A shell colon does nothing. Use it anyway

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 night?day

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.

: "${DATA_DIR:=/var/data}"        # set defaults, : swallows the result
: "${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
( : > 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
if some-command; then
    :                             # command required
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):

: "${DATA_DIR:=/var/data}"          # <- DATA_DIR mentioned once (1)
DATA_DIR="${DATA_DRI:-/var/data}"   # <- oops (2)