Perl Hacks

Hack #20. Read Files Backwards

Process the most recent lines of a file first.

Perl's position in system administration is stable and secure, due in no small part to its fast and flexible text-processing abilities. If you need to slice and dice log files, monitor services, and send out messages, you could glue together the perfect combination of shell and command-line utilities, or you could have Perl do it.

Of course, Perl is a general-purpose language and doesn't always provide every tool you might need by default. For example, if you find yourself processing system logs often, you might wish for a way to read files in reverse order, the most recent line first. Sure, you could slurp all of the lines into an array and read the last one, but on a busy system with lots of huge logs, that can be slow and memory-consuming.

Fortunately, there's more than one way to do it.

The Hack

Yes, you could look up perldoc -F -X and find a file's size and read backwards until you find the appropriate newline and then read forward...but just install File::ReadBackwards from the CPAN instead.

Suppose you have a server process that continually writes its status to a file. You only care about its current status (at least for now), not its historical data. If its status is up, everything is happy. If its status is down, you need to panic and notify everyone, especially if it's 3 a.m. on Boxing Day.

Simulate the program that writes its logs with:

#!/usr/bin/perl use strict; use warnings; use Time::HiRes 'sleep'; ...