journalctl Logs

· 2 min read

The journalctl command is used to query and display messages from the journal, which is a component of the systemd system and service manager. It provides a way to view logs from various sources, including the kernel, system services, and user applications.

Basic Usage:

To view the logs, you can use the following command:

journalctl

This will display the logs in chronological order, with the most recent entries at the bottom. You can scroll through the logs using the arrow keys or Page Up and Page Down. You can also use the --follow option to follow the logs in real-time, similar to the tail -f command:

journalctl --follow

You can view the logs in reverse order (most recent first) using the --reverse option:

journalctl --reverse

Filtering Logs:

You can filter the logs by various criteria, such as time, service, or priority. Here are some common options:

  • --unit=<service>: Show logs for a specific service. For example, to view logs for the mysql service:
journalctl --unit=mysql
  • --since=<time>: Show logs since a specific time. For example, to view logs since yesterday:
journalctl --since="yesterday"
  • --until=<time>: Show logs until a specific time. For example, to view logs until 2 hours ago:
journalctl --until="2 hours ago"
  • --dmesg: Show kernel message log from the current boot. This is useful for debugging hardware issues. journalctl --dmesg is better than dmesg because it shows the logs in a more readable format with timestamps and allows you to filter them.
journalctl --dmesg

My most-used commands are:

journalctl --dmesg --output=short-iso
journalctl --dmesg --output=short-iso --since "10 minutes ago"
journalctl --dmesg --output=short-iso --since "1 hour ago"
journalctl --dmesg --output=short-iso --since "1 day ago"
journalctl --dmesg --reverse --output=short-iso

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