Currently I'm generating text from a command, eg:

  (call-process "my-command" nil t nil "my" "args")
  ;; operate on output in current buffer.

How can I use pipes, something that would be written in shell like this:

my-command my args | wc -l

Note, this is just an example command, doing the same operation without pipes wont address this question.

1 Answer 1


Use the system shell, e.g., M-! ls | wc, there are many other APIs such as shell-command-to-string, call-process-shell-command and start-process-shell-command.

An idea is emulating pipe like the following, it is slow since the second process won't run until the first is done. Though it's possible to use asynchronous process + process filter to avoiding blocking, it's still going to be slow. (Take Eshell for example)

  (call-process "date" nil t)
  (call-process-region nil nil "wc" nil t t )
;; =>
"Tue Apr  7 15:49:17 CST 2020
       1       6      29
  • Can't this be done without using the buffer as an intermediate step?
    – ideasman42
    Apr 7, 2020 at 8:19
  • 1
    @ideasman42 It seems possible using asynchronous process, you get output from process with process-filter, and send input to process with process-send-string, both of them uses string.
    – xuchunyang
    Apr 7, 2020 at 8:56
  • A third option is to create temporary files and pass them as the second and third arguments to call-process. Apart from potentially being faster than using an Emacs buffer, though, I don't see any other benefits to this approach.
    – Basil
    Apr 7, 2020 at 15:40
  • @Basil that's only an option if the process accepts the input as an argument, which isn't always the case.
    – ideasman42
    Apr 8, 2020 at 0:09
  • 1
    @ideasman42 Specifying a file as the stdout of one process and the stdin of another, which the second and third arguments to call-process allow, has nothing to do with what arguments the process accepts.
    – Basil
    Apr 8, 2020 at 0:18

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