Map directive for variable mapping in Nginx - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of Nginx map directive with simple variable mapping

Given the following Nginx configuration snippet, what will be the value of the variable $color when the request URI is /blue?

Nginx

map $uri $color {
    default red;
    /blue blue;
    /green green;
}

A empty string

B red

C green

D blue

💡 Hint

The map directive assigns values based on matching keys; if no match, it uses default.

The map directive checks the $uri. Since the URI is /blue, it matches the key /blue and assigns blue to $color.

🧠 Conceptual

intermediate

Purpose of the Nginx map directive

What is the primary purpose of the Nginx map directive?

A To set up SSL certificates for HTTPS

B To rewrite URLs before processing requests

C To create a new variable by mapping one variable's value to another based on conditions

D To define server blocks for different domains

💡 Hint

Think about how you can assign values dynamically based on another variable's value.

The map directive creates a new variable whose value depends on the value of another variable, allowing conditional logic in configuration.

❓ Configuration

advanced

Correct Nginx map syntax for IP-based access control

Which of the following Nginx map configurations correctly sets $allowed to 1 for IP 192.168.1.100 and 0 for all others?

A

map $remote_addr $allowed {
    192.168.1.100 1;
    default 0;
}

B

map $remote_addr $allowed {
    default 0;
    192.168.1.100 1;
}

C

map $remote_addr $allowed {
    192.168.1.100:1;
    default:0;
}

D

map $remote_addr $allowed {
    192.168.1.100 => 1;
    default => 0;
}

💡 Hint

Check the syntax for key-value pairs inside the map block.

Option A uses the correct syntax: key and value separated by space and ending with semicolon. Other options have syntax errors.

❓ Troubleshoot

advanced

Troubleshooting unexpected variable value in Nginx map

You have this map directive:

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

But $is_mobile is always 0 even when accessing from a mobile browser. What is the most likely cause?

A The variable <code>$http_user_agent</code> is empty or not passed correctly

B The map directive does not support regular expressions

C The <code>~*</code> regex pattern is case-sensitive and should be <code>~</code>

D The map directive must be placed inside the server block, not http block

💡 Hint

Consider if the variable used in the map has the expected value during request processing.

If $http_user_agent is empty or missing, the regex match will never succeed, so $is_mobile stays at default 0.

🔀 Workflow

expert

Order of evaluation in Nginx map directive with overlapping keys

Consider this map directive:

map $uri $page_type {
    /about about_page;
    ~^/about/us about_us_page;
    default other_page;
}

What will be the value of $page_type for the request URI /about/us?

A about_page

B about_us_page

C other_page

D empty string

💡 Hint

The map directive evaluates keys in the order they appear; the first matching key wins.

The map directive checks keys in the order they appear. The URI '/about/us' does not exactly match '/about', but matches the regex '~^/about/us', so $page_type is set to 'about_us_page'.

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