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I am having some trouble understanding how shell-command-on-region works. Sometimes the region is passed to command, sometimes not, as far as I can tell.

Suppose I have an emacs buffer with "word" in the region, then counting the region's length with wc:

shell-command-on-region wc

returns 0 1 4 , as it should. But other commands don't seem to receive the region as input. For instance, both

shell-command-on-region echo

and 

shell-command-on-region locate

produce, respectively, an empty result and an error message ("locate: no pattern to search for specified").

Why is the region passed to the command in the first example and not in the second and third one?

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Why is the region passed to the command in the first example and not in the second and third one?

The region is passed in all three cases; but if the command you pass it to does nothing with its standard input, then you will not get a useful result.

i.e. You are effectively doing this:

printf word | wc printf word | echo printf word | locate

Note that the default key binding M-| is a mnemonic for the fact that you are piping the region as input to the command. That might make it easier to remember.

Edit:

I'm still wondering if there is an emacs command that calls a a command with the region as argument, though.

(defun my-shell-command-with-region-arg (command beginning end) "Prompt for a shell COMMAND, and use the region as an argument. Buffer text from BEGINNING to END is passed as a single argument to COMMAND." (interactive (list (read-shell-command "Shell command: ") (region-beginning) (region-end))) (let ((region (buffer-substring-no-properties beginning end))) (shell-command (concat command " " (shell-quote-argument region)))))
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  • I see, I had indeed misunderstood the command, assuming it would pass the region as argument, not as stdin. What would be needed to have the region passed as argument then? Commented Oct 16, 2019 at 5:22
  • Since the command I am passing the region to is a bash script I wrote, I was able to fix it following the answer by @andy in this SX question: stackoverflow.com/questions/18761209/…. I'm still wondering if there is an emacs command that calls a a command with the region as argument, though. Commented Oct 16, 2019 at 6:08
  • I've added one to the answer. Commented Oct 16, 2019 at 7:56
  • So, in this case, I needed something like xargs -I{} echo {} Commented Mar 26, 2024 at 6:19

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