Map directive for variable mapping in Nginx - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the map directive in nginx?

The map directive creates a new variable whose value depends on the value of another variable. It helps to simplify complex conditional logic by mapping input values to output values.

beginner

How do you define a basic map block in nginx?

You define it with map $input_variable $output_variable { ... } where inside the braces you list input values and their corresponding output values.

intermediate

What happens if no match is found in a map directive?

If no match is found, the map directive uses the value defined by the default keyword if provided, or an empty string otherwise.

intermediate

Can the map directive use regular expressions for matching?

Yes, the map directive supports regular expression matching by prefixing the pattern with ~ for case-sensitive or ~* for case-insensitive matching.

beginner

Where should the map directive be placed in the nginx configuration?

The map directive must be placed in the http context, outside of server or location blocks.

What does the map directive in nginx do?

A Maps one variable's value to another variable's value

B Defines server listening ports

C Sets up SSL certificates

D Configures proxy pass

The map directive creates a new variable based on the value of another variable.

Where must the map directive be placed in nginx config?

A Inside <code>location</code> block

B Inside <code>server</code> block

C Inside <code>events</code> block

D Inside <code>http</code> block

The map directive must be placed in the http context.

How do you specify a default value in a map directive?

A Using the <code>default</code> keyword

B Using <code>else</code> keyword

C Using <code>fallback</code> keyword

D Using <code>catch</code> keyword

The default keyword sets the value if no other match is found.

Which prefix is used for case-insensitive regex matching in map?

A ~

B ~*

C ^~

D *~

The ~* prefix is used for case-insensitive regex matching.

What will be the output variable if no match and no default is set in map?

A Error thrown

B Input variable value

C Empty string

D Null value

If no match and no default, the output variable is an empty string.

Explain how the map directive works in nginx and give an example of its usage.

Think about how you can assign values conditionally without if statements.

Describe where and why you would use the map directive in an nginx configuration.

Consider how nginx handles variables and conditions.

Practice

1. What is the primary purpose of the map directive in nginx configuration?

easy

A. To create new variables based on existing variables

B. To define server blocks for different domains

C. To set up SSL certificates

D. To configure load balancing between servers

Solution

1. Step 1: Understand the role of map in nginx

   The map directive is used to create new variables by mapping values from existing variables.

2. Step 2: Compare with other nginx directives

   Other options like server blocks, SSL, and load balancing serve different purposes unrelated to variable mapping.

3. Final Answer:

   To create new variables based on existing variables -> Option A

4. Quick Check:

   map creates variables [OK]

Hint: Map creates variables from existing ones [OK]

Common Mistakes:

• Confusing map with server block configuration
• Thinking map sets SSL or load balancing
• Assuming map modifies request headers directly

2. Which of the following is the correct syntax to start a map block in nginx?

easy

A. map ($variable, $new_variable) {

B. map $new_variable $variable {

C. map { $variable $new_variable }

D. map $variable $new_variable {

Solution

1. Step 1: Recall the syntax of the map directive

   The correct syntax is map $variable $new_variable { where the first variable is the source and the second is the target.

2. Step 2: Eliminate incorrect syntax

   The incorrect options use wrong order or invalid punctuation for nginx configuration.

3. Final Answer:

   map $variable $new_variable { -> Option D

4. Quick Check:

   map syntax = map $source $target { [OK]

Hint: Map syntax: map $source $target { [OK]

Common Mistakes:

• Swapping source and target variables
• Using parentheses or braces incorrectly
• Missing the $ sign before variables

3. Given this nginx map configuration:

map $http_user_agent $is_mobile {
    default 0;
    ~*Mobile 1;
}

What will be the value of $is_mobile if the user agent is Mozilla/5.0 (Linux; Mobile)?

medium

A. Mobile

B. 0

C. 1

D. default

Solution

1. Step 1: Understand the map pattern matching

   The map uses a case-insensitive regex ~*Mobile to match user agents containing 'Mobile'.

2. Step 2: Check the user agent string

   The user agent contains 'Mobile', so the regex matches and sets $is_mobile to 1.

3. Final Answer:

   1 -> Option C

4. Quick Check:

   Regex match 'Mobile' sets value to 1 [OK]

Hint: Regex ~*Mobile matches user agent containing 'Mobile' [OK]

Common Mistakes:

• Ignoring case-insensitive regex (~*)
• Assuming default value applies despite match
• Confusing variable names

4. Identify the error in this nginx map configuration:

map $request_method $allow_method {
    GET yes;
    POST yes;
    DELETE no
}

medium

A. Missing semicolon after DELETE no

B. Incorrect variable names

C. map directive cannot map request methods

D. Using yes/no instead of 1/0 is invalid

Solution

1. Step 1: Check syntax for each mapping line

   Each line inside map must end with a semicolon. The line DELETE no is missing a semicolon.

2. Step 2: Validate other options

   Variable names are valid, mapping request methods is allowed, and yes/no strings are acceptable values.

3. Final Answer:

   Missing semicolon after DELETE no -> Option A

4. Quick Check:

   Each map line ends with ; [OK]

Hint: Every map entry line ends with a semicolon [OK]

Common Mistakes:

• Omitting semicolons at line ends
• Thinking yes/no are invalid values
• Assuming map can't use request methods

5. You want to create a variable $cache_key that is mobile if the user agent contains 'Mobile', otherwise desktop. Which map configuration correctly achieves this?

hard

A. map $cache_key $http_user_agent {
mobile ~*Mobile;
desktop default;
}

B. map $http_user_agent $cache_key {
~*Mobile mobile;
default desktop;
}

C. map $http_user_agent $cache_key {
default desktop
~*Mobile mobile
}

D. map $http_user_agent $cache_key {
Mobile mobile;
default desktop;
}

Solution

1. Step 1: Confirm correct map syntax and order

   The map directive must have source variable first, target variable second, and each line ends with semicolon.

2. Step 2: Check pattern matching and default

   map $http_user_agent $cache_key { ~*Mobile mobile; default desktop; } uses case-insensitive regex ~*Mobile to match user agents containing 'Mobile' and sets mobile, else desktop.

3. Step 3: Eliminate incorrect options

   map $cache_key $http_user_agent { mobile ~*Mobile; desktop default; } reverses variables, one misses semicolons, another uses exact 'Mobile' without regex and may miss case-insensitive matches.

4. Final Answer:

   map $http_user_agent $cache_key { ~*Mobile mobile; default desktop; } -> Option B

5. Quick Check:

   Correct map syntax with regex and default [OK]

Hint: Use map $source $target with regex and default [OK]

Common Mistakes:

• Swapping source and target variables
• Missing semicolons after lines
• Not using case-insensitive regex for matching