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Update 2020-01-09 22:57:08
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Estimated reading time: 62 minutes

Docker run reference

Docker runs processes in isolated containers. A container is a process which runs on a host. The host may be local or remote. When an operator executes docker run, the container process that runs is isolated in that it has its own file system, its own networking, and its own isolated process tree separate from the host.

This page details how to use the docker run command to define the container’s resources at runtime.

General form

The basic docker run command takes this form:

$ docker run [OPTIONS] IMAGE[:TAG|@DIGEST] [COMMAND] [ARG...]

The docker run command must specify an IMAGE to derive the container from. An image developer can define image defaults related to:

  • detached or foreground running
  • container identification
  • network settings
  • runtime constraints on CPU and memory

With the docker run [OPTIONS] an operator can add to or override the image defaults set by a developer. And, additionally, operators can override nearly all the defaults set by the Docker runtime itself. The operator’s ability to override image and Docker runtime defaults is why run has more options than any other docker command.

To learn how to interpret the types of [OPTIONS], see Option types.

Note: Depending on your Docker system configuration, you may be required to preface the docker run command with sudo. To avoid having to use sudo with the docker command, your system administrator can create a Unix group called docker and add users to it. For more information about this configuration, refer to the Docker installation documentation for your operating system.

Operator exclusive options

Only the operator (the person executing docker run) can set the following options.

  • Detached vs foreground
    • Detached (-d)
    • Foreground
  • Container identification
    • Name (--name)
    • PID equivalent
  • IPC settings (--ipc)
  • Network settings
  • Restart policies (--restart)
  • Clean up (--rm)
  • Runtime constraints on resources
  • Runtime privilege and Linux capabilities

Detached vs foreground

When starting a Docker container, you must first decide if you want to run the container in the background in a detached” mode or in the default foreground mode:

-d=false: Detached mode: Run container in the background, print new container id

Detached (-d)

To start a container in detached mode, you use -d=true or just -d option. By design, containers started in detached mode exit when the root process used to run the container exits, unless you also specify the --rm option. If you use -d with --rm, the container is removed when it exits or when the daemon exits, whichever happens first.

Do not pass a service x start command to a detached container. For example, this command attempts to start the nginx service.

$ docker run -d -p 80:80 my_image service nginx start

This succeeds in starting the nginx service inside the container. However, it fails the detached container paradigm in that, the root process (service nginx start) returns and the detached container stops as designed. As a result, the nginx service is started but could not be used. Instead, to start a process such as the nginx web server do the following:

$ docker run -d -p 80:80 my_image nginx -g 'daemon off;'

To do input/output with a detached container use network connections or shared volumes. These are required because the container is no longer listening to the command line where docker run was run.

To reattach to a detached container, use docker attach command.

Foreground

In foreground mode (the default when -d is not specified), docker run can start the process in the container and attach the console to the process’s standard input, output, and standard error. It can even pretend to be a TTY (this is what most command line executables expect) and pass along signals. All of that is configurable:

-a=[]Attach to `STDIN`, `STDOUT` and/or `STDERR` -tAllocate a pseudo-tty --sig-proxy=true: Proxy all received signals to the process (non-TTY mode only) -iKeep STDIN open even if not attached

If you do not specify -a then Docker will attach to both stdout and stderr . You can specify to which of the three standard streams (STDIN, STDOUT, STDERR) you’d like to connect instead, as in:

$ docker run -a stdin -a stdout -i -t ubuntu /bin/bash

For interactive processes (like a shell), you must use -i -t together in order to allocate a tty for the container process. -i -t


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