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Containers
Containers Logs

Understanding container logging

  • The container application does not connect to a STDOUT, which is why logs, by default, are written to the container
  • Use docker logs mycontainer to get access to the container log
  • Using docker logs is convenient for troubleshooting

Demo

Output
controlplane $ docker run -d mariadb
5c2398dbcbfff2d400f4a3d910e32a3f8ab75171641fb60366e7f95ec259a214
controlplane $ docker ps -a
CONTAINER ID   IMAGE     COMMAND                  CREATED          STATUS                      PORTS                                   NAMES
5c2398dbcbff   mariadb   "docker-entrypoint.s…"   6 seconds ago    Exited (1) 4 seconds ago                                            xander

As you can see the mariadb container exited with status code 1 i.e., with error. To check for logs to inspect the error use docker logs.

Output
controlplane $ docker logs xander
2024-10-18 22:18:28+00:00 [Note] [Entrypoint]: Entrypoint script for MariaDB Server 1:11.5.2+maria~ubu2404 started.
2024-10-18 22:18:29+00:00 [Warn] [Entrypoint]: /sys/fs/cgroup/cpuset:/system.slice/docker-5c2398dbcbfff2d400f4a3d910e32a3f8ab75171641fb60366e7f95ec259a214.scope
2024-10-18 22:18:29+00:00 [Note] [Entrypoint]: Switching to dedicated user 'mysql'
2024-10-18 22:18:29+00:00 [Note] [Entrypoint]: Entrypoint script for MariaDB Server 1:11.5.2+maria~ubu2404 started.
2024-10-18 22:18:29+00:00 [ERROR] [Entrypoint]: Database is uninitialized and password option is not specified
        You need to specify one of MARIADB_ROOT_PASSWORD, MARIADB_ROOT_PASSWORD_HASH, MARIADB_ALLOW_EMPTY_ROOT_PASSWORD and MARIADB_RANDOM_ROOT_PASSWORD

In the log inspection we found that it is asking to provide one of the environment variable to run the container.

Output
controlplane $ docker run -d --name xander  -e MARIADB_ROOT_PASSWORD=password mariadb       
docker: Error response from daemon: Conflict. The container name "/xander" is already in use by container "dff2bf072092f8a2a19029dee93d99c1fbb8a0274828b8e4c8dbf34195c9597d". You have to remove (or rename) that container to be able to reuse that name.
See 'docker run --help'.

Now there is a conflict because container with same name is already present.

Output
controlplane $ docker rm xander
xander

Try again running the same command and this time it worked!!

Output
controlplane $ docker run -d --name xander -e MARIADB_ROOT_PASSWORD=password mariadb
aa39301ca6be70ed02b2e3ebf34e6b4da7330e1987791ba4c31715676ce5a23d
controlplane $ docker ps -a
CONTAINER ID   IMAGE     COMMAND                  CREATED         STATUS         PORTS      NAMES
aa39301ca6be   mariadb   "docker-entrypoint.s…"   6 seconds ago   Up 5 seconds   3306/tcp   xander
Container ImagesContainer Images