What's the meaning of ${1:-0} in Bash?

Question

I seem at example 9-14.

$ echo ${1:-0}
$ 0
$ echo ${2:-32767}
$ 32767

So I cant understand...

Answer 1

Per the Bash Reference Manual, §3.5.3 "Shell Parameter Expansion":

${parameter:-word}

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

So, for example, this script:

echo "${foo:-1}"
foo=2
echo "${foo:-3}"
foo=
echo "${foo:-4}"

prints

1
2
4

Answer 2

The syntax is ${var:-$DEFAULT}. It means if the variable is not set or is null, use the default value.