How to show the different between :- and :=

[tikual]

I know that both of these are used for assigning a default value to the variable. But how can I show the different?

Thanks

[Mark Grant]

newvalue=${variable:-hello} means newvalue will be set to $variable if $variable already has a value. Otherwise newvalue will be set to "hello"

newvalue=${variable:=hello} means newvalue will be set to "hello" and so will $variable if $variable is not set otherwise newvalue will be set to $variable.

[Dietmar Konermann · Accepted solution]

$ unset bla
$ echo ${bla:-huhu}
huhu
$ echo $bla
sh: bla: Parameter not set.
$ echo ${bla:=huhu}
huhu
$ echo $bla
huhu

Best regards...
Dietmar.

[tikual]

My wants is Dietmar's answer. Anyway, thanks all of your help!