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Tip: Simple Regular Expressions For Reviewing Log Files

Cutting Through the Noise With grep

  • November 19, 2008
  • By Carla Schroder

So there you are with all of your Linux servers humming along happily. You have tested, tweaked, and configured until they are performing at their peak of perfection. Users are hardly whining at all. Life is good. You may relax and indulge in some nice, relaxing rounds of TuxKart. After all, you earned it.

Except for one little remaining chore: monitoring your log files. [insert horrible alarming music of your choice here.] You're conscientious, so you know you can't just ignore the logs until there's a problem, especially for public services like Web and mail. Somewhere up in the pointy-haired suites, they may even be plotting to require you to track and analyze all sorts of server statistics.

Crafting clever, complex regular expressions is quite fun, but there are many simple searches that do the job just fine.


Not to worry, for there are many ways to implement data reduction, which is what log parsing is all about. You want to slice and dice your logs to present only the data you're interested in viewing. Unless you wish to devote your entire life to manually analyzing log files. Even if you only pay attention to logfiles when you're debugging a problem, having some tools to weed out the noise is helpful.

Good Ole grep

The simplest method is a keyword search. Suppose you want to separate out the 404 errors in your Apache log, and see if you have any missing files:

$ grep 404 bratgrrl.com-Aug-2004
...
212.27.41.34 - - '30/Aug/2004:02:25:13 -0700' "GET /robots.txt HTTP/1.0" 404 - "-"
Pompos/1.3 http://dir.com/pompos.html"
65.54.188.90 - - '30/Aug/2004:10:32:26 -0700' "GET /robots.txt HTTP/1.0" 404 - "-"
"msnbot/0.11 (+http://search.msn.com/msnbot.htm)"
207.65.113.58 - - '12/Aug/2004:06:49:11 -0700' "GET /favicon.ico HTTP/1.1" 404 - "-"
"Opera/7.21 (X11; Linux i686; U) 'en'"
...

These entries are typical. This site has no robots.txt or favicon, so any requests for these files generate a 404 error. The first two are Web bots. The third entry is probably some random surfer. You can ignore these. So let's screen out robots.txt and favicon, and see what is left:

$ grep 404 bratgrrl.com-Aug-2004 | grep -v -E "favicon.ico|robots.txt"
....
200.16.116.3 - - '29/Aug/2004:20:59:27 -0700' "GET /images/142spacer.gif HTTP/1.0"
404 - "http://www.bratgrrl.com/" "Mozilla/5.0 Galeon/1.2.7 (X11; Linux i686; U;) Gecko/20030131"
200.16.116.3 - - '29/Aug/2004:21:00:08 -0700' "GET /email_crimes.html HTTP/1.0" 404
- "http://www.bratgrrl.com/" "Mozilla/5.0 Galeon/1.2.7 (X11; Linux i686; U;) Gecko/20030131"
....

Now we're getting somewhere. These two files — images/142spacer.gif and email_crimes.html — are referenced somewhere on the Web site, but they do not exist. This is something that should be fixed. How to find the URLs that refer to these files? grep can do this too. Suppose all the site files are in /var/www/bratgrrl:

$ grep -R "142spacer.gif" /var/www/bratgrrl

Here's another cool grep trick for Apache logs. You doubtless noticed that the above examples were referred from http://www.bratgrrl.com. When you're checking to see where your traffic is coming from, you don't care about local referrals. Weed them out with this:

$ cat bratgrrl.com-Aug-2004 | fgrep -v bratgrrl | cut -d\" -f4 | grep -v ^-
http://www.computerbits.com/archive/2004/0800/schroder0408.html
http://www.pdxlinux.org/resources/nw_linux
www.dianagaydon.com/
http://www.netcraft.com/survey/
http://www.techsupportforum.com/computer/topic/3520-1.html
http://us.altavista.com/web/results?tlb=1&kgs=0&ienc=utf8&q=carla+schroder

Now you can see where traffic to your site is coming from, uncluttered by local references. Here's how it works, piece by piece:

  • fgrep -v bratgrrl means "look for the literal string bratgrrl, then exclude lines that contain it."
  • cut -d\" -f4 means "using quotation marks as the delimiter, print only the text in the fourth field." The fourth field is the text between the third and fourth quotation marks.
  • grep -v ^- means "exclude lines that start with a hyphen." Try running the command without this to see why.

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