Configuring Nginx Access and Error Logs, Custom Log Formats, and Log File Caching

Nginx logs are essential for statistics and troubleshooting. There are two main log types: access_log (access logs) and error_log (error logs). Access logs record client requests, while error logs capture problems during request processing.

Setting access_log

The access_log directive records each client request, including IP, browser, referer, request time, URL, etc. The log format can be customized with the log_format directive.

Syntax

access_log path [format [buffer=size] [gzip[=level]] [flush=time] [if=condition]];

path – location of the log file. format – name of the log format (default is combined ). buffer – size of the write buffer (default 64k). gzip – compress log before writing (level 1‑9, default 1). flush – time after which buffered data is flushed. if – condition that determines whether a request is logged.

The special value off disables logging in the current scope.

Scope

access_log

can be used in http, server, location, and limit_except contexts.

Basic Usage

access_log /var/logs/nginx-access.log

Uses the default combined format.

access_log /var/logs/nginx-access.log buffer=32k gzip flush=1m

Specifies a 32k buffer, enables gzip compression, and flushes every minute.

Customizing Log Format with log_format

The predefined combined format can be overridden by defining a new format:

log_format combined '$remote_addr - $remote_user [$time_local] '
                '"$request" $status $body_bytes_sent '
                '"$http_referer" "$http_user_agent"';

To create a custom format, use:

Syntax

log_format name [escape=default|json] string ...;

name – identifier used in access_log . escape – encoding of variables (default or json). string – the actual format string, which may contain Nginx variables.

Example of defining and using a custom format:

access_log /var/logs/nginx-access.log main
log_format main '$remote_addr - $remote_user [$time_local] "$request" '
                '$status $body_bytes_sent "$http_referer" '
                '"$http_user_agent" "$http_x_forwarded_for"';

A sample log line produced by this format is shown in the article.

Setting error_log

The error_log directive records server and request processing errors.

Syntax

error_log file [level];

Default level is error. Levels include debug, info, notice, warn, error, crit, alert, emerg.

Basic Usage

error_log /var/logs/nginx/nginx-error.log

Can be placed in http, server, location, etc.

open_log_file_cache

When log file paths contain variables, opening and closing files for each request can be costly. The open_log_file_cache directive caches file descriptors.

Syntax

open_log_file_cache max=N [inactive=time] [min_uses=N] [valid=time];

max – maximum number of cached descriptors. inactive – time after which unused descriptors are closed (default 10s). min_uses – minimum accesses within inactive to keep cached (default 1). valid – interval to re‑check file names (default 60s). off – disables caching.

Basic Usage

open_log_file_cache max=1000 inactive=20s valid=1m min_uses=2;

Can be set in http, server, or location contexts.

Summary

By configuring access_log and error_log, using log_format for custom log structures, and optionally enabling open_log_file_cache when log paths contain variables, you can efficiently collect and manage Nginx logs. For a full list of log variables, refer to the official Nginx documentation.