Log format customization in Nginx - Step-by-Step Execution

Process Flow - Log format customization

Define log_format directive

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Specify variables and text

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Assign format a name

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Use format name in access_log directive

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Nginx writes logs using custom format

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Logs show customized entries

This flow shows how you define a custom log format, name it, and then use it in the access_log directive so nginx writes logs in your chosen style.

Execution Sample

Nginx

log_format custom '$remote_addr - $remote_user [$time_local] "$request" $status $body_bytes_sent';
access_log /var/log/nginx/access.log custom;

Defines a custom log format named 'custom' and tells nginx to use it for access logs.

Process Table

Step	Directive	Action	Result
1	log_format custom ...	Define custom format with variables	Format 'custom' stored with specified pattern
2	access_log /var/log/nginx/access.log custom;	Set access log to use 'custom' format	Nginx will write logs using 'custom' format
3	Client makes request	Nginx processes request	Nginx logs request using 'custom' format
4	Log entry created	Variables replaced with actual values	Log line example: '192.168.1.1 - - [27/Apr/2024:12:00:00 +0000] "GET / HTTP/1.1" 200 612'
5	Nginx continues serving requests	Logs continue using custom format	Consistent custom log entries
6	Exit	No more config directives	Logging setup complete

💡 All directives processed; nginx uses custom log format for access logs

Status Tracker

Variable	Start	After Step 4 (log entry)
$remote_addr	undefined	192.168.1.1
$remote_user	undefined	-
$time_local	undefined	27/Apr/2024:12:00:00 +0000
$request	undefined	GET / HTTP/1.1
$status	undefined	200
$body_bytes_sent	undefined	612

Key Moments - 3 Insights

Why do we need to name the log format in the log_format directive?

Naming the log format (like 'custom') allows us to reference it later in the access_log directive, telling nginx which format to use for logging. See execution_table step 1 and 2.

What happens if we use a log format name in access_log that was not defined?

Nginx will fail to start or log properly because it cannot find the format. The execution_table shows the importance of defining the format before using it (steps 1 and 2).

How does nginx replace variables in the log format with actual values?

When a request happens, nginx replaces variables like $remote_addr with real data from the request before writing the log line, as shown in execution_table step 4.

Visual Quiz - 3 Questions

Look at the execution_table at step 4, what is the value of $status in the log entry?

A 200

B 404

C 500

D 302

💡 Hint

Check the 'Result' column in step 4 for the example log line.

At which step does nginx start using the custom log format for writing logs?

A Step 1

B Step 2

C Step 3

D Step 5

💡 Hint

Look for when the client request triggers logging using the custom format.

If you change the log_format directive to include $request_time, which step in the execution_table would show a different log entry?

A Step 3

B Step 4

C Step 1

D Step 6

💡 Hint

Variable replacement happens when the log entry is created.

Concept Snapshot

log_format name 'pattern';
access_log path name;

Define variables and text in pattern.
Name format to reuse.
Use in access_log to customize logs.
Nginx replaces variables per request.

Full Transcript

This lesson shows how to customize nginx logs by defining a log format with the log_format directive. You give the format a name and specify variables like $remote_addr and $request. Then you tell nginx to use this format in the access_log directive. When a client makes a request, nginx replaces the variables with real data and writes the log line in your custom style. This helps you see exactly the info you want in your logs.