Docker

[Docker] How to Retrieve Container Logs

To retrieve container logs, follow these detailed steps:

Step 1 : First, run an Nginx container in detached mode so it runs in the background.

docker container run --detach --name nginx nginx

Step 2 : To see the logs produced by the container, use the docker container logs command.

docker container logs nginx

Step 3 : To view only the last few lines of the logs, use the --tail option. This example retrieves the last 5 lines.

docker container logs --tail 5 nginx

Step 4 : To continuously monitor the logs as they are written, use the --follow option along with --tail.

docker container logs --tail 5 --follow nginx

To stop following the logs, press Ctrl + C.

Step 5 : After you are done, you can remove the container using the docker container rm command with the --force option to stop and remove the container.

docker container rm --force nginx

Logging Drivers

Docker supports several logging drivers, with the default being json-file. Available logging drivers include:

  • none: Disables logging for the container.
  • json-file: Logs are stored as JSON files on the host filesystem.
  • journald: Logs are sent to journald (systemd's logging service).
  • syslog: Logs are sent to the syslog daemon.
  • gelf: Logs are sent to a Graylog Extended Log Format (GELF) endpoint.
  • fluentd: Logs are sent to a Fluentd endpoint.
  • awslogs: Logs are sent to Amazon CloudWatch Logs.
  • splunk: Logs are sent to a Splunk endpoint.

Step 6 : You can set a specific logging driver for a container at the time of its creation. For example, to set the none logging driver:

docker container run --name test -it --log-driver none busybox sh -c 'for N in 1 2 3; do echo "Hello $N"; done'

Step 7 : After running the container, you can check the logs using:

docker container logs test

Step 8 : Remove the test container after you are done:

docker container rm test

Changing the Default Logging Driver

To change the default logging driver for Docker on an Ubuntu 22.04 system, follow these steps:

Step 9 : Go to the Docker configuration directory.

cd /etc/docker

Step 10 : Open the daemon.json file in a text editor.

nano daemon.json

Add the following configuration to set local as the logging driver:

{
  "log-driver": "local"
}

- This JSON configuration sets the default logging driver to local, which stores container logs in a custom format designed to be efficient and fast.

Step 11 : Restart Docker to apply the changes:

sudo systemctl restart docker

Step 12 : Check the current logging driver to ensure it has been set correctly:

docker info --format '{{.LoggingDriver}}'

Step 13 : Reopen the daemon.json file to modify the logging driver settings:

nano daemon.json

Update the configuration to set json-file as the logging driver and include additional logging options:

{
  "log-driver": "json-file",
  "log-opts": {
    "max-size": "10m",
    "max-file": "3",
    "labels": "production_status",
    "env": "os,customer"
  }
}

This JSON configuration sets the logging driver to json-file and includes options to rotate log files with a maximum size of 10MB and up to 3 files. It also specifies that log entries should include the production_status label and environment variables os and customer.

Step 14 : Restart Docker to apply the changes:

sudo systemctl restart docker

Step 15 : Check the current logging driver again to ensure the new settings have been applied:

docker info --format '{{.LoggingDriver}}'

Congratulations, you have successfully configured and retrieved Docker container logs!