Bash script what is := for?

Question

Does anyone know what is := for?

I tried googling but it seems google filters all symbol?

I know the below is something like checking if the variable HOME is a directory and then something is not equal to empty string.

  if [ "${HOME:=}" != "" ] && [ -d ${HOME} ]

Answer 1

From Bash Reference Manual:

${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.

Basically it will assign the value of word to parameter if and only if parameter is unset or null.

Answer 2

From the Bash man page:

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.

Man pages are a wonderful thing. man bash will tell you almost everything you want to know about Bash.

Answer 3

Here "${HOME:=}" is a more or less a no-op, since it returns $HOME if it's set and nothing if it's not.

if [ "${HOME:=}" != "" ] && [ -d ${HOME} ]

This would function identically:

if [ "${HOME}" != "" ] && [ -d ${HOME} ]

If you had something after the equal sign, the first part of the if would always be true.

if [ "${HOME:=here}" != "" ] && [ -d ${HOME} ]

whatever happened you'd always get a string that's not equal to "".

However, this would depend on whether $HOME and $DEFAULT were both null:

if [ "${HOME:=$DEFAULT}" != "" ] && [ -d ${HOME} ]