Using Bash Builtin: Complete (Help!)

On your request "help explain the options in the complete command":

It is a longer story to tell all details, hence find a list of hopefully useful documentation references at the end of this post.

About your specific use case to complete $1 as command and $2 as files, I have added a small example in the next section.

Example

Write a completion function that handles positional differences:

# completion function to differentiate type of completion by argument type
_complete_phone () {
  case ${#COMP_WORDS[@]} in
    1) # 1st position is the `phone` command itself
       return;;
    2) # 2nd position are sub-commands, so we use `-W` for word completion
       words="mount unmount"
       COMPREPLY=($(compgen -W "$words" -- "${COMP_WORDS[COMP_CWORD]}"))
       ;;
    *) # other arguments filter files, either
       # any files using option `-f`
       #COMPREPLY=($(compgen -f -- "${COMP_WORDS[COMP_CWORD]}"))
       # and for specific files (i.e. pattern `*.phone`), use words again
       COMPREPLY=($(compgen -W "$(ls *.phone 2>/dev/null)" -- "${COMP_WORDS[COMP_CWORD]}"))
       ;;
  esac
}

Then activate completion by function activated with the -F argument, using your completion function name:

# complete using your function 
complete -F _complete_phone phone

And finally, try (for reproducibility an arbitrary program phone and 'relevant' files have to be created):

# try
phone () { echo "hello, phone program will $@"; }
touch {my,your}.phone
phone m\tab \tab \tab

Documentation

Bash Reference Manual

An excellent go-to resource of documentation when you want more details about bash.

Specific for bash completion, have a look into chapters

• 8.4 Bindable Readline Commands
  • 8.4.6 Letting Readline Type For You
• 8.7 Programmable Completion Builtins
• 8.8 A Programmable Completion Example

Source: www.gnu.org

Bash Manual

Run man bash and go to sections:

• READLINE

  • Readline Variables explaining how to set variables related to completion. For example set completion-ignore-case on to "perform filename matching and completion in case insensitive fashion".
  • Completing default bash completion.
  • Programmable Completion explaining the process of completion, and relevant elements such as methods compgen, variables COMP_WORDS, etc. you find in the example above.

• SHELL BUILTIN COMMANDS

  • compgen to generate completion matches.
  • complete to specify how to complete. Also describes all options, such as -W, -F, -o, etc.
  • compopt to set other completion options.

Source: GNU bash, version 5.1.16(1)-release (x86_64-pc-linux-gnu)