Bash: The Difference Between := and :-

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

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)