Journalctl

journalctl is a command-line utility in Linux used for querying and displaying messages from the journal, which is a centralized logging system. The journal is managed by systemd-journald and is part of the larger systemd system and service manager.

Here are some key points about journalctl:

Viewing Logs:
- journalctl is used to query and display messages from the journal.
- By default, it shows the entire system log, but you can filter and format the output based on various criteria.
Filtering Options:
- You can filter logs based on units (system services), time ranges, priority levels, and more.
- For example, you can use journalctl -u <unit> to view logs for a specific service.
Real-time Monitoring:
- You can use the -f option to follow the journal in real-time, similar to the tail -f command for traditional log files.

Here are some examples of journalctl usage:

To display the entire journal: journalctl
To view logs for a specific service: journalctl -u <unit>
To follow the journal in real-time: journalctl -f
To display logs since a specific time: journalctl --since "2023-01-01 00:00:00"

-x Option:
- Enables additional information in the output, such as the sender executable name and process ID. This can provide more context about the log entries.
-e Option:
- Causes journalctl to immediately exit after displaying the most recent log entries. This is useful for quickly examining recent events without continuously monitoring the journal.

-u vs -t

journalctl -u Option:
- This option is used to filter logs based on a specific unit, where a unit typically corresponds to a systemd service or other systemd-managed entity.
- Example: journalctl -u <unit>
- Replace <unit> with the name of the systemd unit (service) you want to see logs for.
- For instance, if you have a service named "myservice," you can use journalctl -u myservice to display logs specifically related to that service.
journalctl -t Option:
- This option is used to filter logs based on a specific tag, which is essentially a label or identifier associated with the origin of a log message.
- Example: journalctl -t <tag>
- Replace <tag> with the specific tag you want to filter by. This is often used by applications or services to categorize their log messages.
- For example, if an application logs messages with the tag "myapp," you can use journalctl -t myapp to see only those messages.