A shell colon does nothing. Use it anyway

Summary

A shell colon does nothing. Use it anyway.I've written more shell scripts than I can count, but I still stumble upon tricks that honestly blow my mind far too often than I care to admit. Latest thing that blew my head clean off? The shell colon. Published23 July 2026 at 03:53 UTCAuthorFilip RoséenTags 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? Note: Want your mind blown straight away? See Other colons worth remembering. 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, otherwise; 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 a label ...