moby--moby/docs/reference/logging/journald.md

Journald logging driver

The journald logging driver sends container logs to the systemd journal. Log entries can be retrieved using the journalctl command, through use of the journal API, or using the docker logs command.

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.

Usage

You can configure the default logging driver by passing the --log-driver option to the Docker daemon:

docker daemon --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 ...

Options

Users can use the --log-opt NAME=VALUE flag to specify additional journald logging driver options.

labels and env

The labels and env options each take a comma-separated list of keys. If there is collision between label and env keys, the value of the env takes precedence. Both options add additional metadata in the journal with each message.

Note regarding container names

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.

Retrieving log messages with journalctl

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

Retrieving log messages with the journal API

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)