Perl Cookbook

Establishing a Default Value

Problem

You would like to give a default value to a scalar variable, but only if it doesn’t already have one. It often happens that you want a hard-coded default value for a variable that can be overridden from the command-line or through an environment variable.

Solution

Use the || or ||= operator, which work on both strings and numbers:

# use $b if $b is true, else $c
$a = $b || $c;              

# set $x to $y unless $x is already true
$x ||= $y

If 0 or "0" are valid values for your variables, use defined instead:

# use $b if $b is defined, else $c
$a = defined($b) ? $b : $c;

Discussion

The big difference between the two techniques (defined and ||) is what they test: definedness versus truth. Three defined values are still false in the world of Perl: 0, "0", and "". If your variable already held one of those, and you wanted to keep that value, a || wouldn’t work. You’d have to use the clumsier tests with defined instead. It’s often convenient to arrange for your program to care only about true or false values, not defined or undefined ones.

Rather than being restricted in its return values to a mere 1 or as in most other languages, Perl’s || operator has a much more interesting property: It returns its first operand (the left-hand side) if that operand is true; otherwise it returns its second operand. The && operator also returns the last evaluated expression, but is less often used for this property. These operators don’t care whether their operands are strings, ...