question about define variable.

Hi, Unix Gurus,
In our existing file, there is a script like

#!/bin/sh
step=${1:-0}
cur_step=10
if [ $step -le $cur_step ]

...
My question is what's "${1:-0}" mean? I know it defines a variable but I don't know what's (1:-0) mean?

:wall:

Thanks in advance.

If $1 isn't set, it becomes 0.

Thanks for you quick reply.
:b: