In some cases, when delegating work to an external process, it's useful to set a timeout on the command to prevent Emacs from hanging indefinitely.
Unfortunately, the following does not work.

(with-timeout (1 nil)
  (call-process "/usr/bin/bash" nil t nil "-c" "sleep 10"))

The same goes for shell-command.

Is there a way to set a timeout on these synchronous processes?
That is, I want the process to be killed automatically if it doesn't finish within a certain number of seconds. Is that possible?

  • I've never used with-timeout, but I have used kill-process and delete-process.
    – lawlist
    Mar 25, 2015 at 1:06
  • Not sure, but maybe accepting process output would help. I.e. it would tell Emacs to take initiative and execute some Elisp code, supposedly waiting for the process to produce some output, perhaps, that would also be a good time to kill the process if it timed out.
    – wvxvw
    Mar 25, 2015 at 9:57
  • @wvxvw The call-process above is setting the output to be printed on the current buffer (I get the same effect if I pass a different output buffer). Is that what you mean?
    – Malabarba
    Mar 25, 2015 at 12:17
  • Nah, sorry, I confused few things. I meant this: gnu.org/software/emacs/manual/html_node/elisp/… but this is only relevant for the async processes. When I'm home, I'll look into the call-process sources, but now I'm beginning to suspect that there isn't a way to kill it on timeout.
    – wvxvw
    Mar 25, 2015 at 12:52
  • Actually, I tried to look it up on Github: github.com/emacs-mirror/emacs/blob/… and, no, I don't see there any code that would do something like setting timeout for the process.
    – wvxvw
    Mar 25, 2015 at 13:04

2 Answers 2


While call-process is running, emacs will processing events, with-timeout will not work without this:

The timeout is checked whenever Emacs waits for some kind of external event (such as keyboard input, input from subprocesses, or a certain time); if the program loops without waiting in any way, the timeout will not be detected.

You can still use with-timeout with (semi) synchronous processes.

You will actually use a asynchronous process but will synchrously wait while it is running, Emacs will process events when you run sit-for, which you can run for 0 seconds. You can then use the timeout-forms argument of with-timeout to kill the process if it is still running when the timeout fires.

(with-current-buffer (get-buffer-create "*my-proc-buffer*")
  (let ((proc (start-process "myproc" (current-buffer) "bash" "-c" "sleep 4"))) ;; start an async process
    (with-timeout (2 (kill-process proc)) ;; on timeout, kill the process
      (while (process-live-p proc) ;; while process is running
        (sit-for .05)) ;; let emacs read events and run timmers (and check for timeout)
      (message "finished on time!!")))) ;; this will run only if there is no timeout
  • I'm having a small problem with this. The output of the proces isn't guaranteed to be in the output buffer after the loop finishes. I'm working around it by calling another (sit-for 0.2) after the while loop, but that feels unstable. Is there a way to know that the output has been printed on the buffer?
    – Malabarba
    Mar 25, 2015 at 17:22
  • I'm not sure about that. I would assume though that if you hit the timeout, the process is not done anyway so the output is not relevant. What are you doing such that the output is still needed even after a timeout? You could potentially switch to using a process filter and killing the process only X seconds have passed without output rather than after X seconds no matter what. Mar 25, 2015 at 17:40
  • Sorry, I wasn't clear. I don't care about the output when the timeout is hit. I'm saying the output isn't immediately printed after the process finishes successfully.
    – Malabarba
    Mar 25, 2015 at 17:41
  • I think the problem is (sit-for 0), emacs spends too much time letting the process run and not enough listening for events and inserting the text into the buffer. When I change mine to (sit-for .05) things work better and all text gets into my buffer. Even for a short process .05 seconds isn't a concerning overhead. Mar 25, 2015 at 17:57
  • I'm not too worried about overhead in this situation. The process is called 5-8 times and the entire thing is an interactive operation which already takes at least several seconds, so 1 more second won't make much of a difference. What worries me a little is stability, because I had to use (sit-for 0.2) for my local tests to pass, and even then once I pushed the changes the travis tests failed.
    – Malabarba
    Mar 25, 2015 at 19:37

You can achieve this by just adding a GNU timeout invocation to your shell command, which circumvents needing to know any details about Emacs behavior. For example running:

$ timeout 5 sleep 10

Will return in 5 seconds, not 10 (timeout effectively presses Ctrl-C for you).

  • Thanks, I may have to go with this option if I can't achieve stability with the other one. However, I would have preferred not to rely on another utility, so that it would also work on Windows.
    – Malabarba
    Mar 25, 2015 at 19:39

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.