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.

• #shell
• #posix
• #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.

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 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 — 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 and parameter expansion are a good pair to remember.

while : colons are more than punctuation; do
  echo "I love colons"
  sleep 1
done