How to Use the Command `systemd-cat` (with Examples)

systemd-cat is a useful command-line utility in the Linux ecosystem that allows users to connect output streams from programs and pipelines directly to the systemd journal. The systemd journal is a powerful logging framework that captures log entries and system messages, providing an organized way to manage and analyze logs. By using systemd-cat, users can easily redirect program outputs to the systemd journal, making system monitoring, logging, and debugging more efficient and standardized.

Use Case 1: Writing the Output of a Command to the Journal

Code:

systemd-cat command

Motivation:

Imagine you are a system administrator, and you have a custom script or command whose output you want to preserve for later analysis, debugging, or auditing. Using systemd-cat to log this output directly to the systemd journal ensures that you maintain a centralized log repository. This approach also provides the benefit of accessing logs with detailed metadata, such as timestamps and process identifiers, which are automatically recorded by the systemd journal. By using systemd-cat, you can ensure that both the standard output (stdout) and standard error (stderr) streams from your command are captured and logged concurrently, providing a complete overview of its execution.

Explanation:

• systemd-cat: This command is used to route the output of other programs or commands directly into the systemd journal.

• command: This represents any command or script whose output you wish to log. The output from command is captured by systemd-cat and logged.

Example Output:

Upon executing a command with systemd-cat, the output is not displayed on the terminal. Instead, it is captured by the systemd journal. To view it, you would typically use the journalctl command like so:

journalctl -b

Here, you might see entries like:

Oct 15 10:32:15 hostname example-command[12345]: Command output message

This output reflects the structured log entry created in the journal, where hostname is the name of the machine, example-command is a placeholder for the actual command used, and 12345 is the process ID.

Use Case 2: Writing the Output of a Pipeline to the Journal, Keeping stderr in the Terminal

Code:

command | systemd-cat

Motivation:

There are scenarios where you might want to capture the standard output of a pipeline operation in the journal but still view any standard error messages on the terminal. This can be particularly useful for ongoing tasks where errors need immediate attention while standard operations are logged for later inspection. This use case is practical for developers and IT professionals monitoring long-running processes or pipelines but still need visibility into any emerging issues without referring to logs in real-time.

Explanation:

• command: Represents any command or pipeline that generates data through standard output (stdout). The command here can be a sequence of operations, for example, ls -l | grep '.txt'.

• |: This is the pipeline operator in Unix-like systems, used to chain commands together, where the output of the command on the left serves as input to the command on the right.

• systemd-cat: Captures the output (from stdout) of the preceding pipeline command and logs it to the systemd journal. Importantly, stderr (standard error) from the initial command remains printed on the terminal, not redirected unless explicitly specified.

Example Output:

If you run a similar command:

echo "This should go to the journal" | systemd-cat

You won’t see anything in the terminal for the successful output, as it’s logged. However, if there’s an error in the pipeline, such as a command that doesn’t exist, you might see an error message on the terminal like:

bash: non_existing_command: command not found

The standard output message, though, will be in the journal:

journalctl -b

This log entry will showcase the message, ensuring successful outputs are central and errors remain immediate for the user’s attention.

Conclusion:

The systemd-cat command offers a flexible way to handle logging of both standalone commands and complex pipelines. Whether the need is to capture comprehensive logs including both outputs and errors or to separate these for immediate terminal visibility, systemd-cat integrates seamlessly with the systemd journal. This ensures logs are not only captured but are also structured and accessible, embodying efficient log management practices for modern Linux systems.