How does Bash parameter expansion work? [duplicate]

Question

I'm trying to understand a Bash script. I stumbled upon this:

DIR=${1:-"/tmp"}

What does that mean?

Answer 1

:- is actually an operator it says that if $1 (first argument to the script) is not set or is null then use /tmp as the value of $DIR and if it's set assign it's value to $DIR.

DIR=${1:-"/tmp"}

is short for

if [ -z $1 ]; then
        DIR='/tmp'
else
        DIR="$1"
fi

It can be used with any variables not just positional parameters:

$ echo ${HOME:-/tmp} # since $HOME is set it will be displayed.
/home/codaddict
$ unset HOME   # unset $HOME.
$ echo ${HOME:-/tmp} # since $HOME is not set, /tmp will be displayed.
/tmp
$ 

Answer 2

That syntax is parameter expansion:

${parameter:-word}  

If parameter is unset or null, the expansion of word is substituted. Otherwise, the value of parameter is substituted.

So if $1 is unset or null, it evaluates to "/tmp" and to the value of $1 otherwise.