How can I get the return value and matched line by grep in bash at once?

Question

I am learning bash. I would like to get the return value and matched line by grep at once.

if cat 'file' | grep 'match_word'; then
  match_by_grep="$(cat 'file' | grep 'match_word')"
  read a b <<< "${match_by_grep}"
fi

In the code above, I used grep twice. I cannot think of how to do it by grep once. I am not sure match_by_grep is always empty even when there is no matched words because cat may output error message.

match_by_grep="$(cat 'file' | grep 'match_word')"
if [[ -n ${match_by_grep} ]]; then
  # match_by_grep may be an error message by cat.
  # So following a and b may have wrong value.
  read a b <<< "${match_by_grep}"
fi

Please tell me how to do it. Thank you very much.

As a side note, it's preferable to use `grep pattern file` rather than `cat file | grep pattern` — Aaron, Nov 17 '16 at 10:37

score 2 · Accepted Answer · answered Nov 17 '16 at 10:38

You can avoid the double use of grep by storing the search output in a variable and seeing if it is not empty.

Your version of the script without double grep.

#!/bin/bash

grepOutput="$(grep 'match_word' file)"

if [ ! -z "$grepOutput" ]; then
    read a b <<< "${grepOutput}"
fi

An optimization over the above script ( you can remove the temporary variable too)

#!/bin/bash

grepOutput="$(grep 'match_word' file)"

[[ ! -z "$grepOutput" ]] && (read a b <<< "${grepOutput}")

Using double-grep once for checking if-condition and once to parse the search result would be something like:-

#!/bin/bash

if grep -q 'match_word' file; then
    grepOutput="$(grep 'match_word' file)"
    read a b <<< "${grepOutput}"
fi

score 1 · Answer 2 · answered Aug 10 '17 at 20:04

When assigning a variable with a string containing a command expansion, the return code is that of the (rightmost) command being expanded.

In other words, you can just use the assignment as the condition:

if grepOutput="$(cat 'file' | grep 'match_word')"
then
  echo "There was a match"
  read -r a b <<< "${grepOutput}"
  (etc)
else
  echo "No match"
fi

score 0 · Answer 3 · edited May 23 '17 at 10:30

0

Is this what you want to achieve?

grep 'match_word' file ; echo $?

$? has a return value of the command run immediately before.
If you would like to keep track of the return value, it will be also useful to have PS1 set up with $?.

Ref: Bash Prompt with Last Exit Code

edited May 23 '17 at 10:30

Community

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1

answered Nov 17 '16 at 10:30

Ryota

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thank you for answering. I would like to read(or analyze) matched line only when the file has the line including matched word. To do it, I used grep twice, but I feel it is not efficient. I would like to do it by only one grep if possible. – mora Nov 17 '16 at 10:36
@mora sorry misunderstood your intention - if you would like to analyze further on grep output only when matched word is found, you can evaluate $? right after the grep: – Ryota Nov 17 '16 at 11:12
@Ryota : I thank you for answering to my ambiguous question. – mora Nov 17 '16 at 11:25

How can I get the return value and matched line by grep in bash at once?

3 Answers3