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).
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).
Let’s play with the $HOME variable again, but this time without the colon in the operator:
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$ 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, deliberate or not.
