Work the Shell - Analyzing Log Files

by Dave Taylor

If you're running Apache, and you probably are, you've got a file called access_log on your server, probably in /etc/httpd or some similar directory. Find it (you can use locate or find if needed).

First, let's see how many hits you've received—that is, how many individual files have been served up. Use the wc program to do this:

$ wc -l access_log
   83764 access_log

Interesting, but is that for an hour or a month? The way to find out is to look at the first and last lines of the access_log itself, easily done with head and tail:

$ head -1 access_log - - [11/Jul/2006:16:00:59 -0600]
 ↪"GET /favicon.ico HTTP/1.1" 404 36717 "-" "-"
$ tail -1 access_log - - [11/Jul/2006:22:15:14 -0600]
 ↪"GET /individual-entry-javascript.js HTTP/1.1"
 ↪200 2374 "
↪"Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1;
↪.NET CLR 1.1.4322; .NET CLR 2.0.50727)"

These log file lines can be darn confusing, so don't panic if you look at that and become completely baffled. The good news is it's not important to know what every field details. In fact, all we care about is the date and time in square brackets, and the name of the individual file requested after the “GET” line.

Here you can see that the first line in the access log is from 11 July at 16:00:59 and the last line is from 11 July at 22:15:14. Calculate this out, and we're talking a window of about six hours and 15 minutes, or 375 minutes. Divide the total number of hits by this time passage, and we're seeing 223 hits per minute, or a pretty impressive traffic level of 3.7 hits per second.

The Most Popular Files Sent

The second common query is to ascertain which files are requested most often, and that's something we can ascertain with a quick call to awk to split that field from the log file lines, then a combination of sort and uniq with its ever-useful -c option.

Let's take this one step at a time.

If you go back to the log file line shown above, you'll find that it's the seventh field that contains that value, meaning we can extract it like this:

$ head access_log | awk '{print $7}'

When you have a long list of data like this, you can figure out the most popular individual occurrences by sorting everything, then using the uniq command to figure out how often each line occurs. Then use sort again, this time to sort the data from that, prefaced with the largest numeric value to the smallest.

Here's an intermediate result to help you see what's happening:

$ awk '{print $7}' access_log | sort | uniq -c | head
 535 /
26 //favicon.ico
   6 //signup.cgi
   1 /0-blog-pics/MVP-Combo_picture.jpg
   2 /0-blog-pics/address-book-import.jpg
   4 /0-blog-pics/adwords-psp-bids.png
  28 /0-blog-pics/aim-congrats-account.png
  28 /0-blog-pics/aim-create-screen-name.png
  38 /0-blog-pics/aim-delete-screenname-mac.png
  29 /0-blog-pics/aim-forget-password.png

All that's left is to sort it by most popular and axe all but the top few matches:

$ awk '{print $7}' access_log | sort | uniq -c | sort -rn | head
6176 /favicon.ico
5807 /styles-site.css
5733 /Graphics/header-paper2.jpg
5655 /Graphics/pinstripebg.gif
5512 /individual-entry-javascript.js
5458 /Graphics/marker-tray.gif
5366 /Graphics/help-button.jpg
5363 /Graphics/digman.gif
5359 /Graphics/delicious.gif
5323 /0-blog-pics/starbucks-hot-coffee.jpg

The first thing you'll notice is that this isn't pages but graphics. That's not a surprise, because just like most Web sites, my own has graphics shared across all pages, making the graphics more frequently requested than any given HTML page.

Fortunately, we can force the results to be HTML pages by simply using the grep program to filter the final results of the filter sequence:

$ awk '{print $7}' access_log | sort | uniq -c | sort -rn
 ↪| grep "\.html" | head
 446 /motorola_razr_v3c_and_mac_os_x_transfer_pictures_and_wallpaper.html
 355 /how_to_create_new_screen_names_on_aol_america_online.html
 346 /how_do_i_cancel_my_america_online_aol_account.html
 293 /pc_to_sony_psp_how_do_i_download_music.html
 206 /how_do_i_get_photos_and_music_onto_my_sony_psp.html
 198 /how_do_i_get_my_wireless_wep_password_for_my_sony_psp.html
 195 /cant_get_standalone_music_player_to_work_on_myspace.html
 172 /convert_wma_from_windows_media_player_into_mp3_files.html
 166 /sync_motorola_razr_v3c_with_windows_xp_via_bluetooth.html
 123 /how_do_i_create_a_new_screen_name_in_aol_america_online_90.html

(Yes, yes, I know that the URLs on this site are ridiculously long!)

Now, finally, I can see that the articles about the Motorola RAR phone, AOL screen names and Sony PSP are the most popular articles on the site. Remember, this is a slice for only about six hours too, so the RAZR article is actually being requested an impressive once a minute or so. Popular indeed!

I'm going to stop here now that you've had a taste of how basic Linux commands can be combined to extract useful and interesting data from an Apache log file. Next month, we'll look at one more statistic: how much aggregate data we've transferred. Then, we'll start looking at how to build a shell script that does these sorts of calculations with ease.

Dave Taylor is a 26-year veteran of UNIX, creator of The Elm Mail System, and most recently author of both the best-selling Wicked Cool Shell Scripts and Teach Yourself Unix in 24 Hours, among his 16 technical books. His main Web site is at

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