bash Cookbook

Using null As a Valid Default Value

Problem

You need to set a default value, but you want to allow an empty string as a valid value. You only want to substitute the default in the case where the value is unset.

The ${:=} operator has two cases where the new value will be used: first, when the value of the shell variable has previously not been set (or has been explicitly unset); and second, where the value has been set but is empty, as in HOME="" or HOME=$OTHER (where $OTHER had no value).

Solution

The shell can distinguish between these two cases, and omitting the colon (:) indicates that you want to make the substitution only if the value is unset. If you write only ${HOME=/tmp} without the colon, the assignment will take place only in the case where the variable is not set (never set or explicitly unset).

Discussion

Let’s play with the $HOME variable again, but this time without the colon in the operator:

$ echo ${HOME=/tmp}  # no substitution needed
/home/uid002
$ HOME=""       # generally not wise
$ echo ${HOME=/tmp}  # will NOT substitute

$ unset HOME    # generally not wise
$ echo ${HOME=/tmp}     # will substitute
/tmp
$ echo $HOME
/tmp
$

In the case where we simply made the $HOME variable an empty string, the = operator didn’t do the substitution since $HOME did have a value, albeit null. But when we unset the variable, the substitution occurs. If you want to allow for empty strings, use just the = with no colon. Most times, though, the := is used because you can do little with an empty value, ...