:title: Docker Run Reference 
:description: Configure containers at runtime
:keywords: docker, run, configure, runtime

.. _run_docker:

====================
Docker Run Reference
====================

**Docker runs processes in isolated containers**.  When an operator
executes ``docker run``, she starts a process with its own file
system, its own networking, and its own isolated process tree. The
:ref:`image_def` which starts the process may define defaults related
to the binary to run, the networking to expose, and more, but ``docker
run`` gives final control to the operator who starts the container
from the image. That's the main reason :ref:`cli_run` has more options
than any other ``docker`` command.

Every one of the :ref:`example_list` shows running containers, and so
here we try to give more in-depth guidance.

.. contents:: Table of Contents
   :depth: 2

.. _run_running:

General Form
============

As you've seen in the :ref:`example_list`, the basic `run` command
takes this form::

  docker run [OPTIONS] IMAGE[:TAG] [COMMAND] [ARG...]

To learn how to interpret the types of ``[OPTIONS]``, see
:ref:`cli_options`.

The list of ``[OPTIONS]`` breaks down into two groups: 

1. Settings exclusive to operators, including:

   * Detached or Foreground running,
   * Container Identification,
   * Network settings, and
   * Runtime Constraints on CPU and Memory
   * Privileges and LXC Configuration

2. Setting shared between operators and developers, where operators
   can override defaults developers set in images at build time.

Together, the ``docker run [OPTIONS]`` give complete control over
runtime behavior to the operator, allowing them to override all
defaults set by the developer during ``docker build`` and nearly all
the defaults set by the Docker runtime itself.

Operator Exclusive Options
==========================

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

.. contents::
   :local:

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)
.............

In detached mode (``-d=true`` or just ``-d``), all I/O should be done
through network connections or shared volumes because the container is
no longer listening to the commandline where you executed ``docker
run``. You can reattach to a detached container with ``docker``
:ref:`cli_attach`. If you choose to run a container in the detached
mode, then you cannot use the ``--rm`` option.

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 commandline
executables expect) and pass along signals. All of that is
configurable::

   -a=[]           : Attach to ``stdin``, ``stdout`` and/or ``stderr``
   -t=false        : Allocate a pseudo-tty
   --sig-proxy=true: Proxify all received signal to the process (even in non-tty mode)
   -i=false        : Keep STDIN open even if not attached

If you do not specify ``-a`` then Docker will `attach everything
(stdin,stdout,stderr)
<https://github.com/dotcloud/docker/blob/75a7f4d90cde0295bcfb7213004abce8d4779b75/commands.go#L1797>`_. 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 will typically want a tty
as well as persistent standard input (``stdin``), so you'll use ``-i
-t`` together in most interactive cases.

Container Identification
------------------------

Name (--name)
.............

The operator can identify a container in three ways:

* UUID long identifier ("f78375b1c487e03c9438c729345e54db9d20cfa2ac1fc3494b6eb60872e74778")
* UUID short identifier ("f78375b1c487")
* Name ("evil_ptolemy")

The UUID identifiers come from the Docker daemon, and if you do not
assign a name to the container with ``--name`` then the daemon will
also generate a random string name too. The name can become a handy
way to add meaning to a container since you can use this name when
defining :ref:`links <working_with_links_names>` (or any other place
you need to identify a container). This works for both background and
foreground Docker containers.

PID Equivalent
..............

And finally, to help with automation, you can have Docker write the
container ID out to a file of your choosing. This is similar to how
some programs might write out their process ID to a file (you've seen
them as PID files)::

      --cidfile="": Write the container ID to the file

Network Settings
----------------

::

   -n=true   : Enable networking for this container
   --dns=[]  : Set custom dns servers for the container

By default, all containers have networking enabled and they can make
any outgoing connections. The operator can completely disable
networking with ``docker run -n`` which disables all incoming and outgoing
networking. In cases like this, you would perform I/O through files or
STDIN/STDOUT only.

Your container will use the same DNS servers as the host by default,
but you can override this with ``--dns``.

Clean Up (--rm)
---------------

By default a container's file system persists even after the container
exits. This makes debugging a lot easier (since you can inspect the
final state) and you retain all your data by default. But if you are
running short-term **foreground** processes, these container file
systems can really pile up. If instead you'd like Docker to
**automatically clean up the container and remove the file system when
the container exits**, you can add the ``--rm`` flag::

   --rm=false: Automatically remove the container when it exits (incompatible with -d)


Runtime Constraints on CPU and Memory
-------------------------------------

The operator can also adjust the performance parameters of the container::

   -m="": Memory limit (format: <number><optional unit>, where unit = b, k, m or g)
   -c=0 : CPU shares (relative weight)

The operator can constrain the memory available to a container easily
with ``docker run -m``. If the host supports swap memory, then the
``-m`` memory setting can be larger than physical RAM.

Similarly the operator can increase the priority of this container
with the ``-c`` option. By default, all containers run at the same
priority and get the same proportion of CPU cycles, but you can tell
the kernel to give more shares of CPU time to one or more containers
when you start them via Docker.

Runtime Privilege and LXC Configuration
---------------------------------------

::

   --privileged=false: Give extended privileges to this container
   --lxc-conf=[]: (lxc exec-driver only) Add custom lxc options --lxc-conf="lxc.cgroup.cpuset.cpus = 0,1"

By default, Docker containers are "unprivileged" and cannot, for
example, run a Docker daemon inside a Docker container. This is
because by default a container is not allowed to access any devices,
but a "privileged" container is given access to all devices (see
lxc-template.go_ and documentation on `cgroups devices
<https://www.kernel.org/doc/Documentation/cgroups/devices.txt>`_).

When the operator executes ``docker run --privileged``, Docker will
enable to access to all devices on the host as well as set some
configuration in AppArmor to allow the container nearly all the same
access to the host as processes running outside containers on the
host. Additional information about running with ``--privileged`` is
available on the `Docker Blog
<http://blog.docker.io/2013/09/docker-can-now-run-within-docker/>`_.

If the Docker daemon was started using the ``lxc`` exec-driver
(``docker -d --exec-driver=lxc``) then the operator can also specify
LXC options using one or more ``--lxc-conf`` parameters. These can be
new parameters or override existing parameters from the lxc-template.go_.
Note that in the future, a given host's Docker daemon may not use LXC,
so this is an implementation-specific configuration meant for operators
already familiar with using LXC directly.

.. _lxc-template.go: https://github.com/dotcloud/docker/blob/master/execdriver/lxc/lxc_template.go


Overriding ``Dockerfile`` Image Defaults
========================================

When a developer builds an image from a :ref:`Dockerfile
<dockerbuilder>` or when she commits it, the developer can set a
number of default parameters that take effect when the image starts up
as a container.

Four of the ``Dockerfile`` commands cannot be overridden at runtime:
``FROM, MAINTAINER, RUN``, and ``ADD``. Everything else has a
corresponding override in ``docker run``. We'll go through what the
developer might have set in each ``Dockerfile`` instruction and how the
operator can override that setting.

.. contents::
   :local:

CMD (Default Command or Options)
--------------------------------

Recall the optional ``COMMAND`` in the Docker commandline::

  docker run [OPTIONS] IMAGE[:TAG] [COMMAND] [ARG...]

This command is optional because the person who created the ``IMAGE``
may have already provided a default ``COMMAND`` using the ``Dockerfile``
``CMD``. As the operator (the person running a container from the
image), you can override that ``CMD`` just by specifying a new
``COMMAND``.

If the image also specifies an ``ENTRYPOINT`` then the ``CMD`` or
``COMMAND`` get appended as arguments to the ``ENTRYPOINT``.


ENTRYPOINT (Default Command to Execute at Runtime
-------------------------------------------------

::

   --entrypoint="": Overwrite the default entrypoint set by the image

The ENTRYPOINT of an image is similar to a ``COMMAND`` because it
specifies what executable to run when the container starts, but it is
(purposely) more difficult to override. The ``ENTRYPOINT`` gives a
container its default nature or behavior, so that when you set an
``ENTRYPOINT`` you can run the container *as if it were that binary*,
complete with default options, and you can pass in more options via
the ``COMMAND``. But, sometimes an operator may want to run something else
inside the container, so you can override the default ``ENTRYPOINT`` at
runtime by using a string to specify the new ``ENTRYPOINT``. Here is an
example of how to run a shell in a container that has been set up to
automatically run something else (like ``/usr/bin/redis-server``)::

  docker run -i -t --entrypoint /bin/bash example/redis

or two examples of how to pass more parameters to that ENTRYPOINT::

  docker run -i -t --entrypoint /bin/bash example/redis -c ls -l
  docker run -i -t --entrypoint /usr/bin/redis-cli example/redis --help


EXPOSE (Incoming Ports)
-----------------------

The ``Dockerfile`` doesn't give much control over networking, only
providing the ``EXPOSE`` instruction to give a hint to the operator
about what incoming ports might provide services. The following
options work with or override the ``Dockerfile``'s exposed defaults::

   --expose=[]: Expose a port from the container 
               without publishing it to your host
   -P=false   : Publish all exposed ports to the host interfaces
   -p=[]      : Publish a container's port to the host (format: 
                ip:hostPort:containerPort | ip::containerPort | 
                hostPort:containerPort) 
                (use 'docker port' to see the actual mapping)
   --link=""  : Add link to another container (name:alias)

As mentioned previously, ``EXPOSE`` (and ``--expose``) make a port
available **in** a container for incoming connections. The port number
on the inside of the container (where the service listens) does not
need to be the same number as the port exposed on the outside of the
container (where clients connect), so inside the container you might
have an HTTP service listening on port 80 (and so you ``EXPOSE 80`` in
the ``Dockerfile``), but outside the container the port might be 42800.

To help a new client container reach the server container's internal
port operator ``--expose``'d by the operator or ``EXPOSE``'d by the
developer, the operator has three choices: start the server container
with ``-P`` or ``-p,`` or start the client container with ``--link``.

If the operator uses ``-P`` or ``-p`` then Docker will make the
exposed port accessible on the host and the ports will be available to
any client that can reach the host. To find the map between the host
ports and the exposed ports, use ``docker port``)

If the operator uses ``--link`` when starting the new client container,
then the client container can access the exposed port via a private
networking interface. Docker will set some environment variables in
the client container to help indicate which interface and port to use.

ENV (Environment Variables)
---------------------------

The operator can **set any environment variable** in the container by
using one or more ``-e`` flags, even overriding those already defined by the
developer with a Dockefile ``ENV``::

   $ docker run -e "deep=purple" --rm ubuntu /bin/bash -c export
   declare -x HOME="/"
   declare -x HOSTNAME="85bc26a0e200"
   declare -x OLDPWD
   declare -x PATH="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
   declare -x PWD="/"
   declare -x SHLVL="1"
   declare -x container="lxc"
   declare -x deep="purple"

Similarly the operator can set the **hostname** with ``-h``.

``--link name:alias`` also sets environment variables, using the
*alias* string to define environment variables within the container
that give the IP and PORT information for connecting to the service
container. Let's imagine we have a container running Redis::

   # Start the service container, named redis-name
   $ docker run -d --name redis-name dockerfiles/redis
   4241164edf6f5aca5b0e9e4c9eccd899b0b8080c64c0cd26efe02166c73208f3

   # The redis-name container exposed port 6379
   $ docker ps  
   CONTAINER ID        IMAGE                      COMMAND                CREATED             STATUS              PORTS               NAMES
   4241164edf6f        dockerfiles/redis:latest   /redis-stable/src/re   5 seconds ago       Up 4 seconds        6379/tcp            redis-name  

   # Note that there are no public ports exposed since we didn't use -p or -P
   $ docker port 4241164edf6f 6379
   2014/01/25 00:55:38 Error: No public port '6379' published for 4241164edf6f


Yet we can get information about the Redis container's exposed ports
with ``--link``. Choose an alias that will form a valid environment
variable!

::

   $ docker run --rm --link redis-name:redis_alias --entrypoint /bin/bash dockerfiles/redis -c export
   declare -x HOME="/"
   declare -x HOSTNAME="acda7f7b1cdc"
   declare -x OLDPWD
   declare -x PATH="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
   declare -x PWD="/"
   declare -x REDIS_ALIAS_NAME="/distracted_wright/redis"
   declare -x REDIS_ALIAS_PORT="tcp://172.17.0.32:6379"
   declare -x REDIS_ALIAS_PORT_6379_TCP="tcp://172.17.0.32:6379"
   declare -x REDIS_ALIAS_PORT_6379_TCP_ADDR="172.17.0.32"
   declare -x REDIS_ALIAS_PORT_6379_TCP_PORT="6379"
   declare -x REDIS_ALIAS_PORT_6379_TCP_PROTO="tcp"
   declare -x SHLVL="1"
   declare -x container="lxc"

And we can use that information to connect from another container as a client::

   $ docker run -i -t --rm --link redis-name:redis_alias --entrypoint /bin/bash dockerfiles/redis -c '/redis-stable/src/redis-cli -h $REDIS_ALIAS_PORT_6379_TCP_ADDR -p $REDIS_ALIAS_PORT_6379_TCP_PORT'
   172.17.0.32:6379>

VOLUME (Shared Filesystems)
---------------------------

::

   -v=[]: Create a bind mount with: [host-dir]:[container-dir]:[rw|ro]. 
          If "container-dir" is missing, then docker creates a new volume.
   --volumes-from="": Mount all volumes from the given container(s)

The volumes commands are complex enough to have their own
documentation in section :ref:`volume_def`. A developer can define one
or more ``VOLUME``\s associated with an image, but only the operator can
give access from one container to another (or from a container to a
volume mounted on the host).

USER
----

The default user within a container is ``root`` (id = 0), but if the
developer created additional users, those are accessible too. The
developer can set a default user to run the first process with the
``Dockerfile USER`` command, but the operator can override it ::

   -u="": Username or UID

WORKDIR
-------

The default working directory for running binaries within a container is the root directory (``/``), but the developer can set a different default with the ``Dockerfile WORKDIR`` command. The operator can override this with::

   -w="": Working directory inside the container

