A plugin (monitoring-plugin, not nagios-plugin, see also http://is.gd/PP1330) which scans logfiles for patterns.
check_logfiles is a Plugin for Icinga which scans log files for specific patterns.
The conventional plugins which scan log files are not adequate in a mission critical environment. Especially the missing ability to handle logfile rotation and inclusion of the rotated archives in the scan allow gaps in the monitoring. Check_logfiles was written because these deficiencies would have prevented Nagios from replacing a propritetary monitoring system.
nagios$ check_logfiles --logfile /var/adm/messages \
--criticalpattern 'Failed password' --tag ssh
CRITICAL - (4 errors) - May 9 11:33:12 localhost sshd[29742] Failed password for invalid user8 ... |ssh_lines27 ssh_warnings=0 ssh_criticals=4 ssh_unknowns=0
The full documentation can be found here: check_logfiles @ ConSol Labs