Native shell completion in Emacs

I am obsessed with autocompletion in shell mode. Running a shell in shell-mode instead of a terminal emulator has so many advantages. You can treat the whole buffer just like a normal Emacs buffer. You can copy and paste and edit the line normally. You can hook it into native Emacs functionality. You can even display images!

However there is one big disadvantage. You lose access to the state the shell. This means that you have to do tricks like shell-dirtrack-mode just to make sure you are in the right directory. It also means that all the native shell completions are not available. I have tried to approach this problem from multiple angles with packages like this, this, and this. (Yes, I have written a half dozen modes to try solve this.) The most popular package to try and solve this is bash-shell-completion. However all these solutions have the same problem that they don’t actually reflect the internal state of the shell, they are just close approximations.

But the shell knows its own state. In a normal terminal emulator, tab completion will give you access to it. When looking at my shell buffer, it seems that the information I want is just below the surface. If only there was some way to access it. I am not the only one who has had this thought. There is a stackoverflow with over 26,000 views asking this same question. But no one has managed to access the native tab completion before. So I determined to solve this once and for all.

The curious case of bash

If you use csh and send some text followed by the tab character, it will print out all possible completions. But not bash. Try this code and you get nothing.

(comint-simple-send (get-buffer-process (current-buffer)) "git\t\x15")

It works in the terminal but not in the Emacs shell. What conspiracy is this? Turns out that Emacs and bash have put a lot of effort into making sure completion does not work. The first thing to notice is that explicit-bash-args contains the argument --no-editing, which will disable readline completion. Let get rid of that shall we?

(setq explicit-bash-args
          (delete "--noediting" explicit-bash-args))

However removing that still does not enable tab completion. There must be something else going on here. This time it is on the bash side. Looking in the source code we see the follow block.

term = get_string_value ("TERM");
emacs = get_string_value ("EMACS");
inside_emacs = get_string_value ("INSIDE_EMACS");

if (inside_emacs)
    emacs_term = strstr (inside_emacs, ",term:") != 0;
    in_emacs = 1;
 else if (emacs)
     /* Infer whether we are in an older Emacs. */
     emacs_term = strstr (emacs, " (term:") != 0;
     in_emacs = emacs_term || STREQ (emacs, "t");
   in_emacs = emacs_term = 0;

/* Not sure any emacs terminal emulator sets TERM=emacs any more */
no_line_editing |= STREQ (term, "emacs");
no_line_editing |= in_emacs && STREQ (term, "dumb");

For some reason that I can’t explain, bash has special code for running inside Emacs. Lo and behold, if $TERM is dumb and $INSIDE_EMACS is set, then line editing is disabled by the shell itself. Any reason for this? I would love to know. Changing $TERM to something other then dumb fixes the issue, but then programs might not interpret our terminals capabilities correct. The best thing to do is remove the environment variable $INSIDE_EMACS. Doesn’t seem to do anything useful after all.

(advice-add 'comint-term-environment
            :filter-return (lambda (env) (cons "INSIDE_EMACS" env)))

And with that we have working tab completion in bash!

Making completion “Emacsy”

We could stop here and just create a function to send tab to the underlying process. It would behave exactly like the terminal does. But this is Emacs. The whole reason we are using shell-mode in the first place is because things in Emacs are nicer then things in the terminal. We have things like completion-at-point and company-mode that would make a terminal emulators head spin. Makes sense to take advantage of that. So I created a new package called native-complete that talks to the underlying process and gets the actual completions that the shell would natively provide. No more trying to use other packages to guess the shells state, or starting another process that may be out of sync. It even supports invoking subshells! This effort is still work in progress, and many shells have yet to be tested. As with many things, it is not as simple in the implementation as it should be.

I hope this is the shell-mode completion Endgame. I don’t think I can take much more.

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