Monitoring and Alerting on Patterns in Linux Logfiles

As a sysadmin, I've found it's always useful to monitor system logs on your Linux or Unix servers for specific patterns of activity, things that can indicate security or system issues. Even nicer to get alerts when activity occurs. I know there are tools that will do this, but they tend to be all-inclusive and complex. I wrote this perl script that monitors any logfile for a specific pattern and generates email or syslog alerts because I had the need to alert on one specific pattern, and wanted something self-contained that was easy to configure.

Installing Logmon

It needs a few non-core Perl modules to run, namely Mail::Mailer, Proc::Daemon, Unix::Syslog and File::Tail, but these can be installed pretty easily as packaged modules or via CPAN. On Debian/Ubuntu systems, all the needed modules are pre-packaged for you:

apt-get install libmailtools-perl libunix-syslog-perl libfile-tail-perl libproc-daemon-perl

On Red Hat/Fedora servers, you can use yum:

yum install perl-MailTools perl-Unix-Syslog perl-File-Tail perl-Proc-Daemon

To pull in all the modules from CPAN, you can use this one-liner:

for m in Mail::Mailer Proc::Daemon Unix::Syslog File::Tail; do perl -MCPAN -e "install $m"; done

Once the modules are installed, download the script and double check that it will run without error, then copy it to your path and make it executable. You should see no errors about missing modules when you run the script under 'perl -cwT'.

perl -cwT ./ install -m 755 /usr/local/bin/

Running Logmon

It's meant to be both simple and secure, here is the usage summary: synopsis: Daemon that periodically checks logfile for a pattern and send alerts Pattern is always required. If no other options are given, defaults to syslog alerts and monitors /var/log/messages for given pattern. Usage: -p pattern [-m] [-f logfile] [-u run as user] [-g run as group] [-i max interval] [-v] [-d] [-h] -m: Email destination for alerts -f: logfile to monitor -p: Pattern to match against lines in logfile (Perl regexp, match is case-insensitive) -u: Run with permissions of user -g: Run with permissions of group -i: Max time to sleep between checks -d: Debug output to STDERR, do not daemonize -v: Verbose logging (use with caution or you may have endless alerts) -h: This help text

Running the script is pretty straightforward, you specify a pattern to match against with -p (this is the only required parameter), and optionally an email recipient (-m) and logfile to watch (-f). Here is an example. Let's say you want to get alerts whenever MySQL detects a crashed table. The syslog event for this looks like this on my Ubuntu box:

Jul 17 08:02:49 kaylee mysqld[1532]: 110717 8:02:49 [ERROR] /usr/sbin/mysqld: Table './mysql/user' is marked as crashed and should be repaired

And here is the command line usage: -p 'mysqld.+?table.+?crashed' -m -u nobody -g adm -f /var/log/syslog -i 30

Note that the pattern match is case-insensitive. When run this way, Logmon will detach itself from your terminal and run as a daemon, checking /var/log/syslog every 30 seconds for the supplied pattern. I recommend you use the -u and -g options to force Logmon to drop it's privileges, just make sure you specify a user or group that have read-permissions on the specified logfile. On Debian and Ubuntu servers, all the system logs are readable by the group 'adm'.

Other Options and Tips for Testing Logmon

Logmon will dump alerts to your default system log if you leave off the -m option. If you also use the -v option for verbose logging, these syslog entries and alerts will have pattern and match data. If you end up monitoring the same file you are dumping alerts into you'll get an endless series of alerts continuously being added to the system log. For this reason when alerts are sent to syslog, by default they are very generic (email alerts are always verbose).

To test Logmon, use the -v and -d options together. Run this way, Logmon will not daemonize itself, and will just print alerts and activity to your console (STDERR).


Any errors that cause the script to die while it's running as a daemon can be found in your system log.

Startup and Shutdown

I haven't yet written any startup scripts for Logmon, although I plan to. For now, just start it from one of your system's startup scripts, perhaps /etc/rc.local, and if you have to stop it you can just use pkill logmon. Please send any bugs or suggestions for improvement to the email address in the script header, or leave a comment here.