I knew colon command from a bash script which one of my colleagues wrote. He uses it because of usage as here document like
:<<doc usage: foobar <arg1> [arg2] Change arg1 as args, if arg2 is empty... option: ... doc
The colon is a command to do nothing and return exit code zero. That's OK. I understand.
I don't know why it is used here. I'll ask him today.