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.

... 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 .

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 .

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.

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.

set : : : : : : : : : : : : : : : : : : : : : while : colons are more than " ${1:?magic} " ; do echo "$*" && shift done Note : The above example is safe to run locally, try it!

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.

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.

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" .

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.