How to use tmsh command in I-rule and to catch/store the output value in variable

iRule execution is connection-based, meaning (in part) that an iRule only runs while a connection is active. Since a standby unit is not processing connections, no iRules run on it. Also, although it can be done, triggering TMSH commands from an iRule is frowned upon as it can have some serious pitfalls. F5 provides other tools that are better suited to doing what it sounds like you are trying to do, including iCall or iControl.