| `tag`          | optional | Specify template to set `CONTAINER_TAG` and `SYSLOG_IDENTIFIER` value in journald logs. Refer to [log tag option documentation](log_tags.md) to customize the log tag format. |
| `labels`       | optional | Comma-separated list of keys of labels, which should be included in message, if these labels are specified for the container.                                                 |
| `labels-regex` | optional | Similar to and compatible with labels. A regular expression to match logging-related labels. Used for advanced [log tag options](log_tags.md).                                |
| `env`          | optional | Comma-separated list of keys of environment variables, which should be included in message, if these variables are specified for the container.                               |
| `env-regex`    | optional | Similar to and compatible with `env`. A regular expression to match logging-related environment variables. Used for advanced [log tag options](log_tags.md).                    |

If a collision occurs between `label` and `env` options, the value of the `env`
takes precedence. Each option adds additional fields to the attributes of a
logging message.

The following is an example of the logging options required to log to journald.

```console
$ docker run \
    --log-driver=journald \
    --log-opt labels=location \
    --log-opt env=TEST \
    --env "TEST=false" \
    --label location=west \
    your/application
```

This configuration also directs the driver to include in the payload the label
location, and the environment variable `TEST`. If the `--env "TEST=false"`
or `--label location=west` arguments were omitted, the corresponding key would
not be set in the journald log.

## Note regarding container names

The value logged in the `CONTAINER_NAME` field is the name of the container that
was set at startup. If you use `docker rename` to rename a container, the new
name isn't reflected in the journal entries. Journal entries continue
to use the original name.

## Retrieve log messages with `journalctl`

Use the `journalctl` command to retrieve log messages. You can apply filter
expressions to limit the retrieved messages to those associated with a specific
container:

```console
$ sudo journalctl CONTAINER_NAME=webserver
```

You can use additional filters to further limit the messages retrieved. The `-b`
flag only retrieves messages generated since the last system boot:

```console
$ sudo journalctl -b CONTAINER_NAME=webserver
```

The `-o` flag specifies the format for the retrieved log messages. Use `-o json`
to return the log messages in JSON format.

```console
$ sudo journalctl -o json CONTAINER_NAME=webserver
```

### View logs for a container with a TTY enabled

If TTY is enabled on a container you may see `[10B blob data]` in the output
when retrieving log messages.
The reason for that is that `\r` is appended to the end of the line and
`journalctl` doesn't strip it automatically unless `--all` is set:

```console
$ sudo journalctl -b CONTAINER_NAME=webserver --all
```

## Retrieve log messages with the `journal` API

This example uses the `systemd` Python module to retrieve container
logs:

```python
import systemd.journal

reader = systemd.journal.Reader()
reader.add_match('CONTAINER_NAME=web')

for msg in reader:
    print '{CONTAINER_ID_FULL}: {MESSAGE}'.format(**msg)
```