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 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!

And look what happens if we refer to a variable with a proper name — it's the same behavior as previously but easier to spot; the diagnostic includes the name of our variable!

: "${GREET_NAME:?missing argument, aborting.}"
echo "Hello $GREET_NAME!"

$ bash greet.sh
greet.sh: line 1: GREET_NAME: missing argument, aborting.

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

( : < 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

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.

And also.. isn't this — magic?

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)
  

• Why would I do any of these when it hurts readability? #

  There are a few common misconceptions about the — very contrived — examples presented in this article, questions regarding their usefulness, or if one should ever-ever use these things at all.

  Let's take the below as an example.

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

  Of course you can use if ! some-command and get rid of the branching, but that isn't the point of the snippet — the point is to show that the null-command can be used in places where a command is required, but nothing is desired.

  It was never meant to be read as "oh, if I have an if-statement I should use null-command", it is intended to be read as "oh, if I am in a context which requires a command, I can use the null-command to no-op it".

  Clever real-world usage of : in the wild #

  User ifphilipe posted what follows as a comment on news.ycombinator.com, which is the direct equivalent of needing a command which does absolutely nothing.

  I use the colon as EDITOR with Git when I want to do an interactive rebase combined with auto squash without having to edit the todo list. I have an alias[1] for that which I call a quick interactive rebase:

  riq = -c sequence.editor=: rebase --interactive

  Could this article have used the above rather than the so much debated if-statement? If I was clever enough to come up with it — yes.. but would it be as comprehensible in terms of understanding what : really is? I strongly suspect not.