syslog vs dmesg

Understanding /var/log/syslog and its Relation to dmesg

Both /var/log/syslog and dmesg play crucial roles in logging system activity on Linux-based systems, but they have distinct purposes and relationships. Here's a breakdown:

What is /var/log/syslog?

• This file acts as a central repository for various system logs originating from diverse sources like:
  • Users & applications
  • Background services (daemons)
  • System startup and shutdown processes
  • Kernel messages (after the system is fully booted)
• Syslog offers flexibility by allowing different log messages to be directed to specific log files within the /var/log directory based on predefined rules.
• This categorization facilitates targeted troubleshooting for specific areas of the system.

What is dmesg?

• Dmesg is a command-line tool that displays the contents of the kernel's ring buffer, a temporary storage for early boot messages and kernel-related activity.
• This buffer is volatile and resets upon system reboot, meaning dmesg only shows currently ongoing kernel activity.
• Dmesg isn't a file; it's a real-time snapshot of the kernel ring buffer.

Relationship between /var/log/syslog and dmesg:

• While syslog captures various system logs, dmesg focuses specifically on kernel messages.
• Early boot messages, before syslog starts, are only accessible through dmesg.
• Once syslog is operational, kernel messages are also directed to /var/log/syslog by default.
• So, dmesg offers a real-time glimpse of kernel activity, while /var/log/syslog provides a more comprehensive historical record of various system events, including kernel messages.

Key differences:

Feature	/var/log/syslog	dmesg
Scope	Diverse system logs	Kernel messages
Persistence	Persistent across reboots	Volatile, resets on reboot
Real-time view	Not directly	Yes
Specific file	Yes (individual files for categories)	No (command-line tool)

Note: I got the above from google gemini while looking about syslog and dmesg.