How to capture standard output of a shell command in elisp?

Question

I want to run a shell command within Emacs and capture the full output to a variable. Is there a way to do this? For example, I would like to be able to set hello-string to "hello" in the following manner:

(setq hello-string (capture-stdout-of-shell-command "/bin/echo hello"))

Does the function capture-stdout-of-shell-command exist, and if so what is its real name?

score 80 · Accepted Answer · edited Aug 28 '23 at 09:04

80

Does shell-command-to-string meet your purpose?

For example:

(shell-command-to-string "/bin/echo hello")

edited Aug 28 '23 at 09:04

Yash Khare

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answered Feb 16 '11 at 09:52

Ise Wisteria

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Note that `shell-command-to-string` captures both `stdout` _and_ `stderr`. Luckily we can just pipe the stderr to /dev/null: `(shell-command-to-string "/bin/echo hello 2>/dev/null")` – Yorick Sijsling May 08 '20 at 06:31

score 26 · Answer 2 · edited Sep 29 '12 at 01:52

26

I have a suggestion to made that extends Ise Wisteria's answer. Try using something like this:

(setq my_shell_output
  (substring 
    (shell-command-to-string "/bin/echo hello") 
   0 -1))

This should set the string "hello" as the value of my_shell_output, but cleanly. Using (substring) eliminates the trailing \n that tends to occur when emacs calls out to a shell command. It bothers me in emacs running on Windows, and probably occurs on other platforms as well.

edited Sep 29 '12 at 01:52

Brighid McDonnell

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answered Feb 16 '11 at 18:21

Chris McMahan

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I've noticed emacs doing that on both Mac OS X and on Linux, so it's not just NTEmacs. – Brighid McDonnell Sep 29 '12 at 01:38
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`(defun my-shell-command-to-string (&rest cmd) (replace-regexp-in-string "\r?\n$" "" (shell-command-to-string (mapconcat 'identity cmd " "))))` – spacebat Jan 22 '14 at 00:15
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@spacebat You want `\\'` in there, not `$`. – Clément Sep 07 '15 at 22:08
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`echo hello` is `hello\n`. – d12frosted Mar 06 '16 at 08:51
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In linux/osx, you can use ```-n``` to stop outputting the trailing newline., e.g. ```echo -n hello``` – Chuan Ma Apr 29 '16 at 18:46
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Doesn't work when combined with `(shell-command-to-string "/bin/echo hello 2>/dev/null")` per Yorick's comment above. Any tips for handling potentially empty strings? – Marcin Jul 13 '22 at 07:29

How to capture standard output of a shell command in elisp?

2 Answers2