What is the result of ${VAR:=value} in a bash script

Question

I'm looking at several bash scripts that use := in between ${ }. For example, the pattern looks like this:

export VAR=${VAR:=value}

What is this doing? Is it assigning value to VAR if VAR does not exist?

Answer 1

As per documentation:

${parameter:=word}

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.

So it exports the VAR value with its value, or if it is unset/null it is exported with literal value.

Answer 2

From the man page:

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

In other words, it basically lets you specify a default value for a variable. If the variable is unset/null, then it will be set to that value and the value will be used as the expansion as well.