grep drop-down auto-complete is not behaving properly when pattern is not defined yet

[Pouya-moh]

I switched to prezto just today so I'm not sure if this behavior is a bug or it's by design. When I do grep - and tab I have

~ >>> grep -
-- pattern --
-- option --
--after-context    -A     -- specify lines of trailing context
--basic-regexp     -G     -- use basic regular expression
--rest of arguments

I expect with double tab the cursor jumps to the options and I be able to navigate options with arrow keys. But this doesn't work. Instead if I do grep 'my pattern' - and double tab, now it behaves the way that I expect. Note that in this case there is no empty group for -- pattern --.

I'm used to provide the options first, and then the pattern. Is there any work around?

[sorin-ionescu]

Check modules/completion/init.zsh for a line that may be causing it.

[zmwangx]

I see nothing unexpected here. If you take a vanilla Zsh without Prezto, run autoload -Uz compinit && compinit -i to initialize the completion system, and do grep -<TAB><TAB>, you'll basically get the same effect, except that you might be prompted for whether to list completions, and you probably won't see the descriptions -- pattern -- and -- option --.

If you look at the definition of _grep), the basic arguments to _arguments are (L12-15)

  arguments=(
    '(-e --regexp -f --file)1: :_guard "^--*" pattern'
    '*:files:_files'
  )

Along with a whole lot of optspecs.

So, as you can see, '(-e --regexp -f --file)1: :_guard "^--*" pattern' tells you that your grep pattern:

• is the first positional argument (1 means first and first only);
• is incompatible with the -e, --regexp, -f, and --file options;
• its message is empty (the blank in : :); in fact, the description is added by _guard. And
• its associated action is _guard "^--*" pattern.

And

_guard "^--*" pattern

insists that the pattern matches "^--*", i.e., doesn't start with --. Now:

• If you type grep -<TAB>, - matches "^--*" so it is considered a valid pattern, hence you still get -- pattern --; but _guard can't give you suggestions, so you can't tab complete or cycle.
• If you type grep --<TAB>, then -- matches "^--*" and _guard returns a nonzero status immediately, so you are only left with -- options --, where _arguments knows all the options and will let you cycle through with tab.
• If you type grep 'whatever patten' -, as I said before the pattern is the first and first only argument; since the slot is already consumed, all you are left with are the -- options --.

More information in man zshcompsys or the HTML doc.

---

Now it's a separate and perhaps interesting question as to why the pattern in _guard is written as "^--*", as opposed to "^-*". Honestly I can't think of a reason why a pattern should be allowed to begin with - (before --, which terminates the completion of options due to the -S flag to _arguments). Maybe it's a bug, maybe it's for compatibility with some paranoid grep implementation, or maybe I have a blind spot. Anyway, I don't think this is a Prezto problem, and we should probably ask on Zsh's IRC or mailing list.

[zmwangx]

By the way, here's a screencast demonstrating essentially the same behavior with or without Prezto:

[Pouya-moh]

Interesting. I migrated from oh-my-zsh to prezto so perhaps I'm used to a different behavior.

[sorin-ionescu]

Thank you, @zmwangx.