${var:=default} vs ${var:-default} - what is difference? [duplicate]

Question

Seems both make absolutely same actions.

Empty $var returns defaultvalue in both cases:

$ var=
$ echo ${var:-defaultvalue}
defaultvalue

$ var=
$ echo ${var:=defaultvalue}
defaultvalue

Not empty $var return it's value in both cases:

$ var=var
$ echo ${var:-defaultvalue}
var

$ var=var
$ echo ${var:=defaultvalue}
var

$var not set at all - returns defaultvalue in both cases:

$ unset var
$ echo ${var:-defaultvalue}
defaultvalue

$ unset var
$ echo ${var:=defaultvalue}
defaultvalue

Answer 1

They are similar only that ${var:=defaultvalue} assigns value to var as well and not just expand as like it.

Example:

> A='' > echo "${A:=2}" 2 > echo "$A" 2 > A='' > echo "${A:-2}" 2 > echo "$A" (empty) 

Answer 2

Positional or special parameters cannot be assigned using := way. See this example:

args() { v=${1:=one}; echo "$v"; }
args
-bash: $1: cannot assign in this way

And this:

args() { v=${1:-one}; echo "$v"; }
args
one

As per man bash: (emphasis is mine to highlight the difference)

${parameter:-word}
Use Default Values. If parameter is unset or null, the expansion of word is substituted. Otherwise, the value of parameter is substituted.

${parameter:=word}
Assign Default Values. If parameter is unset or null, the expansion of word is assigned to parameter. The value of parameter is then substituted. Positional parameters and special parameters may not be assigned to in this way.