Advanced Techniques for Troubleshooting with journalctl

You can filter logs by unit, which represents a system service or process. This
is useful when you want to focus on logs related to a specific service or
process.

To filter logs by unit, use the -u or –unit option followed by the unit name.
For example, to view logs for the Apache web server, you can use the following
command:

journalctl -u apache2

You can also filter logs based on a specific time range. This is helpful when
you want to analyze logs from a specific period, such as during a system outage
or when a problem occurred.

To filter logs by time, use the –since and –until options followed by the
desired time range. For example, to view logs from the last hour, you can use
the following command:

journalctl –since "1 hour ago"

Another useful filtering option is by log priority. Log messages are assigned
different priorities, such as "emerg" (emergency), "alert", "crit" (critical),
"err" (error), "warning", "notice", "info" (informational), and "debug".
Filtering by priority allows you to focus on logs of a specific severity level.

To filter logs by priority, use the -p or –priority option followed by the
desired priority level. For example, to view only error and critical logs, you
can use the following command:

journalctl -p err -p crit

One way to analyze logs is by searching for specific keywords or patterns. This
can be useful when you are looking for specific error messages or when you want
to identify recurring events.

To search for keywords in logs, use the -t or –grep option followed by the
keyword. For example, to search for logs related to a network issue, you can
use the following command:

journalctl -t network

Another helpful feature of journalctl is the ability to display log statistics.
This provides an overview of the log messages, including the number of log
entries, the number of unique units, and the most frequent log priorities.

To display log statistics, use the –statistics option. For example, to view
log statistics for the entire journal, you can use the following command:

journalctl –statistics

Sometimes, it is necessary to monitor logs in real-time to capture events as
they occur. journalctl allows you to follow logs in real-time, similar to the
tail -f command.

To follow real-time logs, use the -f or –follow option. For example, to
monitor logs for the Apache web server in real-time, you can use the following
command:

journalctl -f -u apache2

Sometimes, it is necessary to share log files with other team members or
analyze them offline. journalctl allows you to export logs to a file for
further analysis.

To export logs to a file, use the -o or –output option followed by the desired
output format and the file path. For example, to export logs in plain text
format to a file named "logs.txt", you can use the following command:

journalctl -o cat > logs.txt

If you are troubleshooting a remote system, you can use journalctl to view its
logs without logging in to the system. This can be done by specifying the
remote system's hostname or IP address.

To view logs from a remote system, use the -b or –boot option followed by the
remote system's hostname or IP address. For example, to view logs from a remote
system with the hostname "remote.example.com", you can use the following
command:

journalctl -b -n 100 –host remote.example.com

journalctl can also be integrated with external tools for more advanced log
analysis. For example, you can use tools like awk, grep, or sed to further
filter or process log messages.

To pipe journalctl output to an external tool, use the pipe (|) operator
followed by the external tool's command. For example, to filter logs for a
specific unit and search for a keyword, you can use the following command:

journalctl -u apache2 | grep "error"