Take the 2-minute tour ×
Unix & Linux Stack Exchange is a question and answer site for users of Linux, FreeBSD and other Un*x-like operating systems.. It's 100% free, no registration required.

So I have a bunch of Apache logs with using the standard log format. I want to get all the log lines that did not come from a web crawler.

So lets say I have a file robot_patterns with entries like


If I run the command grep -f robot_patterns *.log I will get all the entries by bots matching the above patterns. My actual list has ~30 entries of bots and agents that I wish to ignore.

But I want to find all the entries that are NOT from bots. So I try grep -v -f robot_patterns *.log and no results are returned by grep. This is not what I expect or desire, and I am not finding an obvious way to get what I want. When using the -v option combined with multiple patterns in a file, grep will only return a matching line if it matches EVERY pattern.

share|improve this question
When I tried this on my system, grep -v -f had the desired behavior, only returning lines that matched none of the patterns. This was with (GNU grep) 2.14.56-1e3d. What grep are you using? –  wingedsubmariner Dec 11 '13 at 23:36
I am running GNU grep 2.6.3. –  Zoredache Dec 11 '13 at 23:42
I did some more testing, and found that if there is an empty line in the patterns file it will match every line, causing no lines to be returned with -v. This isn't a problem with -F however, and -F might speed up grep for your task - might be worth trying for you. –  wingedsubmariner Dec 11 '13 at 23:59
A trailing empty line! Argh... That seems to be be the issue. If you want, you should add that as answer. –  Zoredache Dec 12 '13 at 0:10

1 Answer 1

up vote 4 down vote accepted

If there is an empty line in the patterns file it will match every line, causing no lines to be returned with -v. This is because the lines are interpreted as regular expressions, and an empty regular expression will always match.

This isn't a problem with -F however, because grep ignores empty lines with -F. -F causes grep to interpret the lines as simple strings to search for, and may speed up grep if regular expressions aren't needed.

share|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.