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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 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 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.

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

Together, the docker run [OPTIONS] give the operator complete control over runtime behavior, 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

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. 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 all standard streams]( 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 (or any other place you need to identify a container). This works for both background and foreground Docker containers.

PID Equivalent

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

Image[:tag]

While not strictly a means of identifying a container, you can specify a version of an image you'd like to run the container with by adding image[:tag] to the command. For example, docker run ubuntu:14.04.

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 container

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 and 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. Note: This gives the container full access to local system services such as D-bus and is therefore considered insecure.
  • 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 traffic 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 command 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, Linux Capabilities, and LXC Configuration

--cap-add: Add Linux capabilities
--cap-drop: Drop Linux capabilities
--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).

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.

In addition to --privileged, the operator can have fine grain control over the capabilities using --cap-add and --cap-drop. By default, Docker has a default list of capabilities that are kept. Both flags support the value all, so if the operator wants to have all capabilities but MKNOD they could use:

$ docker run --cap-add=ALL --cap-drop=MKNOD ...

For interacting with the network stack, instead of using --privileged they should use --cap-add=NET_ADMIN to modify the network interfaces.

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.

Overriding Dockerfile Image Defaults

When a developer builds an image from a Dockerfile 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)

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 instruction. As the operator (the person running a container from the image), you can override that CMD instruction 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 Dockerfile 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>

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 Managing data in containers. 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 instruction, 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