moby--moby/docs/sources/reference/run.md

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:

$ sudo docker run [OPTIONS] IMAGE[:TAG|@DIGEST] [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
  • Detached (-d)
  • Foreground
• Container Identification
  • Name (--name)
  • PID Equivalent
• IPC Settings (--ipc)
• Network Settings
• Restart Policies (--restart)
• Clean Up (--rm)
• Runtime Constraints on CPU and Memory
• Runtime Privilege, Linux Capabilities, 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. 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 (non-TTY mode only)
-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/docker/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:

$ sudo 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. Specifying -t is however forbidden when the client standard output is redirected or pipe, such as in: echo test | docker run -i busybox cat.

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.

Image[@digest]

Images using the v2 or later image format have a content-addressable identifier called a digest. As long as the input used to generate the image is unchanged, the digest value is predictable and referenceable.

PID Settings (--pid)

--pid=""  : Set the PID (Process) Namespace mode for the container,
       'host': use the host's PID namespace inside the container

By default, all containers have the PID namespace enabled.

PID namespace provides separation of processes. The PID Namespace removes the view of the system processes, and allows process ids to be reused including pid 1.

In certain cases you want your container to share the host's process namespace, basically allowing processes within the container to see all of the processes on the system. For example, you could build a container with debugging tools like strace or gdb, but want to use these tools when debugging processes within the container.

$ sudo docker run --pid=host rhel7 strace -p 1234

This command would allow you to use strace inside the container on pid 1234 on the host.

IPC Settings (--ipc)

--ipc=""  : Set the IPC mode for the container,
             'container:<name|id>': reuses another container's IPC namespace
             'host': use the host's IPC namespace inside the container

By default, all containers have the IPC namespace enabled.

IPC (POSIX/SysV IPC) namespace provides separation of named shared memory segments, semaphores and message queues.

Shared memory segments are used to accelerate inter-process communication at memory speed, rather than through pipes or through the network stack. Shared memory is commonly used by databases and custom-built (typically C/OpenMPI, C++/using boost libraries) high performance applications for scientific computing and financial services industries. If these types of applications are broken into multiple containers, you might need to share the IPC mechanisms of the containers.

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
--add-host=""    : Add a line to /etc/hosts (host:IP)
--mac-address="" : Sets the container's Ethernet device's MAC address

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.

By default a random MAC is generated. You can set the container's MAC address explicitly by providing a MAC via the --mac-address parameter (format: 12:34:56:78:9a:bc).

Supported networking modes are:

Mode	Description
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:<name|id>	Use the network stack of another container, specified via its *name* or *id*.

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.

Note

: --net="host" gives the container full access to local system services such as D-bus and is therefore considered insecure.

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.

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

Managing /etc/hosts

Your container will have lines in /etc/hosts which define the hostname of the container itself as well as localhost and a few other common things. The --add-host flag can be used to add additional lines to /etc/hosts.

$ /docker run -ti --add-host db-static:86.75.30.9 ubuntu cat /etc/hosts
172.17.0.22     09d03f76bf2c
fe00::0         ip6-localnet
ff00::0         ip6-mcastprefix
ff02::1         ip6-allnodes
ff02::2         ip6-allrouters
127.0.0.1       localhost
::1	            localhost ip6-localhost ip6-loopback
86.75.30.9      db-static

Restart policies (--restart)

Using the --restart flag on Docker run you can specify a restart policy for how a container should or should not be restarted on exit.

When a restart policy is active on a container, it will be shown as either Up or Restarting in docker ps. It can also be useful to use docker events to see the restart policy in effect.

Docker supports the following restart policies:

Policy	Result
no	Do not automatically restart the container when it exits. This is the default.
on-failure[:max-retries]	Restart only if the container exits with a non-zero exit status. Optionally, limit the number of restart retries the Docker daemon attempts.
always	Always restart the container regardless of the exit status. When you specify always, the Docker daemon will try to restart the container indefinitely.

An ever increasing delay (double the previous delay, starting at 100 milliseconds) is added before each restart to prevent flooding the server. This means the daemon will wait for 100 ms, then 200 ms, 400, 800, 1600, and so on until either the on-failure limit is hit, or when you docker stop or docker rm -f the container.

If a container is succesfully restarted (the container is started and runs for at least 10 seconds), the delay is reset to its default value of 100 ms.

You can specify the maximum amount of times Docker will try to restart the container when using the on-failure policy. The default is that Docker will try forever to restart the container. The number of (attempted) restarts for a container can be obtained via docker inspect. For example, to get the number of restarts for container "my-container";

$ sudo docker inspect -f "{{ .RestartCount }}" my-container
# 2

Or, to get the last time the container was (re)started;

$ docker inspect -f "{{ .State.StartedAt }}" my-container
# 2015-03-04T23:47:07.691840179Z

You cannot set any restart policy in combination with "clean up (--rm)". Setting both --restart and --rm results in an error.

###Examples

$ sudo docker run --restart=always redis

This will run the redis container with a restart policy of always so that if the container exits, Docker will restart it.

$ sudo docker run --restart=on-failure:10 redis

This will run the redis container with a restart policy of on-failure and a maximum restart count of 10. If the redis container exits with a non-zero exit status more than 10 times in a row Docker will abort trying to restart the container. Providing a maximum restart limit is only valid for the on-failure policy.

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)

Security configuration

--security-opt="label:user:USER"   : Set the label user for the container
--security-opt="label:role:ROLE"   : Set the label role for the container
--security-opt="label:type:TYPE"   : Set the label type for the container
--security-opt="label:level:LEVEL" : Set the label level for the container
--security-opt="label:disable"     : Turn off label confinement for the container
--security-opt="apparmor:PROFILE"  : Set the apparmor profile to be applied 
                                     to the container

You can override the default labeling scheme for each container by specifying the --security-opt flag. For example, you can specify the MCS/MLS level, a requirement for MLS systems. Specifying the level in the following command allows you to share the same content between containers.

# docker run --security-opt label:level:s0:c100,c200 -i -t fedora bash

An MLS example might be:

# docker run --security-opt label:level:TopSecret -i -t rhel7 bash

To disable the security labeling for this container versus running with the --permissive flag, use the following command:

# docker run --security-opt label:disable -i -t fedora bash

If you want a tighter security policy on the processes within a container, you can specify an alternate type for the container. You could run a container that is only allowed to listen on Apache ports by executing the following command:

# docker run --security-opt label:type:svirt_apache_t -i -t centos bash

Note:

You would have to write policy defining a svirt_apache_t type.

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

Memory constraints

We have four ways to set memory usage:

Option	Result
memory=inf, memory-swap=inf (default)	There is no memory limit for the container. The container can use as much memory as needed.
memory=L<inf, memory-swap=inf	(specify memory and set memory-swap as -1) The container is not allowed to use more than L bytes of memory, but can use as much swap as is needed (if the host supports swap memory).
memory=L<inf, memory-swap=2*L	(specify memory without memory-swap) The container is not allowed to use more than L bytes of memory, swap *plus* memory usage is double of that.
memory=L<inf, memory-swap=S<inf, L<=S	(specify both memory and memory-swap) The container is not allowed to use more than L bytes of memory, swap *plus* memory usage is limited by S.

CPU share constraint

By default, all containers get the same proportion of CPU cycles. This proportion can be modified by changing the container's CPU share weighting relative to the weighting of all other running containers.

To modify the proportion from the default of 1024, use the -c or --cpu-shares flag to set the weighting to 2 or higher.

The proportion will only apply when CPU-intensive processes are running. When tasks in one container are idle, other containers can use the left-over CPU time. The actual amount of CPU time will vary depending on the number of containers running on the system.

For example, consider three containers, one has a cpu-share of 1024 and two others have a cpu-share setting of 512. When processes in all three containers attempt to use 100% of CPU, the first container would receive 50% of the total CPU time. If you add a fouth container with a cpu-share of 1024, the first container only gets 33% of the CPU. The remaining containers receive 16.5%, 16.5% and 33% of the CPU.

On a multi-core system, the shares of CPU time are distributed over all CPU cores. Even if a container is limited to less than 100% of CPU time, it can use 100% of each individual CPU core.

For example, consider a system with more than three cores. If you start one container {C0} with -c=512 running one process, and another container {C1} with -c=1024 running two processes, this can result in the following division of CPU shares:

PID    container	CPU	CPU share
100    {C0}		0	100% of CPU0
101    {C1}		1	100% of CPU1
102    {C1}		2	100% of CPU2

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
--device=[]: Allows you to run devices inside the container without the --privileged flag.
--lxc-conf=[]: Add custom lxc options

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 or SELinux 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.

If you want to limit access to a specific device or devices you can use the --device flag. It allows you to specify one or more devices that will be accessible within the container.

$ sudo docker run --device=/dev/snd:/dev/snd ...

By default, the container will be able to read, write, and mknod these devices. This can be overridden using a third :rwm set of options to each --device flag:

$ sudo docker run --device=/dev/sda:/dev/xvdc --rm -it ubuntu fdisk  /dev/xvdc

Command (m for help): q
$ sudo docker run --device=/dev/sda:/dev/xvdc:r --rm -it ubuntu fdisk  /dev/xvdc
You will not be able to write the partition table.

Command (m for help): q

$ sudo docker run --device=/dev/sda:/dev/xvdc:w --rm -it ubuntu fdisk  /dev/xvdc
    crash....

$ sudo docker run --device=/dev/sda:/dev/xvdc:m --rm -it ubuntu fdisk  /dev/xvdc
fdisk: unable to open /dev/xvdc: Operation not permitted

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:

$ sudo 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.

$ docker run -t -i --rm  ubuntu:14.04 ip link add dummy0 type dummy
RTNETLINK answers: Operation not permitted
$ docker run -t -i --rm --cap-add=NET_ADMIN ubuntu:14.04 ip link add dummy0 type dummy

To mount a FUSE based filesystem, you need to combine both --cap-add and --device:

$ docker run --rm -it --cap-add SYS_ADMIN sshfs sshfs sven@10.10.10.20:/home/sven /mnt
fuse: failed to open /dev/fuse: Operation not permitted
$ docker run --rm -it --device /dev/fuse sshfs sshfs sven@10.10.10.20:/home/sven /mnt
fusermount: mount failed: Operation not permitted
$ docker run --rm -it --cap-add SYS_ADMIN --device /dev/fuse sshfs
# sshfs sven@10.10.10.20:/home/sven /mnt
The authenticity of host '10.10.10.20 (10.10.10.20)' can't be established.
ECDSA key fingerprint is 25:34:85:75:25:b0:17:46:05:19:04:93:b5:dd:5f:c6.
Are you sure you want to continue connecting (yes/no)? yes
sven@10.10.10.20's password:
root@30aa0cfaf1b5:/# ls -la /mnt/src/docker
total 1516
drwxrwxr-x 1 1000 1000   4096 Dec  4 06:08 .
drwxrwxr-x 1 1000 1000   4096 Dec  4 11:46 ..
-rw-rw-r-- 1 1000 1000     16 Oct  8 00:09 .dockerignore
-rwxrwxr-x 1 1000 1000    464 Oct  8 00:09 .drone.yml
drwxrwxr-x 1 1000 1000   4096 Dec  4 06:11 .git
-rw-rw-r-- 1 1000 1000    461 Dec  4 06:08 .gitignore
....

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.

Note: If you use --lxc-conf to modify a container's configuration which is also managed by the Docker daemon, then the Docker daemon will not know about this modification, and you will need to manage any conflicts yourself. For example, you can use --lxc-conf to set a container's IP address, but this will not be reflected in the /etc/hosts file.

Logging drivers (--log-driver)

You can specify a different logging driver for the container than for the daemon.

Logging driver: none

Disables any logging for the container. docker logs won't be available with this driver.

Log driver: json-file

Default logging driver for Docker. Writes JSON messages to file. docker logs command is available only for this logging driver

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)
• ENTRYPOINT (Default Command to Execute at Runtime)
• EXPOSE (Incoming Ports)
• ENV (Environment Variables)
• VOLUME (Shared Filesystems)
• USER
• WORKDIR

CMD (default command or options)

Recall the optional COMMAND in the Docker commandline:

$ sudo docker run [OPTIONS] IMAGE[:TAG|@DIGEST] [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):

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

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

$ sudo docker run -i -t --entrypoint /bin/bash example/redis -c ls -l
$ sudo 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 or a range of ports 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 or a range of ports to the host 
               format: ip:hostPort:containerPort | ip::containerPort | hostPort:containerPort | containerPort
               Both hostPort and containerPort can be specified as a range of ports. 
               When specifying ranges for both, the number of container ports in the range must match the number of host ports in the range. (e.g., `-p 1234-1236:1234-1236/tcp`)
               (use 'docker port' to see the actual mapping)
--link=""  : Add link to another container (<name or id>:alias)

As mentioned previously, EXPOSE (and --expose) makes ports 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. When using -P, Docker will bind the exposed port to a random port on the host within an ephemeral port range defined by /proc/sys/net/ipv4/ip_local_port_range. To find the mapping 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)

When a new container is created, Docker will set the following environment variables automatically:

Variable	Value
HOME	Set based on the value of USER
HOSTNAME	The hostname associated with the container
PATH	Includes popular directories, such as : /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
TERM	xterm if the container is allocated a psuedo-TTY

The container may also include environment variables defined as a result of the container being linked with another container. See the Container Links section for more details.

Additionally, the operator can set any environment variable in the container by using one or more -e flags, even overriding those mentioned above, or already defined by the developer with a Dockerfile ENV:

$ sudo 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 or id>: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
$ sudo docker run -d --name redis-name dockerfiles/redis
4241164edf6f5aca5b0e9e4c9eccd899b0b8080c64c0cd26efe02166c73208f3

# The redis-name container exposed port 6379
$ sudo 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
$ sudo 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!

$ sudo 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:

$ sudo 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:

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

If you restart the source container (servicename in this case), the recipient container's /etc/hosts entry will be automatically updated.

Note

: Unlike host entries in the /etc/hosts file, IP addresses stored in the environment variables are not automatically updated if the source container is restarted. We recommend using the host entries in /etc/hosts to resolve the IP address of linked containers.

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