How does Bash/Zsh identify the relevant command during completion?

Question

When I start typing something into an interactive login shell (zsh or bash) and click tab, the shell offers me autocomplete suggestions.

I broadly understand how completion specs are defined. For reference it's outlined here:

• Bash: https://www.gnu.org/software/bash/manual/bash.html#Programmable-Completion
• Zsh: http://zsh.sourceforge.net/Doc/Release/Completion-System.html

What I don't understand is how bash/zsh identify which command to complete on when a user presses tab i.e. if I do ls [tab], how does bash/zsh identify ls. Put differently, how do bash/zsh know to generate completions using the ls completion spec.

Some more examples with more complicated parsing which Bash/Zsh still get right:

• ls [tab] -> completes on ls
• echo $( echo $(ls [tab] -> completes on ls
• echo hi; git a[tab] -> completes on git
• echo 'cd [tab] -> does NOT complete on cd
• echo "$(ls [tab] -> completes on ls
• echo $( echo hi | ls [tab] -> completes on ls
  (I know ls doesn't take stdin but this example still illustrates bash/zsh's parsing abilities)

I am presuming bash/zsh are using some sort of parser. However, it's not a normal parser. It completes without a properly structured command. It is aware that something is in quotes or a shell expansion, even if they are not closed.

What function is bash/zsh running to "identify" the relevant command or determine whether or not there is a command to complete on at all?

Answer 1

What function is bash/zsh running to "identify" the relevant command or determine whether or not there is a command to complete on at all?

When you press tab, readline executes the complete function.

You can override this in ~/.inputrc:

"\t": complete

complete is defined in lib/readline/funmap.c as:

static const FUNMAP default_funmap[] = {
[..]
{ "complete", rl_complete },

rl_complete can be found in lib/readline/complete.c (browse source):

/* Complete the word at or before point.  You have supplied the function
   that does the initial simple matching selection algorithm (see
   rl_completion_matches ()).  The default is to do filename completion. */
int
rl_complete (int ignore, int invoking_key)
{
..

This is the starting point. It breaks apart the words, checks which completions are registered and then looks for the relevant completions.

zsh will probably have a similar implementation.

There is also a good explanation of the process here.