parameter expansion default ":=" vs "=" vs ":-" vs "-"

Question

I've seen several different methods of using default values in parameter expansions in Bourne-derived shells: :=, =, :- and -. I'm wondering how they differ. The manual says that - and = handle null values differently from :- and :=. But as far as I can tell, := == :- and = == -. Is this true?

Here it is: https://www.gnu.org/software/bash/manual/html_node/Shell-Parameter-Expansion.html#Shell-Parameter-Expansion — Barmar, Jul 01 '20 at 18:03
The manual describes the behavior of each, but as far as I can tell, :- and := behave exactly the same. I'd like to confirm whether that is really the case. — Jacob Stern, Jul 01 '20 at 18:06
They do not; the equal forms actually change the value of the parameter is the default is used. — chepner, Jul 01 '20 at 18:08
The difference is that `:=` assigns the default value to the variable in addition to returning it. — Barmar, Jul 01 '20 at 18:08
This is also covered in https://wiki.bash-hackers.org/syntax/pe — Charles Duffy, Jul 01 '20 at 18:10

score 3 · Accepted Answer · answered Jul 01 '20 at 18:10

A demonstration of := vs :-:

 $ unset foo
 $ echo ${foo:-bar}
 bar
 $ echo foo
 
 $ echo ${foo:=bye}
 bye
 $ echo $foo
 bye

:- only affects the result of the expansion, leaving the parameter unchanged. := actually assigns the default value to the parameter if it is null or unset.

= works analogously to - regarding unset parameters; it only changes the value of foo if it is unset, not if it has a null value.

score 0 · Answer 2 · answered Jul 01 '20 at 18:12

The documentation is explicit.

With the = forms, emphasis added:

If parameter is unset or null, the expansion of word is assigned to parameter. The value of parameter is then substituted. [...]

With the other forms:

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

No assignment, modifying the parameter's value when later expanded as a variable, takes place without using an = form.

score 0 · Answer 3 · answered Jul 01 '20 at 18:47

0

Even though the question has already been answered, I just want to post a link to the shell bible, where you can find a great explanation not only to the parameter expansion, but to almost everything shell related.

answered Jul 01 '20 at 18:47

dsax7

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parameter expansion default ":=" vs "=" vs ":-" vs "-"

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