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What is the correct way to run multiple async-shell-command invocations serially?

For example, this will execute all at once.

(progn
  (async-shell-command "echo 1; sleep 1")
  (async-shell-command "echo 2; sleep 1")
  (async-shell-command "echo 3; sleep 1"))

But I want each command to wait for the command before it to finish first. (Of course in this case you can just chain them together with bash and &&, but you might like to have different output buffers, etc.) And I don't want to make the user wait, which is why I don't use UI-blocking shell-command function.

The only way I've figured out how to do this is with sentinels and continuation passing, which is very ugly.

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  • 1
    Probably shell-command is no solution for you since you do not want to make the user wait for the commands to finish. If so, please give a note about that. Do you mean by "continuation" to wait for the currently running shell command with a sentinel reacting on finished\\|exited and start the new shell there? That is one way to go. Probably start-process is better for that. You could also use the async package and fire up all shell-command commands synchronously in a separate Emacs with its help.
    – Tobias
    Jan 15, 2019 at 2:47
  • Here are links to a couple of examples of how to do this: stackoverflow.com/a/23071492/2112489 and stackoverflow.com/a/42879986/2112489
    – lawlist
    Jan 15, 2019 at 4:41

2 Answers 2

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The only way I've figured out how to do this is with sentinels and continuation passing, which is very ugly.

Here it is:

(set-process-sentinel
 (start-process-shell-command "Shell" "*Output*" "echo 1; sleep 1")
 (lambda
   (_ _)
   (set-process-sentinel
    (start-process-shell-command "Shell" "*Output*" "echo 2; sleep 1")
    (lambda
      (_ _)
      (start-process-shell-command "Shell" "*Output*" "echo 3; sleep 1")))))

To create the above expression programmatically, you can write a macro, e.g.,

(defmacro multiple-async-shell-commands (&rest commands)
  "Run COMMANDS in sequences, each runs asynchronously."
  (cl-labels ((aux (commands)
                   (pcase commands
                     (`(,command . ,rest)
                      `(set-process-sentinel
                        (start-process-shell-command
                         ,@(if (stringp command) `("Shell" "*Output*" ,command) command))
                        (lambda (_ _)
                          ,(aux rest)))))))
    (aux commands)))

then you can use

(multiple-async-shell-commands "echo 1; sleep 1"
                               "echo 2; sleep 1"
                               "echo 3; sleep 1")

you might like to have different output buffers

The COMMANDS can also be a list of argument of start-process-shell-command, so you can set the output buffer:

(multiple-async-shell-commands ("sh" "*output-1*" "echo 1; sleep 1")
                               ("sh" "*output-2*" "echo 2; sleep 1")
                               ("sh" "*output-3*" "echo 3; sleep 1"))
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  • 1
    In (set-process-sentinel process (lambda (_proc event) ...)) you should check event for a match of "\`finished\\|exited\\|killed". Alternatively you can check (process-status process) for 'exit in the sentinel. Background: The process might also receive other signals such as stop or run.
    – Tobias
    Jan 15, 2019 at 11:12
  • Example for other signals: (let* ((bash-buf (get-buffer-create "*bash*")) (proc (start-process "bash" bash-buf "bash" "-c" "sleep 5; exit -1;"))) (set-process-sentinel proc (lambda (proc event) (message "Proc: %S, Event: %S" proc event))) (stop-process proc) (sleep-for 2) (continue-process proc) (sleep-for 2) (kill-process proc)). (You can play around with the signal generating functions.) Those signals can also be sent with kill from the outside of Emacs.
    – Tobias
    Jan 15, 2019 at 11:13
  • Hm, this doesn't work for me. I get garbage in my process buffers, they live in fundamental-mode, and they don't seem to track processes.
    – Matthew Piziak
    Jan 23, 2019 at 20:05
  • @MatthewPiziak The first code block is straightforward, does it work for you? (It should create buffer *Output* and insert 1\n2\n3\n\nProcess Shell finished\n in the buffer) The rest is based on the first code block.
    – xuchunyang
    Jan 24, 2019 at 4:59
  • @xuchunyang Ah yes, this works for plain text. I don't suppose there is a way to restore ANSI code compatibility?
    – Matthew Piziak
    Jan 24, 2019 at 20:00
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you can use async-defun and await from the emacs-async-await package:

;; -*- lexical-binding: t -*-
;; line above goes at the top of your buffer

(async-defun wait-for()
  (await (shell-command-to-string "echo 1; sleep 1"))
  (await (shell-command-to-string "echo 2; sleep 1"))
  (await (shell-command-to-string "echo 3; sleep 1")))

(wait-for)

EDIT=fixed parentheses EDIT2= added lexical binding

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  • First of all I suspect that your parens don't match here. If I fix them I get Assertion failed: lexical-binding despite the fact that I have (setq lexical-binding t).
    – Matthew Piziak
    Jan 23, 2019 at 19:55
  • Do mind to add ;; -*- lexical-binding: t -*- at the top of your elisp file. I do think that the assert that pops up is off, but I'm not 100% sure
    – Felipe Lema
    Jan 23, 2019 at 20:41
  • @MatthewPiziak What does (setq lexical-binding t) mean? It is almost certainly an error. lexical-binding is a buffer local variable therefore setq-ing it in the initialization does not make sense. In elisp files it must be set as file variable in a comment on the first line.
    – Tobias
    Jan 24, 2019 at 3:41
  • Okay, I've done that and it runs now. But why do you have shell-command-to-string instead of shell-command? There's no way to see the output.
    – Matthew Piziak
    Jan 24, 2019 at 19:51
  • no particular reason. I just happen to use that function a lot and I wrote it without thinking
    – Felipe Lema
    Jan 25, 2019 at 20:43

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