Conditional redirects with if in Nginx - Step-by-Step Execution

Process Flow - Conditional redirects with if

Request Received

↓

Evaluate if condition

↓

Redirect

↓

End Request

Nginx checks the if condition on each request. If true, it redirects; if false, it serves the request normally.

Execution Sample

Nginx

server {
  listen 80;
  if ($host = 'old.example.com') {
    return 301 https://new.example.com$request_uri;
  }
}

Redirect requests from old.example.com to new.example.com preserving the path.

Process Table

Step	Request Host	Condition ($host = 'old.example.com')	Action	Result
1	old.example.com	True	return 301 redirect	Redirect to https://new.example.com$request_uri
2	new.example.com	False	serve normally	Serve requested content
3	other.example.com	False	serve normally	Serve requested content

💡 Requests with host 'old.example.com' redirect; others serve normally.

Status Tracker

Variable	Start	Request 1	Request 2	Request 3
$host	undefined	old.example.com	new.example.com	other.example.com
Condition Result	undefined	True	False	False

Key Moments - 2 Insights

Why does the redirect only happen for 'old.example.com' and not others?

Because the if condition checks if $host equals 'old.example.com'. Only when this is true (see execution_table row 1) does nginx perform the redirect.

What happens if the condition is false?

Nginx skips the redirect and serves the request normally, as shown in execution_table rows 2 and 3.

Visual Quiz - 3 Questions

Look at the execution table, what is the action taken when the request host is 'new.example.com'?

A Serve normally

B Redirect to new.example.com

C Return 404 error

D Redirect to old.example.com

💡 Hint

Check execution_table row 2 under 'Action' column.

At which step does the condition become true?

A Step 2

B Step 1

C Step 3

D None

💡 Hint

Look at the 'Condition' column in execution_table.

If the condition was changed to check for 'new.example.com', what would happen at step 1?

A Redirect occurs

B Error occurs

C Serve normally

D Request is blocked

💡 Hint

Refer to variable_tracker and execution_table logic for condition evaluation.

Concept Snapshot

Nginx conditional redirect syntax:
if (condition) {
  return 301 https://newsite$request_uri;
}
Redirects only if condition is true.
Otherwise, serves request normally.
Use for simple host or path based redirects.

Full Transcript

This visual execution shows how nginx uses the if directive to conditionally redirect requests. When a request arrives, nginx checks if the host matches 'old.example.com'. If yes, it sends a 301 redirect to 'https://new.example.com' preserving the path. If not, it serves the request normally. The execution table traces three requests with different hosts, showing when the redirect happens and when it does not. The variable tracker shows the $host value and condition result for each request. Key moments clarify why only matching hosts redirect and what happens otherwise. The quiz tests understanding of condition evaluation and redirect behavior.