Perl for System Administration

Unix Process Control

Strategies for Unix process control offer another multiple-choice situation. Luckily, these choices aren’t nearly as complex to introduce as those offered by NT. When we speak of process control under Unix, we’re referring to three operations:

1. Enumerating the list of running processes on a machine

2. Changing their priority or process group

3. Terminating the processes

For the final two of these operations, there are Perl functions to do the job: setpriority( ), setpgrp( ), and kill( ). The first one offers us a few options. To list running processes, you can:

• Call an external program like ps

• Take a crack at deciphering /dev/kmem

• Look through the /proc filesystem

• Use the Proc::ProcessTable module

Let’s discuss each of these approaches. For the impatient reader, I’ll reveal right now that Proc::ProcessTable is my preferred technique, and you might just skip directly to the discussion of that module. But I recommend reading about the other techniques anyway, since they may come in handy in the future.

Calling an External Program

Common to all modern Unix variants is a program called ps, used to list running processes. However, ps is found different places in the filesystem on different Unix variants and the command-line switches it takes are also not consistent across variants. Therein lies one problem with this option: it lacks portability.

An even more annoying problem is the difficulty in parsing the output (which also varies from variant to variant). Here’s a snippet ...