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This question already has an answer here:

What I want is printing the output the first line ( table head ) of ps aux and the grep result.

After search, I come up with following.

ps aux | tee >(head -1 > /dev/tty) | grep mongo

But I find the stdin of grep mongo is cut off.

Also, if I omit > /dev/tty, what will the stdout of head -1 direct to? Why isn't it the console?

(Yes, I know I can achive my purpose by command awk. I am just curious about why my command doesn't work?)

marked as duplicate by don_crissti, Networker, jimmij, Braiam, slm Dec 22 '14 at 14:49

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • This doesn't appear to be a dupe of the question it was closed as a dupe of. This one is asking why something doesn't work as expected, The other is asking how to accomplish a specific task. – Faheem Mitha Dec 22 '14 at 15:34
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    Take a look at Thor's answer to the other question; he covers why your approach doesn't work, and a way to make it (somewhat) work. (I don't blame you for missing it, it's nowhere near the top. I missed it myself at first, too.) – derobert Dec 22 '14 at 15:37
  • Also see a followup asked about that: How do I use tee to redirect to grep? – derobert Dec 22 '14 at 15:40
  • Thanks @derobert the two links solve my question. I think I have more specific keyword to search to understand it. – Sisyphus Dec 23 '14 at 5:25
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You could use awk to match two things at the same time:

  • The first line.
  • The line containing "mongo".

There you go:

$ ps aux | awk 'NR == 1 || /mongo/ {print $0}'
  • The NR == 1 condition matches the first line.
  • The /mongo/ condition matches lines containing "mongo".
  • {print $0} is the action associated with the two previous conditions, in this case: print the whole line.

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