Another grep question - UNIX for Dummies Questions & Answers

kingdbag · #1 (**permalink**) 10-26-2006

Say I want to grep for a line with EXEC in it

grep EXEC blah.conf

thats cool but what if i want to grep for EXEC and if it finds a line with that in it to also grab the two lines above it?

BOFH · #2 (**permalink**) 10-26-2006

Depends on the OS. On my mac

Code:

$ grep -B 2 EXEC testfile
this is a test line 3
this is a test line 4
EXEC
--
this is a test line 7
this is a test line 8
EXEC
--
this is a test line 13
this is a test line 14
EXEC

Same with Linux (Mandrake was the test box).

Carl

blowtorch · #3 (**permalink**) 10-26-2006

That won't work anywhere else. You will have to probably use a combo of 'grep -n' and then sed to print the lines before your match. You end up scanning the input file too many times which makes it quite slow.

BOFH · #4 (**permalink**) 10-26-2006

Yep, that's what happens when you don't offer the OS you're talking about

It's also why I'll typically note which OS I'm offering the answer for. Technically you should be able to get GNU/grep for the major OSs' (like sunfreeware.com).

My Sun boxes aren't up and I don't want to VPN to work and check out the HP-UX or AIX boxes I have available to me for this question. If it's still up when I get back on-line to work tomorrow, I'll check them as well.

Carl

Ygor · #5 (**permalink**) 10-26-2006

If you don't have GNU grep...

Code:

[n]awk '{a[NR%p]=$0}$0~s{for(c=p;c>0;c--)print a[(NR-c+1)%p]}' p=3 s=EXEC blah.conf

kingdbag · #6 (**permalink**) 10-26-2006

Oh, I'm using Solaris 9 sorry about that!

ghostdog74 · #7 (**permalink**) 10-26-2006

Quote:

Originally Posted by kingdbag

Say I want to grep for a line with EXEC in it

grep EXEC blah.conf

thats cool but what if i want to grep for EXEC and if it finds a line with that in it to also grab the two lines above it?

Python alternative:
Sample input:
abc
def
ghi
EXEC
klm
nop
qrs
tuv
wxy

Code:

#!/usr/bin/python
data = open("input.txt").readlines()
for linenum , line in enumerate(data):
 	if 'EXEC' in line:
 		print ''.join(data[linenum - 2 : linenum])

output:

Code:

def
ghi

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