Importing a pre-exisiting iRule into an existing virtual server using TMSH or Bash

The best thing to do is to use tmsh sys config merge function, for which you need to make sure the text of irule in the file start from "ltm rule ..." e.g:

ltm rule hello_world { 
  when RULE_INIT { 
     log local0. "HELLO WORLD!" 
     } 
}

e.g. save this in hello_world.txt file, now you can verify it and "load merge" it:

First verify:

tmsh load sys config verify file hello_world.txt merge 
Validating configuration... hello_world.txt

now merge for real:

tmsh load sys config file hello_world.txt merge 

Loading configuration... hello_world.tcl 

That's it!