monitor a pool that consists of Linux Squid Servers

As per the original script you mentioned, the receive string must be tested using the script and not from the GUI config (RECV = the expected response).

"if [ $? -eq 0 ]" checks if the last command was executed without errors, which seems to be always the case with your command.

Additionally, whatever your script send to the standard output (wether UP or Down or Simpsons...) it will cause the member to be marked up. For a member to be marked Down the script should output nothing.

So, to summarize, you must test for the receive string within your script, and you must not send data to standard output if the pool member has to be marked down.

More on external monitors here: Overview of BIG-IP EAV external monitors (f5.com)