how to capture logs using TCPDUMP

Adding to IheartF5's answer, the tcpdump command is used primarily to capture network traffic. Logs, on the other hands (system and module), are generated and stored in the /var/log directory. Here is a list of the common set of syslog facilities:

sol13317: Configuring the level of information that syslog-ng sends to log files (11.x)

A common iRules troubleshooting tool is to run the tail command in "follow" mode from the shell to see errors and inserted log statements. For example, this is to tail the ltm log:

tail -f /var/log/ltm

You could then insert log statements in your iRules at strategic points and see those in the above stream of messages. Example:

when HTTP_REQUEST {
    log local0. "URI = [HTTP::uri]"
}