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.

Published

23 July 2026 at 03:53 UTC

Modified

26 July 2026 at 02:59 UTC

Author

Filip Roséen

Tags

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 more colons in the limelight.

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 1: 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 and Unix's very first comment marker.

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

More colons in the limelight #

Perhaps we have already established that there is more to : than meets the eye, but to prove the real magic of the null-command — here are a few usages that blew my mind.

: "${DATA_DIR:=/var/data}"       
: "${RETRIES:=3}"                

: > error.log                    
: > error.log > access.log       

( : < dataset.json ) && echo YES 
( : >> result.json ) && echo YES 

trap : INT                       
sleep 60                         

set -u                           
: "$DEPLOY_ENV" "$HOST"          

if some-command; then
    :                            
else
    echo "command failed"
fi

Con-colon-sion #

So, if you are like me and prefer less typing (gotta go fast) — the null command and parameter expansion are a pair worth studying before your coffee goes cold.

set  : : : : : : : : : : : : : : : : : : : : :

while : colons are more than "${1:?magic}"; do
    echo "$*" && shift
done

Note: The above example is safe to run locally, try it!

Frequently Asked Questions #

After reading a few comments online, it seems I skipped over some things worth explaining. I will keep this section updated as questions come up.

• Why do I need the null-command? Doesn't the expansion happen without the colon? #

  The parameter expansion will happen regardless, but without a null-command or similar usage the shell will treat the resulting string as a command to run.

  % ${HELLO:=123}
  zsh: command not found: 123
  

  If we prefix our parameter-expansion with the null-command, the result is discarded, but the expression is still evaluated (setting HELLO to 123).

  % : ${HELLO:=123}
  % echo $HELLO
  123
  

• Why use the null-command when I could do VAR=${VAR:-default-value}? #

  This at its core boils down to personal preference, but using our beloved colon we can shrink the number of potential typos to one (rather than two):

  : "${DATA_DIR:=/var/data}"           
  DATA_DIR="${DATA_DRI:-/var/data}"