By default, Docker captures the standard output (and standard error) of all your containers, and writes them in files using the JSON format. The JSON format annotates each line with its origin (stdout or stderr) and its timestamp. Each log file contains information about only one container.

{
  "log": "Log line is here\n",
  "stream": "stdout",
  "time": "2019-01-01T11:11:11.111111111Z"
}

[!WARNING]

The json-file logging driver uses file-based storage. These files are designed to be exclusively accessed by the Docker daemon. Interacting with these files with external tools may interfere with Docker's logging system and result in unexpected behavior, and should be avoided.

Usage#

To use the json-file driver as the default logging driver, set the log-driver and log-opts keys to appropriate values in the daemon.json file, which is located in /etc/docker/ on Linux hosts or C:\ProgramData\docker\config\ on Windows Server. If the file does not exist, create it first. For more information about configuring Docker using daemon.json, see daemon.json.

The following example sets the log driver to json-file and sets the max-size and max-file options to enable automatic log-rotation.

{
  "log-driver": "json-file",
  "log-opts": {
    "max-size": "10m",
    "max-file": "3"
  }
}

[!NOTE]

log-opts configuration options in the daemon.json configuration file must be provided as strings. Boolean and numeric values (such as the value for max-file in the example above) must therefore be enclosed in quotes (").

Restart Docker for the changes to take effect for newly created containers. Existing containers don't use the new logging configuration automatically.

You can set the logging driver for a specific container by using the --log-driver flag to docker container create or docker run:

$ docker run \
      --log-driver json-file --log-opt max-size=10m \
      alpine echo hello world

Options#

The json-file logging driver supports the following logging options:

Option	Description	Example value
max-size	The maximum size of the log before it is rolled. A positive integer plus a modifier representing the unit of measure (k, m, or g). Defaults to -1 (unlimited).	--log-opt max-size=10m
max-file	The maximum number of log files that can be present. If rolling the logs creates excess files, the oldest file is removed. Only effective when max-size is also set. A positive integer. Defaults to 1.	--log-opt max-file=3
labels	Applies when starting the Docker daemon. A comma-separated list of logging-related labels this daemon accepts. Used for advanced log tag options.	--log-opt labels=production_status,geo
labels-regex	Similar to and compatible with labels. A regular expression to match logging-related labels. Used for advanced log tag options.	--log-opt labels-regex=^(production_status\|geo)
env	Applies when starting the Docker daemon. A comma-separated list of logging-related environment variables this daemon accepts. Used for advanced log tag options.	--log-opt env=os,customer
env-regex	Similar to and compatible with env. A regular expression to match logging-related environment variables. Used for advanced log tag options.	--log-opt env-regex=^(os\|customer)
compress	Toggles compression for rotated logs. Default is disabled.	--log-opt compress=true

Examples#

This example starts an alpine container which can have a maximum of 3 log files no larger than 10 megabytes each.

$ docker run -it --log-opt max-size=10m --log-opt max-file=3 alpine ash