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I'm using start-process to run mplayer and when playing videos I get a ton of output to stderr from "vdp_...". On the command line I usually just 2>/dev/null but start-process runs it directly without a shell. (I tried start-process-shell-command but it simply does not work at all). call-process lets you provide a list for separate stdout and stderr but it seems start-process does not. I don't see an option to mplayer to inhibit the output. If only there were a way to do with start-process what you can do with call-process. Is there some way I'm not seeing to do this?

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  • The second argument in start-process is the output buffer's name, which can be nil to suppress the output buffer entirely. Is that what you are asking? Commented Oct 15, 2015 at 6:15
  • No, I still want to see stdout since I grab that time stamp at times. That all works great if it's just music, only the video produces the junk to stderr. Commented Oct 15, 2015 at 6:34
  • In that case, you can set up a filter to only print the strings or portions of strings that you want to see -- you'll need to identify either the stuff you want to see or the stuff you don't want to see and set up some rules -- e.g., with regexp and string= etc. gnu.org/software/emacs/manual/html_node/elisp/… Commented Oct 15, 2015 at 6:51
  • Thanks lawlist, I look forward to playing with filters. I might even try that for this project since it seems to make sense for what I'm doing. Commented Oct 16, 2015 at 6:51

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I know this is an emacs forum, but maybe the simplest thing to do is to write a simple shell script that filters stderr. For example in file nostderr.sh:

#!/bin/sh $@ 2>/dev/null exit $?

And then run that in your start-process.

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  • 1
    In fact this is what the docstring says as well: If you want to separate standard output from standard error, invoke the command through a shell and redirect one of them using the shell syntax Commented Oct 15, 2015 at 23:29
  • Yes, that's one of the work-arounds I'd thought of, but good catch on finding that in the docstring. For that I'll give you the win. What I ended up doing was just searching further back and found what I wanted from the stdout. But in cases when what you are searching for cannot be separated from stderr, this is probably best. Commented Oct 16, 2015 at 6:48
  • As for this being an emacs forum, your suggestion was perfectly apropos to the problem which involves command lines. Commented Oct 16, 2015 at 6:55
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Emacs 25 introduced the function make-process for creating asynchronous processes, which is lower-level than start-process. In particular, you can separate stderr from stdout by passing a buffer or pipe as the function's :stderr keyword argument. For example:

(make-process :name "mplayer" :buffer (generate-new-buffer "*mplayer out*") :command '("mplayer" "/path/to/video") :connection-type 'pipe :stderr (generate-new-buffer "*mplayer err*"))

The standard output and error streams of mplayer will now appear in buffers with names like *mplayer out* and *mplayer err*, respectively.

See (info "(elisp) Asynchronous Processes") for more information.

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  • I have tried to replace start-process with make-process but I keep getting errors and am unfortunately not well-versed in elisp... an example might be nice here Commented Nov 23, 2020 at 12:58
  • @Xerus Done. What kind of errors are you getting? Commented Nov 23, 2020 at 13:40

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