Cultured Perl: Automating UNIX System Administration with Perl

By Teodor Zlatanov
2004-07-15

Task Automation

Task automation is an extremely broad topic. I will limit this section to only simple automation of non-interactive UNIX commands. For automation of interactive commands, Expect is the best tool currently available. You should either learn its syntax, or use the Perl Expect.pm module. You can get Expect.pm from CPAN; see Resources for more details.

With cfengine, you can automate almost any task based on arbitrary criteria. Its functionality, however, is a lot like the Makefile functionality in that complex operations on variables are hard to do. When you find that you need to run commands with parameters obtained from a hash, or through a separate function, it's usually best to switch to a shell script or to Perl. Perl is probably the better choice because of its functionality. You shouldn't discard shell scripts as an alternative, though. Sometimes Perl is overkill and you just need to run a simple series of commands.

Automating user addition is a common problem. You can write your own adduser.pl script, ), --> or you can use the adduser program provided with most modern UNIX systems. Make sure the syntax is consistent between all the UNIX systems you will use, but don't try to write a universal adduser program interface. It's too hard, and sooner or later someone will ask for a Win32 or MacOS version when you thought you had all the UNIX variants covered. This is one of the many problems that you just shouldn't solve entirely in Perl, unless you are very ambitious. Just have your script ask for user name, password, home directory, etc. and invoke adduser with a system() call.

Listing 4: Invoking adduser with a simple script

#!/usr/bin/perl -w



use strict;



my %values;                             # will hold the values to fill in



# these are the known adduser switches

my %switches = ( home_dir => '-d', comment => '-c', group => '-G',

                 password => '-p', shell => '-s', uid => '-u');



# this location may vary on your system

my $command = '/usr/sbin/adduser ';



# for every switch, ask the user for a value

foreach my $setting (sort keys %switches, 'username')

{

 print "Enter the $setting or press Enter to skip: ";

 $values{$setting} = ;

 chomp $values{$setting};

 # if the user did not enter data, kill this setting

 delete $values{$setting} unless length $values{$setting};

}



die "Username must be provided" unless exists $values{username};



# for every filled-in value, add it with the right switch to the command

foreach my $setting (sort keys %switches)

{

 next unless exists $values{$setting};

 $command .= "$switches{$setting} $values{$setting} ";

}



# append the username itself

$command .= $values{username};



# important - let the user know what's going to happen

print "About to execute [$command]";



# return the exit status of the command

exit system($command);

Another task commonly done with Perl is monitoring and restarting processes. Usually, this is done with the Proc::ProcessTable CPAN module, which can go through the entire process table, and give the user a list of processes with many important attributes. Here, however, I must recommend cfengine. It offers much better process monitoring and restarting options than a quick Perl tool does, and if you get serious about writing such a tool, you are just reinventing the wheel (and cfengine is stealing your hubcaps). If you do not want to use cfengine for your own reasons, consider the pgrep and pkill utilities that come with most modern UNIX systems. pkill -HUP inetd will do in one concise command as much as a Perl script four or more lines long. This said, you should definitely use Perl if the process monitoring you are doing is very complex or time sensitive.

For the sake of completeness, here is a Proc::ProcessTable example that shows how to use the kill() Perl function. The "9" as a parameter is the strongest kill() argument, meaning roughly "kill process with extreme prejudice, then feed it to the piranhas." Do not run this as root, unless you really want to kill your inetd processes.

Listing 5: Running through the processes, and killing all inetds

use Proc::ProcessTable;

$t = new Proc::ProcessTable;

foreach $p (@{$t->table})
{
# note that we will also kill "xinetd" and all processes
# whose command line contains "inetd"
kill 9, $p->pid if $p->cmndline =~ 'inetd';
}

Tutorial Pages:
» A Centralized Configuration File Strategy
» The Tool Cfengine
» Configuration File Management
» Task Automation
» Summary
» Resources

First published by IBM DeveloperWorks

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