Question

I am working on a UNIX box, and trying to run an application, which gives some debug logs to the standard output. I have redirected this output to a log file, but now wish to get the lines where the error is being shown.

My problem here is that a simple

cat output.log | grep FAIL 

does not help out. As this shows only the lines which have FAIL in them. I want some more information along with this. Like the 2-3 lines above this line with FAIL. Is there any way to do this via a simple shell command? I would like to have a single command line (can have pipes) to do the above.

Answer 1

grep -A $NUM

This will print $NUM lines of trailing context after matches.

-B $NUM prints leading context.

man grep is your best friend.

So in your case:

cat log | grep -A 3 -B 3 FAIL