Extending Eshell: Completion

December 18, 2020

Emacs supports programmable completion in a couple of modes through a feature called pcomplete. This article goes into more detail. For reference, this is what I have in my Emacs config file (code mostly from that article):

(defun pcmpl-git-commands ()
  "Return the most common git commands by parsing the git output."
    (call-process-shell-command "git" nil (current-buffer) nil "help" "--all")
    (goto-char 0)
    (search-forward "Main Porcelain Commands")
    (let (commands)
      (while (re-search-forward
	      nil t)
	(push (match-string 1) commands)
	(when (match-string 2)
	  (push (match-string 2) commands)))
      (sort commands #'string<))))

(defconst pcmpl-git-commands (pcmpl-git-commands)
  "List of `git' commands.")

(defvar pcmpl-git-ref-list-cmd "git for-each-ref refs/ --format='%(refname)'"
  "The `git' command to run to get a list of refs.")

(defun pcmpl-git-get-refs (type)
  "Return a list of `git' refs filtered by TYPE."
    (insert (shell-command-to-string pcmpl-git-ref-list-cmd))
    (goto-char (point-min))
    (let (refs)
      (while (re-search-forward (concat "^refs/" type "/\\(.+\\)$") nil t)
	(push (match-string 1) refs))
      (nreverse refs))))

(defun pcmpl-git-remotes ()
  "Return a list of remote repositories."
  (split-string (shell-command-to-string "git remote")))

(defun pcomplete/git ()
  "Completion for `git'."
  ;; Completion for the command argument.
  (pcomplete-here* pcmpl-git-commands)
   ((pcomplete-match "help" 1)
    (pcomplete-here* pcmpl-git-commands))
   ((pcomplete-match (regexp-opt '("pull" "push")) 1)
    (pcomplete-here (pcmpl-git-remotes)))
   ;; provide branch completion for the command `checkout'.
   ((pcomplete-match "checkout" 1)
    (pcomplete-here* (append (pcmpl-git-get-refs "heads")
			     (pcmpl-git-get-refs "tags"))))
    (while (pcomplete-here (pcomplete-entries))))))

Unfortunately, git help doesn’t give a machine-parsable list of commands, so the first function which tries to identify them is sort of a hack. But, as you can see, it’s fairly straightforward to add your own complete functions. I think I’ll use it as an excuse to learn more Emacs Lisp.