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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.

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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

2 Answers 2

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.

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GNU fgrep ignoring that trailing empty string was a bug that was fixed in 2.19 (commit 2d3832e1ff772dc1a374bfad5dcc1338350cc48b , so you shouldn't rely on it. – Stéphane Chazelas Oct 29 at 11:17

You can try:

grep -vE 'Googlebot|msnbot-media|YandexBot|bingbot' yourlogfile
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Welcome to Unix & Linux.  The OP has a list of approximately 30 strings that he wants to ignore, and the four that he presented as examples have an average length of ten characters each, so your command is likely to be over 300 characters long.  This is likely to be hard to maintain (and even to read).  Can you modify your answer to be driven by the OP’s list of strings?  … … … … … … … … … … … … … … … … … … … …  P.S. Did you notice that the answer has been found? — The OP has learned how to get his original approach to work. – G-Man Oct 30 at 11:33
Why negatively evaluate my response? : / – Orsius Oct 31 at 10:17

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