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colon command

I knew colon command from a bash script which one of my colleagues wrote. He uses it because of usage as here document like

usage: foobar <arg1> [arg2]
Change arg1 as args, if arg2 is empty...

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.