bash Cookbook

Setting Default Values

Problem

Your script may rely on certain environment variables, either widely used ones (e.g., $USER) or ones specific to your own business. If you want to build a robust shell script, you should make sure that these variables do have a reasonable value. You want to guarantee a reasonable default value. How?

Solution

Use the assignment operator in the shell variable reference the first time you refer to it to assign a value to the variable if it doesn’t already have one, as in:

cd ${HOME:=/tmp}

Discussion

The reference to $HOME in the example above will return the current value of $HOME unless it is empty or not set at all. In those cases (empty or not set), it will return the value /tmp, which will also be assigned to $HOME so that further references to $HOME will have this new value.

We can see this in action here:

$ echo ${HOME:=/tmp}
/home/uid002
$ unset HOME # generally not wise to do
$ echo ${HOME:=/tmp}
/tmp
$ echo $HOME
/tmp
$ cd ; pwd
/tmp
$

Once we unset the variable it no longer had any value. When we then used the := operator as part of our reference to it, the new value (/tmp) was substituted. The subsequent references to $HOME returned its new value.

One important exception to keep in mind about the assignment operator: this mechanism will not work with positional parameter arguments (e.g., $1 or $*). For those cases, use :- in expressions like ${1:-default}, which will return the value without trying to do the assignment.

As an aside, it might help you ...