How to caputre packet pertaining to a irule using tmsh

You may be better off configuring logging in the iRule.

If you must capture packets, use tcpdump ([tmsh] run util tcpdump).

If you are unfamiliar with tcpdump, this series of blogs will help: ">http://packetpushers.net/masterclass-tcpdump-basics/" target="_blank">">http://packetpushers.net/masterclass-tcpdump-basics/