journalctl Command: Tutorial & Examples
Managing log files
journalctl command is an indispensable tool in the arsenal of a Linux administrator. It is used to view and manage
the logs generated by the systemd journal. The systemd journal is a system service that collects
and stores logging data. It creates and maintains structured, indexed journals based on logging information that is
received from the kernel, the init system, and other user processes.
journalctl command allows you to filter the logs by different criteria such as date, service, priority and more.
Understanding how to use this command effectively can help you troubleshoot issues
like high load or network failure more efficiently.
Why is journalctl important?
The importance of
journalctl lies in its ability to provide a detailed view of what's happening in the system. Linux
logs almost everything that happens in the system, and these logs can be instrumental in figuring out what went wrong
when an issue arises. By using
journalctl, you can filter out the noise and focus on the logs that matter.
How to use the journalctl command
To use the
journalctl command, you simply need to type
journalctl in your shell. This will
display the entire systemd journal, starting with the oldest logs. However, the real power of
journalctl comes from
its versatile filtering options.
Here are some examples:
To view the logs in reverse order (newest first), you can use the
To display the logs from a specific unit, you can use the
-uoption. For example, to view the logs from the sshd service, you would use:
journalctl -u sshd
You can use the
-boption to view the logs since the last boot:
The typical output of the
journalctl command will look something like this:
-- Logs begin at Thu 2019-04-11 14:52:38 PDT, end at Wed 2020-03-11 08:38:42 PDT. --
Mar 11 08:34:17 localhost.localdomain systemd: Started Daily Cleanup of Temporary Directories.
Mar 11 08:34:17 localhost.localdomain systemd: Starting Daily Cleanup of Temporary Directories...
Each line represents a single log entry, with the timestamp, the host, the process that generated the log, and the message.
Common problems and solutions using journalctl
One common problem that administrators face is the overwhelming amount of logs that Linux systems generate.
journalctl command, with its filtering capabilities, can help alleviate this problem.
For example, if you are facing a high load on your server and you suspect that a particular service (like httpd) is causing it, you could use the following command to view only the logs generated by the httpd service:
journalctl -u httpd
Similarly, if your system is experiencing a network failure, you might want to check the logs for the network service:
journalctl -u network
In conclusion, the
journalctl command is a powerful tool for managing and troubleshooting Linux systems. Whether you
are a seasoned administrator or a newbie, understanding how to use
journalctl effectively will undoubtedly make your