Re: [Help-bash] What is : good for

[Greg Wooledge]

On Fri, Feb 19, 2016 at 12:27:08AM -0500, David Niklas wrote:
> I tried a couple of things with it, but I can't think of a use.

It's used any place the grammar requires a command but you don't actually
want to do anything.

There are three places it is traditionally used.

1) In the Bourne shell, which had no ! prefix, it was used with "if":

if something
then
  :
else
  stuff
fi

In Bash, we'd simply write:

if ! something; then
  stuff
fi


2) It is used with the "assign a default value" parameter expansion:

: "${foovar:=foodefault}"


3) In a "case" branch whose only existence is an exclusion from a later
   branch:

case $thing in
  x*y*) : ;;
  x*)  die "Error: x without y" ;;
esac


It can also be used in place of "true", when you want to loop for a
really long time.  For example:

while ! thing you are polling for; do
  :
done

while :; do
  something
  something else
  some check && break
done

I'd use "while true; do" in the last one, but some people like : more.