Use of nginx custom variables and built-in predefined variables

Overview

Nginx can use variables to simplify configuration and improve configuration flexibility. All variable values ​​can be referenced in this way:

$variable name

There are two types of variables in nginx: custom variables and built-in predefined variables

Built-in variables

statement

You can use the set command (non-unique) to declare variables in tags such as server, http, location, etc. The syntax is as follows

set $variable name variable value

Note that all variables in nginx must start with $.

Visibility

All variables used in the nginx configuration file must be declared, otherwise nginx will fail to start and print related exception logs

An interesting feature of nginx variables is that none of them are globally visible, yet they are not global variables. For example, the following example

location a/
 return 200 $a
}

location b/ {
 set $a hello nginx
 return 200 $a
}

Since the variable is globally visible, nginx will not report an error when it starts. However, the specific value of $a is unknown in the first location, so the returned response result is an empty string.

The visibility rules for variables declared in tags at different levels are as follows:

• The variables declared in the location tag are visible to this location block.
• Variables declared in the server tag are visible to the server block and all sub-blocks in the server block.
• Variables declared in the http tag are visible to the http block and all sub-blocks in the http block.

Built-in predefined variables

Built-in predefined variables are variables that can be used without declaration, usually including the value of a part of the content in an HTTP request or response. The following are some commonly used built-in predefined variables

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