What is the use case of noop [:] in bash?

Question

I searched for noop in bash (:), but was not able to find any good information. What is the exact purpose or use case of this operator?

I tried following and it\'s w

Answer 1

Two of mine.

Embed POD comments

A quite funky application of : is for embedding POD comments in bash scripts, so that man pages can be quickly generated. Of course, one would eventually rewrite the whole script in Perl ;-)

Run-time function binding

This is a sort of code pattern for binding functions at run-time. F.i., have a debugging function to do something only if a certain flag is set:

#!/bin/bash
# noop-demo.sh 
shopt -s expand_aliases

dbg=${DBG:-''}

function _log_dbg {
    echo >&2 "[DBG] $@"
}

log_dbg_hook=':'

[ "$dbg" ] && log_dbg_hook='_log_dbg'

alias log_dbg=$log_dbg_hook


echo "Testing noop alias..."
log_dbg 'foo' 'bar'

You get:

$ ./noop-demo.sh 
Testing noop alias...
$ DBG=1 ./noop-demo.sh 
Testing noop alias...
[DBG] foo bar