How to view Docker container logs?

Viewing Docker Container Logs

Monitoring and troubleshooting Docker containers is an essential task for any Docker user or administrator. One of the primary ways to gain insight into a container's behavior is by examining its logs. Docker provides several commands and options to view and manage container logs, which we'll explore in this answer.

Accessing Container Logs

The primary command to view the logs of a Docker container is docker logs. This command allows you to access the standard output (stdout) and standard error (stderr) streams of a running container.

Here's the basic syntax:

docker logs [OPTIONS] CONTAINER

Some common options you can use with the docker logs command include:

• -f or --follow: Follows the log output in real-time, similar to the tail -f command.
• --tail="n": Displays the last n lines of the log.
• -t or --timestamps: Includes timestamps in the log output.

For example, to view the logs of a container named my-app with timestamps and follow the log output in real-time, you would run:

docker logs -f -t my-app

This will continuously display the log output of the my-app container, including timestamps, until you interrupt the command (e.g., by pressing Ctrl+C).

Viewing Logs for Stopped Containers

Even if a container has stopped running, you can still access its logs using the docker logs command. This can be useful for troubleshooting issues that occurred during the container's lifetime.

To view the logs of a stopped container, simply use the same docker logs command, but with the container's ID or name:

docker logs my-stopped-container

Logging Drivers

Docker supports different logging drivers, which determine how container logs are handled and stored. The default logging driver is json-file, which stores logs in JSON format on the host's filesystem.

You can view the current logging driver used by your Docker daemon by running:

docker info | grep "Logging Driver"

If you need to change the logging driver, you can do so by modifying the Docker daemon configuration file (typically located at /etc/docker/daemon.json) and restarting the Docker service.

Here's an example of how to configure the Docker daemon to use the syslog logging driver:

{
  "log-driver": "syslog"
}

After making this change, restart the Docker service for the new logging driver to take effect.

Mermaid Diagram: Docker Logging Workflow

This diagram illustrates the overall workflow of Docker container logging. The Docker container sends its logs to the Docker daemon, which then passes the logs to the configured logging driver. The logging driver stores the logs in the appropriate location (e.g., local file system, syslog, or a remote logging service), and the docker logs command allows you to access and view the stored logs.

Conclusion

Viewing and managing Docker container logs is a crucial skill for any Docker user or administrator. The docker logs command provides a straightforward way to access and monitor the logs of your containers, whether they are running or stopped. Additionally, understanding the different logging drivers and how to configure them can help you better manage and analyze your container's behavior and troubleshoot any issues that may arise.