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Perl One-Liner of the Month: The Adventure of the Spicy Blonde By Ben Okopnik

It was a blonde. A blonde to make a bishop kick a hole in a stained glass window. 
 -- Raymond Chandler

Frink staggered into Woomert's office, somewhat pop-eyed.

- "Well, Frink - that's quite a description. First, take a deep breath... that's it, again... one more time... good. Now, what's it all about?"

- "Right. Well, for one thing, she's really upset; her Web server died, her hard drive crashed, her SCSI controller bit the dust - before you say anything, though, the hardware's been fixed already. The problem is that they still can't get the server up - it keeps dying on startup - and she says that someone's been stealing her domain names! That's all I got, though: she's, uh, a little distracting, especially when she's angry... you'll see what I mean when you talk to her."

- "When you talk to her and I observe, you mean. I meant it when I said that I wanted you to be my partner, Frink; you'll need to learn all the aspects of being an Internet Detective, and this sounds like the perfect opportunity."

Woomert stood up and walked across the room, and stood by the door, obviously waiting for Frink to go through first.

Frink looked at him for a few seconds, then took a deep breath, squared his shoulders and nodded. In all the time that the two had known each other, Woomert had never known the younger man to look so serious, or so intent on doing the right thing. He stepped into the living room just after Frink, and quietly closed the door.

Woomert's first thought was that Frink had not exaggerated. The woman in front of him, dressed in a red leather ultra-miniskirt and a white silk blouse that was at least one size too small, was quite distracting - stunning, actually. Woomert's eyes briefly swept over her generous curves and came to rest on her eyes, blue as sapphires of the first water... and, just like those sapphires, full of fire. She had been paying only cursory attention to Frink, and now turned the full force of her personality on Woomert.

- "Just a moment, Ms. Bombshell." Woomert stood easily, hands in his pockets and leaning against the wall. "You seem to be laboring under a misconception; I am indeed the senior partner here, but I've recently" - there was no need to specify just how recently - "begun acting in a largely advisory capacity, while my partner, Frink Oooblick, does most of the work." He smiled slightly. "Rank hath its privileges, time goes on. Fear not: Frink is quite capable, and can call on me if the need should arise - although I don't expect that to happen."

- "Ms. Bombshell." There was obvious frost in Woomert's voice. "If you wish to deal with us, then you need to know this: I and my partner behave in a civil manner toward our clients, and we require the same in return. I have stated what is on offer; it is up to you to accept or find help elsewhere. Being offensive will simply limit your options to the latter."

- "Then I suggest you abide by the rules I've stated." Woomert crossed to the pantry and began filling the kettle with spring water. "I'll be making tea. Shall I make you a cup, or have you decided to leave?"

- "Two teaspoons of brown sugar, if you've got it. Or I can live with the white stuff if I have to. You're a tough customer, you know that?"

Auburn snorted and settled back into the armchair, cradling her cup. "OK, so this isn't exactly business attire. I just thought you boys would be a little more... manageable this way." She glanced at Frink, who was still standing by the table in the center of the room. "Come on over and sit down next to me - Frink, is it? I won't bite." Her eyes glittered with amusement. "Not with Mr. Tough here watching, anyway. Or without you asking me first."

Frink walked over and sat down on the far end of the couch. Auburn glanced at him appraisingly for a moment, then spoke. Her voice held no trace of levity, and she was all business.

- "Well, as I was saying - I host a bunch of these things, and now Apache won't come up any more! My techs have tried and tried, but they got nowhere: it seems to be beyond them. They don't know which sites are down without bringing up the server, and they can't bring up the server without... well, you can see the problem. One thing they've mentioned is that there's certainly more than one dead site, at least a dozen - but not much more than that. All I want for now is to get the working ones up, and I'll get the broken ones fixed later - in fact, I know I'll have to restore some of them from tape, since the hardware took a chunk of data with it when it died." Auburn shifted in the chair until she was looking directly at Frink. "What do you think, Big Guy? Can you handle it? The SSH server is still running, so we can still log in."

- "Well, Miss Bombshell..."

- "Right. Well... Auburn... let's just log in and see what's going on." Frink walked over to his machine and logged in. After a bit of quick exploration of relevant files, he tried to start the webserver which failed as advertised, and looked through the log file - indeed, there was a list of error messages, but all of them pointed to the first site that was failing. If he were to fix that one, the server still would not come up - the errors would now simply report the next broken site, an iterative process that could take a long, long time. He then looked through the server configuration file, ``/etc/apache/httpd.conf''.

As he was about to start typing, Frink felt a tap on his shoulder. Looking around he saw Woomert extending... oh, good grief! Those were Woomert's favorite typing gloves! Frink took a deep breath, nodded at Woomert and pulled them on, very conscious of the implied honor.

perl -00wne'/^<V/||next;open F,">",++$a or die"$a: $!\n";print F' httpd.conf

perl -i -wlpe'if(/^<V/){print"Include $_"for 1..509;exit}' httpd.conf

- "Excellent, Frink!" Woomert was smiling. "What's your next move?"

- "Well, now that I've isolated the vhosts, I get to do a little trouble-shooting - that is, I need to figure out which of these hosts are stopping Apache from coming up. Fortunately, from what Auburn told us, there aren't that many - so it makes sense for me to run a binary split, a.k.a. ``divide and conquer''. First, let's make sure that Apache itself is OK, i.e., that it will come up when all the vhosts are disabled:

perl -i -wpe's/^(?=Include)/# /' httpd.conf

- "Won't that take a long time?"

- "All right - now we know there's a bad vhost somewhere in this bunch. So, we'll comment out half of the vhosts we've just enabled, and... the server comes up, which tells us that the bad vhost is in the group that we've just commented out. Re-enable half of that, and..."

Frink grinned and blushed again, typing all the while. After a few minutes, he had located fourteen broken vhosts, which he commented out. With a sigh of satisfaction, he started the Apache server for the last time.

- "Oh, I'm sure of it - now that you've fixed the main problem. Say, what about somebody stealing my domain names? I tried a few of them while they were down, and came up with some kind of an advertising page! What's going on there?"

Auburn squealed happily, reached around the chair, hugged Frink, and planted a kiss on his cheek, then jumped up and grabbed her purse.

Frink sat on the arm of the couch and flipped over it to the other side, ending up sitting comfortably. He looked somewhat pensive.

- "Oh, nothing significant; a matter of style. Five hundred-some vhosts isn't a terribly large number - I think I would have run a loop that added vhost ``includes'' one at a time to ``httpd.conf'', restarted the server, and commented the last entry if it failed. This way, the identification would be completely automatic, and it wouldn't matter if the number of bad vhosts was high relative to the total - a scenario where a binary search would become more time-intensive than a linear one. However, your approach works better where the total is large and the number of bad spots relatively small, which was the case here.

Frink straightened up, smiling.

All right, then, here we go. We'll start with my first one-liner:

perl -00wne'/^<V/||next;open F,">",++$a or die"$a: $!\n";print F' httpd.conf

<VirtualHost ip.address.of.host.somedomain.com>
    ServerAdmin webmaster@host.somedomain.com
    DocumentRoot /www/docs/host.somedomain.com
    ServerName host.somedomain.com
    ErrorLog logs/host.somedomain.com-error.log
    CustomLog logs/host.somedomain.com-access.log common
</VirtualHost>

<VirtualHost ip.address.of.host.somedomain.com>
    ServerAdmin webmaster@host.somedomain.com
    DocumentRoot /www/docs/host.somedomain.com
    ServerName host.somedomain.com
    ErrorLog logs/host.somedomain.com-error.log
    CustomLog logs/host.somedomain.com-access.log common
</VirtualHost>

...

- "Well, I wanted to skip everything in there until I did get to the "vhosts" section. Having glanced through Auburn's ``httpd.conf'', I knew that nothing in there began with ``<V'' until you got to ``vhosts'' - so I just told Perl to skip everything until then; ``/^<V/||next'' says ``unless the current line begins with a "<V", jump to the next line.'' Once it got to the right section, it would actually become a double-check: all the paragraphs we wanted were supposed to begin with a ``<V''.

Woomert laughed happily. "I'm proud of you, Frink. You've come a long way, just as I'd said before. Go ahead and run over the rest of what you've done; I have a wonderful lunch almost ready for us, but we've still got a few minutes."

"Well, the rest is even easier - although admittedly a bit more dangerous.

perl -i -wlpe'if(/^<V/){print"Include $_"for 1..509;exit}' httpd.conf

- "Well, the last one is pretty simple - although it uses a feature of the regular expression set that beginners might not know about: one of the so-called "extended" patterns, a ``positive look-ahead assertion''.

perl -i -wpe's/^(?=Include)/# /' httpd.conf

Now, for lunch, we've got a bit of Dutch-Indonesian cuisine: I'm setting out a rijstaffel - ``rice table'' to the uninitiated, the idea being something like a Swedish smorgasbord, a pick-and-choose sort of thing. There are eight different kinds of satay (tasty skewers of various meats), ajam panggang (a sort of barbecued chicken) with nasi goreng (fried rice), gado-gado salad, and of course lots of peanut sauce, ketjap manis, and chili paste if you want a bit more spice, always a doubtful proposition with, erm, properly-seasoned Indonesian food. There's also Thai iced coffee to follow - not Dutch or Indonesian, but I find it does well for putting out the fire!"

Ben is a Contributing Editor for Linux Gazette and a member of The Answer Gang.

Ben was born in Moscow, Russia in 1962. He became interested in electricity at age six--promptly demonstrating it by sticking a fork into a socket and starting a fire--and has been falling down technological mineshafts ever since. He has been working with computers since the Elder Days, when they had to be built by soldering parts onto printed circuit boards and programs had to fit into 4k of memory. He would gladly pay good money to any psychologist who can cure him of the resulting nightmares.

Ben's subsequent experiences include creating software in nearly a dozen languages, network and database maintenance during the approach of a hurricane, and writing articles for publications ranging from sailing magazines to technological journals. Having recently completed a seven-year Atlantic/Caribbean cruise under sail, he is currently docked in Baltimore, MD, where he works as a technical instructor for Sun Microsystems.

Ben has been working with Linux since 1997, and credits it with his complete loss of interest in waging nuclear warfare on parts of the Pacific Northwest.