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Bash Completion

TO get a list of the current completion function calls...
  complete -p

Tutorials
  https://debian-administration.org/article/316/An_introduction_to_bash_completion_part_1


COMP_WORDS  array containing all individual words in the current command line.
COMP_CWORD  index of the word containing the current cursor position.
COMPREPLY   output array from which Bash reads the possible completions.

# reduce the list of words that could be completed
compgen -W "list of words to select from" -- "${cur}

The Standard installation comes from
  https://github.com/scop/bash-completion/blob/master/bash_completion

Completion scripts are placed in /usr/share/bash-completion/completions
As per the command: pkg-config --variable=completionsdir bash-completion
These are loaded automatically only when needed.

OR in user personal directory  ~/.local/share/bash-completion 
which is set by $BASH_COMPLETION_USER_DIR, or $XDG_DATA_HOME/bash-completion

Files in the old backward compatibility directory: /etc/bash_completion.d
are pre-loaded! As pre command: pkg-config --variable=compatdir bash-completion


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Basic Example of implementing BASH Completion.

Based on...
  https://askubuntu.com/questions/68175/#483149

Example Script you want completion for...
  script [-options] cmd1|cmd2|cmd3
  script --help-list-options    # list defined options
  script --help-list-commands   # list defined commands


=======8<--------CUT HERE----------axes/crowbars permitted---------------
_script() {

  COMPREPLY=()
  cur="${COMP_WORDS[COMP_CWORD]}"       # the word being completed
  prev="${COMP_WORDS[COMP_CWORD-1]}"    # the previous word (option)
  opts="--help --verbose --version"

  _script_options=$(script --help-list-options)
  _script_commands=$(script --help-list-commands)

  if [[ ${cur} == -* ]] ; then
    COMPREPLY=( $(compgen -W "${_script_options}" -- ${cur}) )
    return 0
  fi
  COMPREPLY=( $(compgen -W "${_script_commands}" -- ${cur}) )
  return 0
}
complete -o nospace -F _script admin.sh
=======8<--------CUT HERE----------axes/crowbars permitted---------------

Lots more complete examples in /etc/bash_completion.d/ of course.


If the command only has simple words... complete can do the job directly

  complete -W "cmd1 cmd2 cmd3" admin.sh

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