How do I fetch lines before/after the grep result in bash?
BashShellUbuntuBash Problem Overview
I want a way to search in a given text. For that, I use grep:
grep -i "my_regex"
That works. But given the data like this:
This is the test data
This is the error data as follows
. . .
. . . .
. . . . . .
. . . . . . . . .
Error data ends
Once I found the word error (using grep -i error data), I wish to find the 10 lines that follow the word error. So my output should be:
. . .
. . . .
. . . . . .
. . . . . . . . .
Error data ends
Are there any way to do it?
Bash Solutions
Solution 1 - Bash
You can use the -B and -A to print lines before and after the match.
grep -i -B 10 'error' data
Will print the 10 lines before the match, including the matching line itself.
Solution 2 - Bash
This prints 10 lines of trailing context after matching lines
grep -i "my_regex" -A 10
If you need to print 10 lines of leading context before matching lines,
grep -i "my_regex" -B 10
And if you need to print 10 lines of leading and trailing output context.
grep -i "my_regex" -C 10
Example
user@box:~$ cat out
line 1
line 2
line 3
line 4
line 5 my_regex
line 6
line 7
line 8
line 9
user@box:~$
Normal grep
user@box:~$ grep my_regex out
line 5 my_regex
user@box:~$
Grep exact matching lines and 2 lines after
user@box:~$ grep -A 2 my_regex out
line 5 my_regex
line 6
line 7
user@box:~$
Grep exact matching lines and 2 lines before
user@box:~$ grep -B 2 my_regex out
line 3
line 4
line 5 my_regex
user@box:~$
Grep exact matching lines and 2 lines before and after
user@box:~$ grep -C 2 my_regex out
line 3
line 4
line 5 my_regex
line 6
line 7
user@box:~$
Reference: manpage grep
-A num
--after-context=num
Print num lines of trailing context after matching lines.
-B num
--before-context=num
Print num lines of leading context before matching lines.
-C num
-num
--context=num
Print num lines of leading and trailing output context.
Solution 3 - Bash
The way to do this is near the top of the man page
grep -i -A 10 'error data'
Solution 4 - Bash
Try this:
grep -i -A 10 "my_regex"
-A 10 means, print ten lines after match to "my_regex"