I have two files: one containing a list of IDs (file A) and other containing a list of IDs plus their corresponding text -which is always on the next line- (file B).

File A:


File B:

more text
more and more text
you get the idea

How can I extract only the IDs + their corresponding text from file B that match the IDs from file A?

Expected output:




With GNU grep, FreeBSD grep:

grep -A1 -Ff fileA fileB
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  • Tried it on my files and didn't work.. Gives me a blank output. Is there any other way? – Manuel May 7 '18 at 3:48
  • @Manuel does fileA contains id the same string as id in fileB? -F causes the id line in fileA matches the id line in fileB, if it's substring, or pattern, then it doesn't match – cuonglm May 7 '18 at 3:51
  • FileA's and B's IDs are exact same strings. Not substrings nor patterns. – Manuel May 7 '18 at 4:02
  • @Manuel So checking if you have trailing whitespaces? I tested with your input and it give me desired output. Also make sure fileA is the one contains list of ids, and the fileA fileB order in command line is a must. – cuonglm May 7 '18 at 4:06
  • Yeah, I also tried with the example I provided and it worked. Weird. Already checked for whitespaces and everything is ok. FileA contains 535 IDs and fileB contains 3736 IDs + 3736 lines of text, one after each ID. Can size be an issue when using your command? – Manuel May 7 '18 at 4:16

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