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I'm trying to run strace trough ccze, and the pipe doesn't work as expected.

The command-line I'm running to test is sudo strace -p $(pgrep apache2) | grep open, and all lines are output, ignoring grep.

Is there something special about strace that causes this behavior?

Andrei
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1 Answers1

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strace prints its traces on standard error, not on standard output. That's because it's common to want to redirect the standard output of the program, but usually not a problem that strace's stderr and the program's stderr are mixed.

So you should redirect strace's stderr to stdout to be able to pipe it:

sudo strace -p $(pgrep apache2) 2>&1 | grep open

except that what you're really looking for is

sudo strace -p $(pgrep apache2) -e open
Gilles 'SO- stop being evil'
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  • it works!! thank you! - I wasn't actually trying to look only at open calls, I just put that as an example, what I'm really trying to do is color highlighting – Andrei Sep 15 '12 at 00:03
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    I am getting strace: Invalid process id: '-e' with the last command.

    I am on version 4.8. 2010-03-30.

     – Elijah Lynn Jun 29 '17 at 19:13
  • Ahh, I actually have httpd (RHEL). – Elijah Lynn Jun 29 '17 at 19:21
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    @ElijahLynn Replace apache2 by the name of the process you're interested in. Check that pgrep prints a single PID, otherwise pick one and run e.g. sudo strace -p 1234 -e open – Gilles 'SO- stop being evil' Jun 29 '17 at 20:34
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    @Andrei, what about using vim color syntax highlighting? strace $CMD 2>&1 > /dev/null | vim -c ':set syntax=strace' -. – Pablo A Aug 27 '17 at 14:49