How does command-line tab completion work?

I’ve written a diary CLI program which lets me write a new entry or read an old entry. Here’s a simplified implementation:

#!/bin/bash
function usage {
  echo "usage: $(basename ${0}) write"
  echo "       $(basename ${0}) read <timestamp>"
  exit 1
}
case "${1}" in
  write )
    mkdir -p ~/diary
    nano ~/diary/"$(date '+%Y-%m-%d')"
    ;;
  read )
    if [ "${2}" = "" ]; then
      usage
    fi
    cat ~/diary/"${2}"
    ;;
  * )
    usage
    ;;
esac

This program prints its “usage” if the user gets it wrong:

$ mydiary
usage: mydiary write
       mydiary read <timestamp>
$ mydiary write
[nano opens for editing]
$ mydiary read
usage: mydiary write
       mydiary read <timestamp>
$ mydiary read 2018-01-14
Today I ate a crisp sandwich

But we can do better! With tab-completion, the user can see the possible commands without running them, and can complete the names of diary entries without typing them out fully.

This page copyright James Fisher 2018. Content is not associated with my employer. Found an error? Edit this page.

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