The irb Command

From Metasploit Unleashed - Mastering The Framework

Running the 'irb' command will drop you into a live Ruby interpreter shell where you can issue commands and create Metasploit scripts on the fly. This feature is also very useful for understanding the internals of the Framework.

msf > irb
[*] Starting IRB shell...

>> puts "Hello, metasploit!"
Hello, metasploit!
=> nil
>> Framework::Version
=> "3.8.0-dev"
>> framework.modules.keys.length
=>1336