How to use tail -f with grep to show surrounding lines

Question

I would like to see the output in a logfile greped by only one domain but also the following two lines.

Example:

tail -f /var/log/apache2/modsec_audit.log |grep mydomain.de

this shows all lines, that contain "mydomain.de" but the important information is in the line below the line, where the domain is included

Answer 1

grep has extra options to define how many lines before and after the result:

• -A (after)
• -B (before)
• -C (context [before + after])

So in your case you need -A:

YOUR_COMMAND |grep -A NUMBER YOURDOMAIN

the above command prints NUMBER of lines after YOURDOMAIN in file.

Answer 2

Try passing number of context lines to grep.

| grep -C 5

will print 5 lines before and after the match.