How to wait / capture the output of aysnchronous shell command in emacs lisp?

Question

If I execute a shell command asynchronously in emacs lisp as follows:

(shell-command "mycommand &")

Is there any way to wait for the command to generate output before continuing? For my current application, it is probably enough to wait until the command prints any output at all, but ideally I would like to capture the output for additional processing. Is it possible?

Answer 1

You should use the variable comint-output-filter-functions , which contains the function to call after the output is inserted into the buffer.

For example, you can:

 (add-hook 'comint-output-filter-functions '(lambda (txt) (message "hello"))) 

NB: From Emacs 23.2, you have a new async-shell-command , globally linked to M- & . This executes an asynchronous command without requiring an ampersand. The output of your command is sent to the *Async Shell Command* buffer.

Answer 2

Perhaps you need to register a Process Filter to give you the callback time you need? See 37.9 Getting Process Output in the Elisp Manual (I see this in my copy for Emacs 22.3).

Here is an example of triggering a callback when you get the first output of a process and also save it in a "linked buffer". Copy it to the *scratch* and eval-region buffers, but be sure to separate it and show the *Messages* buffer so you can see what happens.

 ;; this is emacs lisp (and a comment line) (defvar my-callback-got-some-already nil) (defun my-callback () (message "callback ran at %s" (current-time-string))) (defun my-filter-waits-for-first-time-input (proc string) (unless my-callback-got-some-already (setq my-callback-got-some-already t) ;; do your one-time thing (my-callback)) ;; insert into the associated buffer as if no process filter was ;; registered (with-current-buffer (process-buffer proc) (let ((moving (= (point) (process-mark proc)))) (save-excursion ;; Insert the text, advancing the process marker. (goto-char (process-mark proc)) (insert string) (set-marker (process-mark proc) (point))) (if moving (goto-char (process-mark proc)))))) (defun async-callback-test-harness () (interactive) (let ((process-handle "async-callback-test") (associated-process-buffer "*async-callback-test*") (command "ls") (busy-loop-var "")) (setq my-callback-got-some-already nil) (message "start test %s" (current-time-string)) (start-process process-handle associated-process-buffer command) ;; Supposedly async but Emacs doesn't get the input until ;; "Emacs is waiting" so the following set-process-filter ;; can be registered in time. ;; To prove the point, make emacs busy loop to show that the ;; emacs doesn't drop its input and ;; the callback will get the unskipped input. (switch-to-buffer associated-process-buffer) (dotimes (k 2000) ; about two seconds on my machine (setq busy-loop-var (concat busy-loop-var "busy looping..."))) (message "done busy waiting %s" (current-time-string)) (set-process-filter (get-process process-handle) 'my-filter-waits-for-first-time-input) nil)) ;; run it! (async-callback-test-harness)