a command.

-A action — autocomplete with:

• alias aliases (= -a).
• arrayvar array variable names.
• binding readline bindings.
• builtin builtin command names (= -b).
• command command names (= -c).
• directory directories (= -d).
• disabled disabled builtin commands.
• enabled enabled builtin commands.
• export exported variables (= -e).
• file files (= -f).
• function functions.
• group groups (= -g).
• helptopic built-in help topics.
• hostname hosts (host file is specified by the HOSTFILE variable).
• job jobs (= -j).
• keyword shell keywords (= -k).
• running running tasks.
• service services (?) (= -s).
• setopt valid arguments for the shell -o option.
• shopt options for shopt.
• signal signals.
• stopped stopped jobs.
• user users (logins) (= -u).
• variable variables.

For example, complete -A directory dothis will complete with directory names.

Dynamic completion

Let’s change the script:

#/usr/bin/env bash
_dothis_completions()
{
  COMPREPLY+=("now")
  COMPREPLY+=("tomorrow")
  COMPREPLY+=("never")
}

complete -F _dothis_completions dothis

This produces the same result as the original version — three completion options. However, if we type dothis ne<tab><tab>, we get all three options instead of the expected single never.

Let’s fix this. In the function called for autocompletion, the following variables are available:

• COMP_WORDS array of words entered after the program name;
• COMP_CWORD index of the word in the previous array where the cursor currently is;
• COMP_LINE the current command line.

Additionally, the built-in compgen function, which accepts the same flags as complete, can produce a completion list filtered by the beginning of a word. For example, compgen -W "now tomorrow never" n outputs now and never.

Therefore, the basic functionality can be obtained like this:

#/usr/bin/env bash
_dothis_completions()
{
  COMPREPLY=($(compgen -W "now tomorrow never" "${COMP_WORDS[1]}"))
}

complete -F _dothis_completions dothis

Example of completing the -u flag with user names:

_dothis_completions()
{
  local cur prev opts
  COMPREPLY=()
  cur="${COMP_WORDS[COMP_CWORD]}"
  prev="${COMP_WORDS[COMP_CWORD-1]}"

  if [[ ${prev} == -u ]] ; then
    COMPREPLY=( $(compgen -u -- ${cur}) )
    return 0
  fi
}

All variables available in autocompletion:

• COMP_CWORD Index of the current word in ${COMP_WORDS}
• COMP_LINE The entire command line
• COMP_POINT Cursor position relative to the beginning of the command line. If the cursor is at the very end, this equals ${#COMP_LINE}.
• COMP_WORDBREAKS Set of characters considered word delimiters for autocompletion (do not modify it — it’s a global variable!).
• COMP_WORDS Array of words from the current ${COMP_LINE}.
• COMPREPLY Array of words for autocompletion — the output of the completion script.

#bash#cli