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I am parsing text on the command line. How can I take the output of one command and use it in two places in the next command?

some_command | head -n $( awk '/search_term/ {print FNR} output_of_some_command ) output_of_some_command

I am truncating the output of some_command using head, but I need to search the output of some_command (using awk in this case) to find the line number at which to truncate.

Can I do this, or do I need to write to a temporary file?

  • Thinking this through I could use sed instead of head to truncate the text... but I still wonder if it could be done this way. – Ashley G Oct 15 '14 at 22:07
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For this particular task, you could use sed instead of head – or you could use awk instead of head:

some_command | awk '{print} /search_term/{exit}'

Print lines, but, when you find one that matches the search term, exit (and don’t process any more lines after that).

A more general solution to the problem you describe is

some_command | tee >(command2) | command3

or

some_command | tee >(command2) >(command3)

These will work in bash, but probably not too many other shells (including sh).  >(command) gives you a file handle to a pipe to a process running command.  As you probably know, tee lets you take a data stream and write it multiple places.

But your problem is subtlety more complicated than what you said.  You’re not trying to do

                                          A→B and A→C in parallel

you’re trying to do

                A→B→C and A→C in parallel

where the head command (corresponding to C in the diagram) takes input from “some_command” (A) but also has a command-line argument set from the output of awk (B).  I suspect that a general solution for that would be much trickier.

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