set_if_empty
The `set_if_empty` directive sets a variable to a specified value if that variable is currently empty.
Description
The set_if_empty directive is useful in NGINX for conditional variable assignment. It checks the specified variable, and if it is empty (not defined or has a length of zero), it assigns a default value to it, which is specified as the second argument of the directive. This feature is helpful in situations where you want to ensure that a variable has a value before it is used further down in processing requests, such as in rules, conditions, or other directives that depend on that variable's value. The directive can be utilized in several contexts including http, server, location, or within if statements inside these contexts.
The syntax for set_if_empty requires two arguments: the first is the variable to be checked (which must be prefixed with a dollar sign '$') and the second is the value that should be assigned if the first variable is empty. This directive enhances the flexibility of variable management, allowing for cleaner and more robust configurations that can handle various scenarios without the need for complex conditional logic elsewhere in the configuration files.
Config Example
location /foo {
set $a $arg_a;
set_if_empty $a 56;
# GET /foo?a=32 will yield $a == 32
# while GET /foo and GET /foo?a= will yield $a == 56
}Overriding existing values: Be mindful that set_if_empty will override the variable if it is empty, which might not be desired in every case.
Nesting: Avoid using set_if_empty within other conditional directives, as it can lead to unexpected behaviors.
Variable scope: The directive's context limits its use, particularly if the variable is expected in a different scope.