Difference between : and := with variables in bash files

Question

I came across a bash file which has something like this

MYVAR := SomeVal

I know after reading some tutorials you set values as such

MYVAR = SomeVal

What is the difference between the above two ?

Answer 1

You probably saw a parameter expansion to set a default value.

The difference is whether the variable is unset or it is set with a null value:

1. when the variable is unset, there's no difference

   $ unset MYVAR; : ${MYVAR:=SomeVal}; echo "$MYVAR"
   SomeVal
   $ unset MYVAR; : ${MYVAR=SomeVal}; echo "$MYVAR"
   SomeVal
   

2. when the variable has a value, there's no difference

   $ MYVAR="foo"; : ${MYVAR:=SomeVal}; echo "$MYVAR"
   foo
   $ MYVAR="foo"; : ${MYVAR=SomeVal}; echo "$MYVAR"
   foo
   

3. when the variable has null value:

   $ MYVAR=""; : ${MYVAR:=SomeVal}; echo "$MYVAR"
   SomeVal
   $ MYVAR=""; : ${MYVAR=SomeVal}; echo "$MYVAR"
   # empty line