page_title: Docker Run Reference
page_description: Configure containers at runtime
page_keywords: docker, run, configure, runtime

# 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 [*Image*](/terms/image/#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 [*run*](/reference/commandline/cli/#cli-run) has more options than any
other `docker` command.

## General Form

The basic `docker run` command takes this form:

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

To learn how to interpret the types of `[OPTIONS]`,
see [*Option types*](/reference/commandline/cli/#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.

 - [Detached vs Foreground](#detached-vs-foreground)
     - [Detached (-d)](#detached-d)
     - [Foreground](#foreground)
 - [Container Identification](#container-identification)
     - [Name (--name)](#name-name)
     - [PID Equivalent](#pid-equivalent)
 - [Network Settings](#network-settings)
 - [Clean Up (--rm)](#clean-up-rm)
 - [Runtime Constraints on CPU and
    Memory](#runtime-constraints-on-cpu-and-memory)
 - [Runtime Privilege and LXC
    Configuration](#runtime-privilege-and-lxc-configuration)

## 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 command line where you executed `docker run`.
You can reattach to a detached container with `docker`
[*attach*](/reference/commandline/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 command line 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
[*links*](/userguide/dockerlinks/#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

    --dns=[]        : Set custom dns servers for the container
    --net="bridge"  : Set the Network mode for the container
                                 'bridge': creates a new network stack for the container on the docker bridge
                                 'none': no networking for this container
                                 'container:<name|id>': reuses another container network stack
                                 'host': use the host network stack inside the contaner

By default, all containers have networking enabled and they can make any
outgoing connections. The operator can completely disable networking
with `docker run --net none` 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`.

Supported networking modes are: 

* none - no networking in the container
* bridge - (default) connect the container to the bridge via veth interfaces
* host - use the host's network stack inside the container
* container - use another container's network stack

#### Mode: none
With the networking mode set to `none` a container will not have a access to 
any external routes.  The container will still have a `loopback` interface 
enabled in the container but it does not have any routes to external traffic.

#### Mode: bridge
With the networking mode set to `bridge` a container will use docker's default
networking setup.  A bridge is setup on the host, commonly named `docker0`, 
and a pair of veth interfaces will be created for the container.  One side of 
the veth pair will remain on the host attached to the bridge while the other 
side of the pair will be placed inside the container's namespaces in addition 
to the `loopback` interface.  An IP address will be allocated for containers 
on the bridge's network and trafic will be routed though this bridge to the
container.

#### Mode: host
With the networking mode set to `host` a container will share the host's
network stack and all interfaces from the host will be available to the 
container.  The container's hostname will match the hostname on the host 
system.  Publishing ports and linking to other containers will not work 
when sharing the host's network stack.  

#### Mode: container
With the networking mode set to `container` a container will share the 
network stack of another container.  The other container's name must be 
provided in the format of `--net container:<name|id>`.

Example running a redis container with redis binding to localhost then 
running the redis-cli and connecting to the redis server over the 
localhost interface.

    $ docker run -d --name redis example/redis --bind 127.0.0.1
    $ # use the redis container's network stack to access localhost
    $ docker run --rm -ti --net container:redis example/redis-cli -h 127.0.0.1

## 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](
https://github.com/dotcloud/docker/blob/master/daemon/execdriver/lxc/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](
https://github.com/dotcloud/docker/blob/master/daemon/execdriver/lxc/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.

## Overriding Dockerfile Image Defaults

When a developer builds an image from a [*Dockerfile*](/reference/builder/#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.

 - [CMD (Default Command or Options)](#cmd-default-command-or-options)
 - [ENTRYPOINT (Default Command to Execute at Runtime)](
    #entrypoint-default-command-to-execute-at-runtime)
 - [EXPOSE (Incoming Ports)](#expose-incoming-ports)
 - [ENV (Environment Variables)](#env-environment-variables)
 - [VOLUME (Shared Filesystems)](#volume-shared-filesystems)
 - [USER](#user)
 - [WORKDIR](#workdir)

## 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'sexposed 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>

Docker will also map the private IP address to the alias of a linked
container by inserting an entry into `/etc/hosts`.  You can use this
mechanism to communicate with a linked container by its alias:

    $ docker run -d --name servicename busybox sleep 30
    $ docker run -i -t --link servicename:servicealias busybox ping -c 1 servicealias

## 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 [*Share Directories via Volumes*](/userguide/dockervolumes/#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

> **Note:** if you pass numeric uid, it must be in range 0-2147483647.

## 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
