What is the purpose of the : (colon) GNU Bash builtin?

user_237955 0
Viewed 163916

What is the purpose of a command that does nothing, being little more than a comment leader, but is actually a shell builtin in and of itself?

It's slower than inserting a comment into your scripts by about 40% per call, which probably varies greatly depending on the size of the comment. The only possible reasons I can see for it are these:

# poor man's delay function
for ((x=0;x<100000;++x)) ; do : ; done

# inserting comments into string of commands
command ; command ; : we need a comment in here for some reason ; command

# an alias for `true'
while : ; do command ; done

I guess what I'm really looking for is what historical application it might have had.

built-in shell bash
12 Answers

A useful application for : is if you're only interested in using parameter expansions for their side-effects rather than actually passing their result to a command.

In that case, you use the parameter expansion as an argument to either : or false depending upon whether you want an exit status of 0 or 1. An example might be

: "${var:=$1}"

Since : is a builtin, it should be pretty fast.

user_4954510

Another way, not yet mentioned here is the initialisation of parameters in infinite while-loops. Below is not the cleanest example, but it serves it's purpose.

#!/usr/bin/env bash
[ "$1" ] && foo=0 && bar="baz"
while : "${foo=2}" "${bar:=qux}"; do
    echo "$foo"
    (( foo == 3 )) && echo "$bar" && break
    (( foo=foo+1 ))
done
user_83440600
Related