Missing required arguments in shell scripts? Use the colon. | Filip Roséen - refp.se

Table of Contents

• In a land far-far away..
• Checking for required arguments
* Parameter expansion and the story of :?
• That.. other colon
• Con-colon-sion

In a land far-far away.. #

... there was once a far too cold cup of coffee next to a freshly brewed far-too-hot one. Four different terminals where three could have been closed an 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 a script, it takes a few arguments, and some of them are mandatory; alright, an if-statement like so many times before:

if [ -z "$1" ]; then
   echo "missing argument, aborting!" 1>&2 
   exit 1
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
example.sh: line 3: 1: missing argument, aborting!
$ bash example.sh refp
Hello refp!

Parameter expansion and the story of `:?` #

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

• The syntax `${name:?diagnostic}` checks whether `$name` is unset or empty — if it is, the diagnostic is printed to stderr and the shell exits with a 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 the beginning of the line?

• `:` is the null command— a builtin that does nothing but evaluate its arguments and discard the result.
• `:` is old — it goes all the way back to the 1971 Thompson shell where it doubled as Unix's very first comment 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 command and parameter expansion are a good pair to remember.

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