A shell colon does nothing. Use it anyway

# A shell colon does nothing. Use it anyway. Source: [https://refp.se/articles/your-shell-and-the-magic-colon](https://refp.se/articles/your-shell-and-the-magic-colon) 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 UTCAuthorTags- [\#shell](https://refp.se/articles/tagged/shell) - [\#posix](https://refp.se/articles/tagged/posix) - [\#unix](https://refp.se/articles/tagged/unix) ## 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](https://refp.se/articles/your-shell-and-the-magic-colon#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](https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_06_02): - 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`$name`\. ## That\.\. other colon So that's one colon, but what about that other one, the one who sits alone at the beginning of the line? - `:`is the[null command](https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_16)— a builtin that does nothing but evaluate its arguments and discard the result\. - `:`is old — it goes all the way back to the[1971 Thompson shell](https://en.wikipedia.org/wiki/Thompson_shell#Design)where it doubled as a[label](https://www.in-ulm.de/~mascheck/bourne/PWB/goto.1.html)and Unix's very first comment marker\. - `:`two eyes staring at you in the dark, with love\. ## Other colons worth remembering Perhaps we have already established that there is more to`:`than meets the eye, but to prove the real magic of the[null\-command](https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_16)— here are a few usages that blew my mind\. ``` : "${DATA_DIR:=/var/data}" # set defaults, : swallows the result : "${RETRIES:=3}" # instead of running it as a command ``` ``` : > error.log # truncate error.log : > error.log > access.log # truncate both error.log and access.log ``` ``` ( : < dataset.json ) && echo YES # is dataset.json readable? ( : >> result.json ) && echo YES # is result.json writable? ``` ``` trap : INT # trap requires a command sleep 60 # sleep is interruptible ``` ``` set -u # error on unset variables : "$DEPLOY_ENV" "$HOST" # check DEPLOY_ENV and HOST ``` ## Con\-colon\-sion So, if you are like me and prefer less typing \(gotta go fast\) — the[null command](https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_16)and[parameter expansion](https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_06_02)are a good pair to remember\. ``` while : colons are more than punctuation; do echo "I love colons" sleep 1 done ```