Wednesday, November 17, 2010

Emacs: Copy Environment Variable from Shell

Sometimes I get annoyed when my emacs session has different environment variable settings than some shell buffer I have running in the session. The most painful is when a Makefile depends on environment settings that I don't have in my .profile. Command-name completion in a shell buffer can also be painful if you have changed your path. And of course I'd like gdb to start up with the correct environment every time -- you can set the variables inside gdb, but that gets old on those days when gdb itself crashes again and again.

I kept pasting export FOO=bar into the *scratch* buffer and editing it into (setenv "FOO" "bar") and eval'ing that. After the 1000th time of doing that, I decided to automate it. Turns out emacs already has an interactive function for copying an environment variable from the shell, but you have to type in the variable name. I decided to write a little function to look for the last export, or the last echo $FOO, and copy that variable:

(defun engisneering-shell-copy-env-var ()
  (let* ((expat "\\(export +\\([^=\n]+\\)=\\(.+\\)\\)")
         (echpat "\\(echo +\\$\\(.+\\)\n\\(.+\\)\\)")
         (cshpat "\\(setenv +\\([^ \n]+\\) +\\(.+\\)\\)")
         (patt (concat shell-prompt-pattern
                       "\\(" expat "\\|" echpat "\\|" cshpat "\\)")))
      (if (re-search-backward patt)
          (let* ((m (or (and (match-beginning 2) 3)
                        (and (match-beginning 5) 6) 9))
                 (var (buffer-substring (match-beginning m) (match-end m)))
                 (oldval (or (getenv var) " ")))
            (shell-copy-environment-variable var)
            (setq val (getenv var))
            (message "Old %s=%s; New %s=%s" var oldval var val))))))

Run this function inside your shell buffer, and it will search backwards for the last environment variable action and bring that variable setting into the emacs session. It's also nice because it gives you a message in the minibuffer showing the change. Add a local-set-key -- I like C-c C-v -- in your shell-mode-hook, and you're good to go.

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