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I have a long list of strings separated by newlines and each section separated by a string starting with ~ (there are no other ~ in the file). I want to extend grep to find the ~ text right below the match.

Input:

abc
ads
acb
abc
acsa
acfs
~notthis
abc
ads
acb
xyz                <-- pattern
acsa
acfs
~this              <-- output
abc
ads
acb
abc
acsa
acfs
~no

Output when searching for xyz:

~ this

With awk I'd do something like

awk '/xyz/{x=1}x&&/~/{print;exit}' file

but I want to use grep if it can give me a performance advantage

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2 Answers 2

Reset to default
4

At least with GNU grep 2.12:

$ grep -Pzo 'xyz(?:.*\n)*?\K~this' file
~this

If your grep version does not allow -P with -z, you can use pcregrep instead:

$ pcregrep -Mo 'xyz(?:.*\n)*?\K~this' file
~this
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10
  • 1
    Thanks, but I get grep: The -P and -z options cannot be combined with GNU grep 2.5.2. Also, I don't know the text after ~ like you searched for ~this- this is my desired output (I only know it's following a ~)
    – confused00
    Oct 29, 2014 at 10:30
  • I replaced ~this in your pattern with .* and I get the right output, thanks. Do you know whether there's a performance disadvantage for using pcregrep instead of grep?
    – confused00
    Oct 29, 2014 at 10:38
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    I have a hard time to believe it is working. If one can use ~this literally then simple grep '~this' file will do the job, but this is not the point, right? To my mind it should be pcregrep -Mo 'xyz[^~]*?\K~.*' file | head -1. Also I don't believe any kind of grep expression can beat awk.
    – jimmij
    Oct 29, 2014 at 11:25
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    And after replacing ~this to .* does it work as expected for you, i.e. returns ~this? My version of pcregrep returns ~no two times, what is understandable due to greedy nature of *, so one has to at least add ? after *.
    – jimmij
    Oct 29, 2014 at 11:42
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    Test once again more carefully, most probably it matches many occurrences of the pattern, and you want only first, right?
    – jimmij
    Oct 29, 2014 at 12:06
1 
cat "$file" | grep xyz -A 10000 | grep \~ -m1

or, equivalently,

grep -A 10000 xyz "$file" | grep -m1 '~'
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3
  • This looks like it will produce the correct output, but I wonder whether it really performs better (i.e., runs faster) than awk. Have you tested that? Please post your benchmark results.
    – G-Man Says 'Reinstate Monica'
    Jun 30, 2022 at 17:08
  • I have not benchmarked it but I found a link that says, basically, since awk has more to do per line that grep is faster unix.stackexchange.com/questions/88503/using-grep-vs-awk
    – MarkTheRusty
    Jul 4, 2022 at 5:57
  • Well, yeah, but the OP’s suggested command runs one awk process, and your answer runs two grep processes.  So this raises the question: how much faster is grep than awk?  Twice as fast?  Also, the OP’s command exits immediately upon finding the desired match, whereas yours potentially goes 9999 lines beyond it.
    – G-Man Says 'Reinstate Monica'
    Jul 4, 2022 at 22:15

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