Bash completion: the three variables that matter

The quick answer

Programmable completion functions inspect the current command line and populate COMPREPLY with candidate completions.

COMP_WORDS  COMP_CWORD  COMPREPLY

What's going on?

Programmable completion functions inspect the current command line and populate COMPREPLY with candidate completions.

Understanding the underlying mechanism makes troubleshooting much easier than memorising a command in isolation.

Dig a little deeper

COMP_WORDS is an array of parsed words; COMP_CWORD is the index of the word under the cursor; COMPREPLY is the output array. COMP_LINE and COMP_POINT help with finer-grained parsing.

Hands-on commands

Try these in a suitable test environment. Read each command before running it, especially if you have elevated privileges.

complete -p | head
help complete
help compgen