I couldn't find an example online but I'm sure I've seen shell coders use ${1:--} to accept user input. Here's an example:
#!/bin/bash
var="${1:--}"
echo "$var"
Then, run it:
$ ./test.sh "this is a test"
My question is: how is using "${1:--}" to accept user input different from "$1"?
${1:--} will expand to the string "-" if there is no parameter one or if the parameter is empty.
So ./test.sh "" will return the string "-" as will the command ./test.sh This is considered to be a useful default in many circumstances where an argument of "-" can mean stdin or stdout. Also it makes sure scripts don't break when a parameter is not explicitly set.
Under bash's parameter expansion you can use default value of the variable and this is what ${1:--} does to thirst command argument, namely it assigns value - if and only if $1 is unset or null. It works with any other parameters too, test this example:
echo "${var:-X}" # var is not defined so X will be echoed
var=Y # lets define var
echo "${var:-X}" # var is defined so its value Y will be printed
