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 working like this for me:

[mandy@root]$ a=11
[mandy@root]$ b=20
[mandy@root]$ c=30
[mandy@root]$ echo $a; : echo $b ; echo $c
10
30

Please let me know, any use case of this operator in real time or any place where it is mandatory to use it.

Answer 1

It’s there more for historical reasons. The colon builtin : is exactly equivalent to true. It’s traditional to use true when the return value is important, for example in an infinite loop:

while true; do
  echo 'Going on forever'
done

It’s traditional to use : when the shell syntax requires a command but you have nothing to do.

while keep_waiting; do
  : # busy-wait
done

The : builtin dates all the way back to the Thompson shell, it was present in Unix v6. : was a label indicator for the Thompson shell’s goto statement. The label could be any text, so : doubled up as a comment indicator (if there is no goto comment, then : comment is effectively a comment). The Bourne shell didn’t have goto but kept :.

A common idiom that uses : is : ${var=VALUE}, which sets var to VALUE if it was unset and does nothing if var was already set. This construct only exists in the form of a variable substitution, and this variable substitution needs to be part of a command somehow: a no-op command serves nicely.

See also What purpose does the colon builtin serve?.