2

I've got a command foo that outputs a list of filenames:

$ foo
file1.a
file2.b
file3.a

And a command bar that accepts the names of .a files as arguments and does some processing:

$ bar file1.a file3.a
Great success!
$ bar file2.b
FAILURE

I'd like to combine the two with a pipe like foo | xargs bar, but I need to filter out all filenames that don't end in .a. How can I do this? Ideally I want something simple I can stick between the two commands in a pipe, like foo | filter-lines ".a" | xargs bar.

  • 5
    You should read up on the 'text processing' tag, and learn some or all of the following tools: grep, sed, awk; and also cut, paste, join. For the postgraduate text-processing tool you can learn Perl, but in 99% of cases you can accomplish everything you want with a simple grep or sed one-liner. – Wildcard Jun 6 '16 at 20:58
  • how is it even possible to know about something relatively obscure like xargs but not grep or sed? – cas Jun 7 '16 at 4:28
  • @cas Completely forgot that grep would view the standard input as a file rather than checking each file's contents. – Jack Jun 7 '16 at 10:26
7

You can use grep to grab all files within foo that end with .a.

foo | grep "\.a$" | xargs -d'\n' -r bar

  • Hmm, wouldn't grep \\.a$ be better? Oh, I see someone else got there first – infixed Jun 6 '16 at 20:59
  • +1. you should use xargs -d'\n' bar for safety, in case any filenames have spaces or other annoying characters in them. If you're using GNU xargs, you should also use -r or --no-run-if-empty so that xargs doesn't run bar if the input is empty: xargs -d'\n' -r bar – cas Jun 7 '16 at 4:26
  • @cas - Thank you for the suggestions. I have edited the answer. – Peschke Jun 7 '16 at 4:44
  • if foo produced NUL-separated output, you could use xargs -0r (and grep -z) and it would work even with filenames that have embedded newlines...but you have to work with what you get. – cas Jun 7 '16 at 23:46

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