The journald logging driver sends container logs to the systemd journal. Log entries can be retrieved using the journalctl command or through use of the journal API.
In addition to the text of the log message itself, the journald log driver stores the following metadata in the journal with each message:
| Field | Description |
|---|---|
CONTAINER_ID | The container ID truncated to 12 characters. |
CONTAINER_ID_FULL | The full 64-character container ID. |
CONTAINER_NAME | The container name at the time it was started. If you use docker rename to rename a container, the new name is not reflected in the journal entries. |
You can configure the default logging driver by passing the --log-driver option to the Docker daemon:
docker --log-driver=journald
You can set the logging driver for a specific container by using the --log-driver option to docker run:
docker run --log-driver=journald ...
The value logged in the CONTAINER_NAME field is the container name that was set at startup. If you use docker rename to rename a container, the new name will not be reflected in the journal entries. Journal entries will continue to use the original name.
You can use the journalctl command to retrieve log messages. You can apply filter expressions to limit the retrieved messages to a specific container. For example, to retrieve all log messages from a container referenced by name:
# journalctl CONTAINER_NAME=webserver
You can make use of additional filters to further limit the messages retrieved. For example, to see just those messages generated since the system last booted:
# journalctl -b CONTAINER_NAME=webserver
Or to retrieve log messages in JSON format with complete metadata:
# journalctl -o json CONTAINER_NAME=webserver
This example uses the systemd Python module to retrieve container logs:
import systemd.journal
reader = systemd.journal.Reader()
reader.add_match('CONTAINER_NAME=web')
for msg in reader:
print '{CONTAINER_ID_FULL}: {MESSAGE}'.format(**msg)