Grep this but not that with -C option

Question

The twist is that, I d like to grep the files around my search with -C5.

So the grep this myfile | grep -v that won't work.

example: let say myfile is as follows:

**
**
alpha
**
**
##
##
alpha beta
##
##

I d like to grep :

**
**
alpha
**
**

how would I achieve this?

anubhava · Accepted Answer · 2018-06-22T08:19:49.653

2

You can use this gnu grep with a PCRE regex:

grep -C2 -P '^(?!.*beta).*alpha' file

**
**
alpha
**
**

Regex '^(?!.*beta).*alpha' uses a negative lookahead to match a line that contains alpha anywhere but doesn't contain beta.

If gnu grep is not available:

awk '/alpha/ && !/beta/{print line[NR-2]; print line[NR-1]; n=NR}
     n && NR<=n+2; {line[NR]=$0}' file

**
**
alpha
**
**

edited Jun 22 '18 at 08:19

answered Jun 22 '18 at 07:10

anubhava

2

Your regex solution and its explanation is not correct. It must be `grep -C2 -P '^(?!.*beta).*alpha' file`, because `(?!.*beta)alpha` matches any line with `alpha` word that is not followed with `beta` anwhere *after* `alpha`, and `beta alpha` would get returned. – Wiktor Stribiżew Jun 22 '18 at 08:17
Yes good point Wiktor. It should have been `'^(?!.*beta).*alpha'` to match `alpha` anywhere. – anubhava Jun 22 '18 at 08:20

1 Answers1