Fire irule when condition met

Sample implementation:

/config/alert_user.conf

TestMe

vs

/var/log/ltm

alert st_exec_command "I sense something" {
  exec command="tmsh modify ltm virtual vs rules { TestMe }"
}

Send a test log message:

 logger -p local0.info "I sense something; a presence I have not felt since..."

Check if the iRule is attached:

 tmsh list ltm virtual vs rules
ltm virtual vs {
    rules {
        TestMe
    }
}

Access the virtual. See if it fired. In this example, the

TestMe

Jul 26 12:18:36 ltm1310 info tmm[12214]: Rule /Common/TestMe : Obi-Wan is here. The Force is with him.

You need to find the exact log message that trigers the iRule. If the virtual already had other iRules, the logic may become more complex. In that case, you may want to write a tmsh script.