During the process of migrating a Conda environment and mitigating confliction, I noticed Bash programmable completion was experiencing an issue when the environment was not yet incomplete. I seemingly showed no curiousity, proceeding to enter the command despite STDERR for the rest of migration. This post was created after that since there are no Marginalia search engine-style sources on Bash programmable completion.

Context-dependent completion is easy to spot. Git, the first instance coming to my mind, completes options and hash strings when you are hasty writing down an arbitrary hexadecimal ID. For practical scripts, completion functions serve as a valuable aid in recalling details of commands.

The builtin command complete determines how arguments are to be completed by Readline. In the absence of arguments, it outputs available completion functions in a ready-to-load format. To relate a function to a command, use

complete -F <completion_function> <command>

Here is an example of completion function:

_mycmd_completion() {
    # retrieve words on the current line
    local cur prev
    cur="${COMP_WORDS[COMP_CWORD]}"
    prev="${COMP_WORDS[COMP_CWORD-1]}"

    # complete according to the previous word
    case "$prev" in
        mycmd)
            COMPREPLY=( $(compgen -W "start stop" -- "$cur") )
            ;;
        *)
            COMPREPLY=()
            ;;
    esac
}
# enable the completion function
complete -F _mycmd_completion mycmd

The function retrieves precedent words from variables. The current line, COMP_LINE is split into words in COMP_WORDS. In the function, the target is the last word indexed as COMP_CWORD. Use COMP_LINE for the word being edited.

compgen is another builtin function that generates completions, intended to be used in a completion function. The options of compgen denote the type and lexical criteria of completion, as exemplified in the table:

Therefore, the expression above completes start and/or stop according to the partially typed word. You can also pipe compgen output to grep to further filter the results.

In the end, the completion function is attached to a command.

For Python scripts, packages like shtab, argcomplete, and prompt_toolkit are responsible for completions. click, an alternative to builtin argument parsing library argparse, is a versatile tool supporting completion.

Using compgen and complete in Bash, you may create completion rules for filenames, commands, options, and more. Custom completion allows you to enhance the usability of command-line tools and scripts. Fish offers more nuanced completion generators invoked in a similar procedure; you may read the super-friendly documentation.