Missing required arguments in shell scripts? Use the colon. | Filip Roséen - refp.se<br>Missing required arguments in shell scripts? Use the colon.<br>I've written more shell scripts than I can count, but I still stumble on neat tricks that make the everyday chores smoother. This time it's the colon.

Published23 July 2026 at 03:53 UTCAuthorFilip RoséenTags#shell<br>#posix<br>#unix<br>Table of Contents<br>In a land far-far away..<br>Checking for required argumentsParameter expansion and the story of :?

That.. other colon<br>Con-colon-sion

In a land far-far away.. #

... there was once a far too cold cup of coffee next to a freshly brewed<br>far-too-hot one. Four different terminals where three could have been closed an<br>hour ago, and a shell script which I really (really) did not want to write.

Who would have thought a single colon would be the one to save the day night?

Checking for required arguments #

This is a familiar dance, it's pretty much muscle memory by this point. You have<br>a script, it takes a few arguments, and some of them are mandatory; alright, an<br>if-statement like so many times before:

if [ -z "$1" ]; then<br>echo "missing argument, aborting!" 1>&2<br>exit 1<br>fi

echo "Hello: $1!"

Though.. what if I told you the above four lines could be replaced by just... one?

: ${1:?missing argument, aborting!}

echo "Hello $1!"

$ bash example.sh<br>example.sh: line 3: 1: missing argument, aborting!

$ bash example.sh refp<br>Hello refp!

Parameter expansion and the story of :? #

There are two things going on in the previous snippet, and you are correct in<br>identifying that one part is using parameter expansion:

The syntax ${name:?diagnostic} checks whether $name is unset or empty<br>— if it is, the diagnostic is printed to stderr and the shell exits with a<br>non-zero status, and;

if the variable is set, it is equivalent to $name.

That.. other colon #

So that's one colon, but what about that other one, the one who sits alone at<br>the beginning of the line?

: is the null command<br>— a builtin that does nothing but evaluate its arguments and discard the<br>result.

: is old — it goes all the way back to the 1971 Thompson<br>shell where it doubled as Unix's very first comment<br>marker.

: two eyes staring at you in the dark, with love.

Con-colon-sion #

So, if you are like me and prefer less typing (gotta go fast) — the null<br>command and parameter expansion are a good<br>pair to remember.

while : "do the hustle"; do<br>echo "I love colons"<br>sleep 1<br>done