JSON File logging driver

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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.

Usage

To use the json-file driver as the default logging driver, set the log-driver and log-opt keys to appropriate values in the daemon.json file, which is located in /etc/docker/ on Linux hosts or C:\ProgramData\docker\config\daemon.json on Windows Server. For more about +configuring Docker using daemon.json, see +daemon.json.

The following example sets the log driver to json-file and sets the max-size option.

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

Restart Docker for the changes to take effect.

You can set the logging driver for a specific container by using the --log-driver flag to docker 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 will accept. Used for advanced log tag options. --log-opt labels=production_status,geo
env Applies when starting the Docker daemon. A comma-separated list of logging-related environment variables this daemon will accept. 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).

Note: If max-size and max-file are set, docker logs only returns the log lines from the newest log file.

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
json-file, docker, logging, driver