github.com/tompao/docker@v1.9.1/docs/reference/run.md

<!--[metadata]>
+++
title = "Docker run reference"
description = "Configure containers at runtime"
keywords = ["docker, run, configure,  runtime"]
[menu.main]
parent = "mn_reference"
+++
<![end-metadata]-->

<!-- TODO (@thaJeztah) define more flexible table/td classes -->
<style>
.content-body table .no-wrap {
    white-space: nowrap;
}
</style>
# 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*](glossary.md#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
 * privileges and LXC configuration

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*](commandline/run.md) has more options than any
other `docker` command.

To learn how to interpret the types of `[OPTIONS]`, see [*Option
types*](commandline/cli.md#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-vs-foreground)
     - [Detached (-d)](#detached-d)
     - [Foreground](#foreground)
 - [Container identification](#container-identification)
     - [Name (--name)](#name-name)
     - [PID equivalent](#pid-equivalent)
 - [IPC settings (--ipc)](#ipc-settings-ipc)
 - [Network settings](#network-settings)
 - [Restart policies (--restart)](#restart-policies-restart)
 - [Clean up (--rm)](#clean-up-rm)
 - [Runtime constraints on resources](#runtime-constraints-on-resources)
 - [Runtime privilege, Linux capabilities, and LXC configuration](#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)

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. A container in detached mode cannot be automatically
removed when it stops, this means you cannot use the `--rm` option with `-d` option.

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*](commandline/attach.md) 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`
    -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:

    $ 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` is often written `-it`
as you'll see in later examples.  Specifying `-t` is forbidden when the client
standard output is redirected or piped, such as in:
`echo test | docker run -i busybox cat`.

>**Note**: A process running as PID 1 inside a container is treated
>specially by Linux: it ignores any signal with the default action.
>So, the process will not terminate on `SIGINT` or `SIGTERM` unless it is
>coded to do so.

## 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. If you do not assign a
container name with the `--name` option, then the daemon generates a random
string name for you. Defining a `name` can be a handy way to add meaning to a
container. If you specify a `name`, you can use it  when referencing the
container within a Docker network. This works for both background and foreground
Docker containers.

**Note**: Containers on the default bridge network must be linked to communicate by name.  

### 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.

    $ 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.

## UTS settings (--uts)

    --uts=""  : Set the UTS namespace mode for the container,
           'host': use the host's UTS namespace inside the container

The UTS namespace is for setting the hostname and the domain that is visible
to running processes in that namespace.  By default, all containers, including
those with `--net=host`, have their own UTS namespace.  The `host` setting will
result in the container using the same UTS namespace as the host.

You may wish to share the UTS namespace with the host if you would like the
hostname of the container to change as the hostname of the host changes.  A
more advanced use case would be changing the host's hostname from a container.

> **Note**: `--uts="host"` gives the container full access to change the
> hostname of the host and is therefore considered insecure.

## 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"   : Connects a container to a network
                        '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
                        'NETWORK': connects the container to user-created network using `docker network create` command
    --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.

Publishing ports and linking to other containers only works with the the default (bridge). The linking feature is a legacy feature. You should always prefer using Docker network drivers over linking.

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

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

Supported networks :

<table>
  <thead>
    <tr>
      <th class="no-wrap">Network</th>
      <th>Description</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td class="no-wrap"><strong>none</strong></td>
      <td>
        No networking in the container.
      </td>
    </tr>
    <tr>
      <td class="no-wrap"><strong>bridge</strong> (default)</td>
      <td>
        Connect the container to the bridge via veth interfaces.
      </td>
    </tr>
    <tr>
      <td class="no-wrap"><strong>host</strong></td>
      <td>
        Use the host's network stack inside the container.
      </td>
    </tr>
    <tr>
      <td class="no-wrap"><strong>container</strong>:&lt;name|id&gt;</td>
      <td>
        Use the network stack of another container, specified via
        its *name* or *id*.
      </td>
    </tr>
    <tr>
      <td class="no-wrap"><strong>NETWORK</strong></td>
      <td>
        Connects the container to a user created network (using `docker network create` command)
      </td>
    </tr>
  </tbody>
</table>

#### Network: none

With the network is `none` a container will not have
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.

#### Network: bridge

With the network 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.

Containers can communicate via their IP addresses by default. To communicate by
name, they must be linked.

#### Network: host

With the network 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.  Note that `--add-host` `--hostname`  `--dns` `--dns-search`
`--dns-opt` and `--mac-address` are invalid in `host` netmode.

Compared to the default `bridge` mode, the `host` mode gives *significantly*
better networking performance since it uses the host's native networking stack
whereas the bridge has to go through one level of virtualization through the
docker daemon. It is recommended to run containers in this mode when their
networking performance is critical, for example, a production Load Balancer
or a High Performance Web Server.

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

#### Network: container

With the network 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>`. Note that `--add-host`
`--hostname` `--dns` `--dns-search` `--dns-opt` and `--mac-address` are
invalid in `container` netmode, and `--publish` `--publish-all` `--expose` are
also invalid in `container` netmode.

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 -it --net container:redis example/redis-cli -h 127.0.0.1

#### User-defined network

You can create a network using a Docker network driver or an external network
driver plugin. You can connect multiple containers to the same network. Once
connected to a user-defined network, the containers can communicate easily using
only another container's IP address or name.  

For `overlay` networks or custom plugins that support multi-host connectivity,
containers connected to the same multi-host network but launched from different
Engines can also communicate in this way.

The following example creates a network using the built-in `bridge` network
driver and running a container in the created network

```
$ docker network create -d overlay my-net
$ docker run --net=my-net -itd --name=container3 busybox
```

### 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 -it --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

If a container is connected to the default bridge network and `linked`
with other containers, then the container's `/etc/hosts` file is updated
with the linked container's name.

If the container is connected to user-defined network, the container's
`/etc/hosts` file is updated with names of all other containers in that
user-defined network.

> **Note** Since Docker may live update the container’s `/etc/hosts` file, there
may be situations when processes inside the container can end up reading an
empty or incomplete `/etc/hosts` file. In most cases, retrying the read again
should fix the problem.

## 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`](commandline/ps.md). It can also be
useful to use [`docker events`](commandline/events.md) to see the
restart policy in effect.

Docker supports the following restart policies:

<table>
  <thead>
    <tr>
      <th>Policy</th>
      <th>Result</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td><strong>no</strong></td>
      <td>
        Do not automatically restart the container when it exits. This is the
        default.
      </td>
    </tr>
    <tr>
      <td>
        <span style="white-space: nowrap">
          <strong>on-failure</strong>[:max-retries]
        </span>
      </td>
      <td>
        Restart only if the container exits with a non-zero exit status.
        Optionally, limit the number of restart retries the Docker
        daemon attempts.
      </td>
    </tr>
    <tr>
      <td><strong>always</strong></td>
      <td>
        Always restart the container regardless of the exit status.
        When you specify always, the Docker daemon will try to restart
        the container indefinitely. The container will also always start
        on daemon startup, regardless of the current state of the container.
      </td>
    </tr>
    <tr>
      <td><strong>unless-stopped</strong></td>
      <td>
        Always restart the container regardless of the exit status, but
        do not start it on daemon startup if the container has been put
        to a stopped state before.
      </td>
    </tr>
  </tbody>
</table>

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 successfully 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`](commandline/inspect.md). For example, to get the number of restarts
for container "my-container";

    $ 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)"](#clean-up-rm). Setting both `--restart` and `--rm`
results in an error.

### Examples

    $ 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.

    $ 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)

> **Note**: When you set the `--rm` flag, Docker also removes the volumes
associated with the container when the container is removed. This is similar
to running `docker rm -v my-container`.

## 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.

## Specifying custom cgroups

Using the `--cgroup-parent` flag, you can pass a specific cgroup to run a
container in. This allows you to create and manage cgroups on their own. You can
define custom resources for those cgroups and put containers under a common
parent group.

## Runtime constraints on resources

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

| Option                     |  Description                                                                                |
|----------------------------|---------------------------------------------------------------------------------------------|
| `-m`, `--memory="" `       | Memory limit (format: `<number>[<unit>]`, where unit = b, k, m or g)                        |
| `--memory-swap=""`         | Total memory limit (memory + swap, format: `<number>[<unit>]`, where unit = b, k, m or g)   |
| `--memory-reservation=""`  | Memory soft limit (format: `<number>[<unit>]`, where unit = b, k, m or g)                   |
| `--kernel-memory=""`       | Kernel memory limit (format: `<number>[<unit>]`, where unit = b, k, m or g)                 |
| `-c`, `--cpu-shares=0`     | CPU shares (relative weight)                                                                |
| `--cpu-period=0`           | Limit the CPU CFS (Completely Fair Scheduler) period                                        |
| `--cpuset-cpus="" `        | CPUs in which to allow execution (0-3, 0,1)                                                 |
| `--cpuset-mems=""`         | Memory nodes (MEMs) in which to allow execution (0-3, 0,1). Only effective on NUMA systems. |
| `--cpu-quota=0`            | Limit the CPU CFS (Completely Fair Scheduler) quota                                         |
| `--blkio-weight=0`         | Block IO weight (relative weight) accepts a weight value between 10 and 1000.               |
| `--oom-kill-disable=false` | Whether to disable OOM Killer for the container or not.                                     |
| `--memory-swappiness=""  ` | Tune a container's memory swappiness behavior. Accepts an integer between 0 and 100.        |

### User memory constraints

We have four ways to set user memory usage:

<table>
  <thead>
    <tr>
      <th>Option</th>
      <th>Result</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td class="no-wrap">
          <strong>memory=inf, memory-swap=inf</strong> (default)
      </td>
      <td>
        There is no memory limit for the container. The container can use
        as much memory as needed.
      </td>
    </tr>
    <tr>
      <td class="no-wrap"><strong>memory=L&lt;inf, memory-swap=inf</strong></td>
      <td>
        (specify memory and set memory-swap as <code>-1</code>) 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).
      </td>
    </tr>
    <tr>
      <td class="no-wrap"><strong>memory=L&lt;inf, memory-swap=2*L</strong></td>
      <td>
        (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.
      </td>
    </tr>
    <tr>
      <td class="no-wrap">
          <strong>memory=L&lt;inf, memory-swap=S&lt;inf, L&lt;=S</strong>
      </td>
      <td>
        (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.
      </td>
    </tr>
  </tbody>
</table>

Examples:

    $ docker run -ti ubuntu:14.04 /bin/bash

We set nothing about memory, this means the processes in the container can use
as much memory and swap memory as they need.

    $ docker run -ti -m 300M --memory-swap -1 ubuntu:14.04 /bin/bash

We set memory limit and disabled swap memory limit, this means the processes in
the container can use 300M memory and as much swap memory as they need (if the
host supports swap memory).

    $ docker run -ti -m 300M ubuntu:14.04 /bin/bash

We set memory limit only, this means the processes in the container can use
300M memory and 300M swap memory, by default, the total virtual memory size
(--memory-swap) will be set as double of memory, in this case, memory + swap
would be 2*300M, so processes can use 300M swap memory as well.

    $ docker run -ti -m 300M --memory-swap 1G ubuntu:14.04 /bin/bash

We set both memory and swap memory, so the processes in the container can use
300M memory and 700M swap memory.

Memory reservation is a kind of memory soft limit that allows for greater
sharing of memory. Under normal circumstances, containers can use as much of
the memory as needed and are constrained only by the hard limits set with the
`-m`/`--memory` option. When memory reservation is set, Docker detects memory
contention or low memory and forces containers to restrict their consumption to
a reservation limit.

Always set the memory reservation value below the hard limit, otherwise the hard
limit takes precedence. A reservation of 0 is the same as setting no
reservation. By default (without reservation set), memory reservation is the
same as the hard memory limit.

Memory reservation is a soft-limit feature and does not guarantee the limit
won't be exceeded. Instead, the feature attempts to ensure that, when memory is
heavily contended for, memory is allocated based on the reservation hints/setup.

The following example limits the memory (`-m`) to 500M and sets the memory
reservation to 200M.

```bash
$ docker run -ti -m 500M --memory-reservation 200M ubuntu:14.04 /bin/bash
```

Under this configuration, when the container consumes memory more than 200M and
less than 500M, the next system memory reclaim attempts to shrink container
memory below 200M.

The following example set memory reservation to 1G without a hard memory limit.

```bash
$ docker run -ti --memory-reservation 1G ubuntu:14.04 /bin/bash
```

The container can use as much memory as it needs. The memory reservation setting
ensures the container doesn't consume too much memory for long time, because
every memory reclaim shrinks the container's consumption to the reservation.

By default, kernel kills processes in a container if an out-of-memory (OOM)
error occurs. To change this behaviour, use the `--oom-kill-disable` option.
Only disable the OOM killer on containers where you have also set the
`-m/--memory` option. If the `-m` flag is not set, this can result in the host
running out of memory and require killing the host's system processes to free
memory.

The following example limits the memory to 100M and disables the OOM killer for
this container:

    $ docker run -ti -m 100M --oom-kill-disable ubuntu:14.04 /bin/bash

The following example, illustrates a dangerous way to use the flag:

    $ docker run -ti --oom-kill-disable ubuntu:14.04 /bin/bash

The container has unlimited memory which can cause the host to run out memory
and require killing system processes to free memory.

### Kernel memory constraints

Kernel memory is fundamentally different than user memory as kernel memory can't
be swapped out. The inability to swap makes it possible for the container to
block system services by consuming too much kernel memory. Kernel memory includes：

 - stack pages
 - slab pages
 - sockets memory pressure
 - tcp memory pressure

You can setup kernel memory limit to constrain these kinds of memory. For example,
every process consumes some stack pages. By limiting kernel memory, you can
prevent new processes from being created when the kernel memory usage is too high.

Kernel memory is never completely independent of user memory. Instead, you limit
kernel memory in the context of the user memory limit. Assume "U" is the user memory
limit and "K" the kernel limit. There are three possible ways to set limits:

<table>
  <thead>
    <tr>
      <th>Option</th>
      <th>Result</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td class="no-wrap"><strong>U != 0, K = inf</strong> (default)</td>
      <td>
        This is the standard memory limitation mechanism already present before using
        kernel memory. Kernel memory is completely ignored.
      </td>
    </tr>
    <tr>
      <td class="no-wrap"><strong>U != 0, K &lt; U</strong></td>
      <td>
        Kernel memory is a subset of the user memory. This setup is useful in
        deployments where the total amount of memory per-cgroup is overcommitted.
        Overcommitting kernel memory limits is definitely not recommended, since the
        box can still run out of non-reclaimable memory.
        In this case, the you can configure K so that the sum of all groups is
        never greater than the total memory. Then, freely set U at the expense of
        the system's service quality.
      </td>
    </tr>
    <tr>
      <td class="no-wrap"><strong>U != 0, K &gt; U</strong></td>
      <td>
        Since kernel memory charges are also fed to the user counter and reclamation
        is triggered for the container for both kinds of memory. This configuration
        gives the admin a unified view of memory. It is also useful for people
        who just want to track kernel memory usage.
      </td>
    </tr>
  </tbody>
</table>

Examples:

    $ docker run -ti -m 500M --kernel-memory 50M ubuntu:14.04 /bin/bash

We set memory and kernel memory, so the processes in the container can use
500M memory in total, in this 500M memory, it can be 50M kernel memory tops.

    $ docker run -ti --kernel-memory 50M ubuntu:14.04 /bin/bash

We set kernel memory without **-m**, so the processes in the container can
use as much memory as they want, but they can only use 50M kernel memory.

### Swappiness constraint

By default, a container's kernel can swap out a percentage of anonymous pages.
To set this percentage for a container, specify a `--memory-swappiness` value
between 0 and 100. A value of 0 turns off anonymous page swapping. A value of
100 sets all anonymous pages as swappable. By default, if you are not using
`--memory-swappiness`, memory swappiness value will be inherited from the parent.

For example, you can set:

    $ docker run -ti --memory-swappiness=0 ubuntu:14.04 /bin/bash

Setting the `--memory-swappiness` option is helpful when you want to retain the
container's working set and to avoid swapping performance penalties.

### 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. If 0 is set, the system will ignore the
value and use the default of 1024.

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 fourth 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

### CPU period constraint

The default CPU CFS (Completely Fair Scheduler) period is 100ms. We can use
`--cpu-period` to set the period of CPUs to limit the container's CPU usage.
And usually `--cpu-period` should work with `--cpu-quota`.

Examples:

    $ docker run -ti --cpu-period=50000 --cpu-quota=25000 ubuntu:14.04 /bin/bash

If there is 1 CPU, this means the container can get 50% CPU worth of run-time every 50ms.

For more information, see the [CFS documentation on bandwidth limiting](https://www.kernel.org/doc/Documentation/scheduler/sched-bwc.txt).

### Cpuset constraint

We can set cpus in which to allow execution for containers.

Examples:

    $ docker run -ti --cpuset-cpus="1,3" ubuntu:14.04 /bin/bash

This means processes in container can be executed on cpu 1 and cpu 3.

    $ docker run -ti --cpuset-cpus="0-2" ubuntu:14.04 /bin/bash

This means processes in container can be executed on cpu 0, cpu 1 and cpu 2.

We can set mems in which to allow execution for containers. Only effective
on NUMA systems.

Examples:

    $ docker run -ti --cpuset-mems="1,3" ubuntu:14.04 /bin/bash

This example restricts the processes in the container to only use memory from
memory nodes 1 and 3.

    $ docker run -ti --cpuset-mems="0-2" ubuntu:14.04 /bin/bash

This example restricts the processes in the container to only use memory from
memory nodes 0, 1 and 2.

### CPU quota constraint

The `--cpu-quota` flag limits the container's CPU usage. The default 0 value
allows the container to take 100% of a CPU resource (1 CPU). The CFS (Completely Fair
Scheduler) handles resource allocation for executing processes and is default
Linux Scheduler used by the kernel. Set this value to 50000 to limit the container
to 50% of a CPU resource. For multiple CPUs, adjust the `--cpu-quota` as necessary.
For more information, see the [CFS documentation on bandwidth limiting](https://www.kernel.org/doc/Documentation/scheduler/sched-bwc.txt).

### Block IO bandwidth (Blkio) constraint

By default, all containers get the same proportion of block IO bandwidth
(blkio). This proportion is 500. To modify this proportion, change the
container's blkio weight relative to the weighting of all other running
containers using the `--blkio-weight` flag.

The `--blkio-weight` flag can set the weighting to a value between 10 to 1000.
For example, the commands below create two containers with different blkio
weight:

    $ docker run -ti --name c1 --blkio-weight 300 ubuntu:14.04 /bin/bash
    $ docker run -ti --name c2 --blkio-weight 600 ubuntu:14.04 /bin/bash

If you do block IO in the two containers at the same time, by, for example:

    $ time dd if=/mnt/zerofile of=test.out bs=1M count=1024 oflag=direct

You'll find that the proportion of time is the same as the proportion of blkio
weights of the two containers.

> **Note:** The blkio weight setting is only available for direct IO. Buffered IO
> is not currently supported.

## Additional groups
    --group-add: Add Linux capabilities

By default, the docker container process runs with the supplementary groups looked
up for the specified user. If one wants to add more to that list of groups, then
one can use this flag:

    $ docker run -ti --rm --group-add audio  --group-add dbus --group-add 777 busybox id
    uid=0(root) gid=0(root) groups=10(wheel),29(audio),81(dbus),777

## 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](
https://github.com/docker/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 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](http://blog.docker.com/2013/09/docker-can-now-run-within-docker/).

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.

    $ 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:

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

    Command (m for help): q
    $ 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

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

    $ 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. The following table lists the Linux capability options which can be added or dropped.

| Capability Key | Capability Description |
| -------------- | ---------------------- |
| SETPCAP | Modify process capabilities. |
| SYS_MODULE| Load and unload kernel modules. |
| SYS_RAWIO | Perform I/O port operations (iopl(2) and ioperm(2)). |
| SYS_PACCT | Use acct(2), switch process accounting on or off. |
| SYS_ADMIN | Perform a range of system administration operations. |
| SYS_NICE | Raise process nice value (nice(2), setpriority(2)) and change the nice value for arbitrary processes. |
| SYS_RESOURCE | Override resource Limits. |
| SYS_TIME | Set system clock (settimeofday(2), stime(2), adjtimex(2)); set real-time (hardware) clock. |
| SYS_TTY_CONFIG | Use vhangup(2); employ various privileged ioctl(2) operations on virtual terminals. |
| MKNOD | Create special files using mknod(2). |
| AUDIT_WRITE | Write records to kernel auditing log. |
| AUDIT_CONTROL | Enable and disable kernel auditing; change auditing filter rules; retrieve auditing status and filtering rules. |
| MAC_OVERRIDE | Allow MAC configuration or state changes. Implemented for the Smack LSM. |
| MAC_ADMIN | Override Mandatory Access Control (MAC). Implemented for the Smack Linux Security Module (LSM). |
| NET_ADMIN | Perform various network-related operations. |
| SYSLOG | Perform privileged syslog(2) operations.  |
| CHOWN | Make arbitrary changes to file UIDs and GIDs (see chown(2)). |
| NET_RAW | Use RAW and PACKET sockets. |
| DAC_OVERRIDE | Bypass file read, write, and execute permission checks. |
| FOWNER | Bypass permission checks on operations that normally require the file system UID of the process to match the UID of the file. |
| DAC_READ_SEARCH | Bypass file read permission checks and directory read and execute permission checks. |
| FSETID | Don't clear set-user-ID and set-group-ID permission bits when a file is modified. |
| KILL | Bypass permission checks for sending signals. |
| SETGID | Make arbitrary manipulations of process GIDs and supplementary GID list. |
| SETUID | Make arbitrary manipulations of process UIDs. |
| LINUX_IMMUTABLE | Set the FS_APPEND_FL and FS_IMMUTABLE_FL i-node flags. |
| NET_BIND_SERVICE  | Bind a socket to internet domain privileged ports (port numbers less than 1024). |
| NET_BROADCAST |  Make socket broadcasts, and listen to multicasts. |
| IPC_LOCK | Lock memory (mlock(2), mlockall(2), mmap(2), shmctl(2)). |
| IPC_OWNER | Bypass permission checks for operations on System V IPC objects. |
| SYS_CHROOT | Use chroot(2), change root directory. |
| SYS_PTRACE | Trace arbitrary processes using ptrace(2). |
| SYS_BOOT | Use reboot(2) and kexec_load(2), reboot and load a new kernel for later execution. |
| LEASE | Establish leases on arbitrary files (see fcntl(2)). |
| SETFCAP | Set file capabilities.|
| WAKE_ALARM | Trigger something that will wake up the system. |
| BLOCK_SUSPEND | Employ features that can block system suspend. |

Further reference information is available on the [capabilities(7) - Linux man page](http://linux.die.net/man/7/capabilities)

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.

    $ 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 daemon --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/docker/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.

> **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)

The container can have a different logging driver than the Docker daemon. Use
the `--log-driver=VALUE` with the `docker run` command to configure the
container's logging driver. The following options are supported:

| `none`      | Disables any logging for the container. `docker logs` won't be available with this driver.                                    |
|-------------|-------------------------------------------------------------------------------------------------------------------------------|
| `json-file` | Default logging driver for Docker. Writes JSON messages to file.  No logging options are supported for this driver.           |
| `syslog`    | Syslog logging driver for Docker. Writes log messages to syslog.                                                              |
| `journald`  | Journald logging driver for Docker. Writes log messages to `journald`.                                                        |
| `gelf`      | Graylog Extended Log Format (GELF) logging driver for Docker. Writes log messages to a GELF endpoint likeGraylog or Logstash. |
| `fluentd`   | Fluentd logging driver for Docker. Writes log messages to `fluentd` (forward input).                                          |
| `awslogs`   | Amazon CloudWatch Logs logging driver for Docker. Writes log messages to Amazon CloudWatch Logs                               |

The `docker logs` command is available only for the `json-file` and `journald`
logging drivers.  For detailed information on working with logging drivers, see
[Configure a logging driver](logging/overview.md).


## Overriding Dockerfile image defaults

When a developer builds an image from a [*Dockerfile*](builder.md)
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|@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`):

    $ 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 following `run` command options work with container networking:

    --expose=[]: Expose a port or a range of ports inside the container.
                 These are additional to those exposed by the `EXPOSE` instruction
    -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, for example:
                       -p 1234-1236:1234-1236/tcp

                   When specifying a range for hostPort only, the
                   containerPort must not be a range.  In this case the
                   container port is published somewhere within the
                   specified hostPort range. (e.g., `-p 1234-1236:1234/tcp`)

                   (use 'docker port' to see the actual mapping)

    --link=""  : Add link to another container (<name or id>:alias or <name or id>)

With the exception of the `EXPOSE` directive, an image developer hasn't
got much control over networking. The `EXPOSE` instruction defines the
initial incoming ports that provide services. These ports are available
to processes inside the container. An operator can use the `--expose`
option to add to the exposed ports.

To expose a container's internal port, an operator can start the
container with the `-P` or `-p` flag. The exposed port is accessible on
the host and the ports are available to any client that can reach the
host.

The `-P` option publishes all the ports to the host interfaces. Docker
binds each exposed port to a random port on the host. The range of
ports are within an *ephemeral port range* defined by
`/proc/sys/net/ipv4/ip_local_port_range`. Use the `-p` flag to
explicitly map a single port or range of ports.

The port number inside the container (where the service listens) does
not need to match the port number exposed on the outside of the
container (where clients connect). For example, inside the container an
HTTP service is listening on port 80 (and so the image developer
specifies `EXPOSE 80` in the Dockerfile). At runtime, the port might be
bound to 42800 on the host. To find the mapping between the host ports
and the exposed ports, use `docker port`.

If the operator uses `--link` when starting a new client container, then the
client container can access the exposed port via a private networking interface.
Linking is a legacy feature that is only supported on the default bridge
network. You should prefer the Docker networks feature instead. For more
information on this feature, see the [*Docker network
overview*""](../userguide/networking/index.md)).

### ENV (environment variables)

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

<table>
 <tr>
  <th>Variable</th>
  <th>Value</th>
 </tr>
 <tr>
  <td><code>HOME</code></td>
  <td>
    Set based on the value of <code>USER</code>
  </td>
 </tr>
 <tr>
  <td><code>HOSTNAME</code></td>
  <td>
    The hostname associated with the container
  </td>
 </tr>
 <tr>
  <td><code>PATH</code></td>
  <td>
    Includes popular directories, such as :<br>
    <code>/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin</code>
  </td>
 <tr>
  <td><code>TERM</code></td>
  <td><code>xterm</code> if the container is allocated a pseudo-TTY</td>
 </tr>
</table>

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`:

    $ 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`.

### VOLUME (shared filesystems)

    -v=[]: Create a bind mount with: [host-dir:]container-dir[:<options>], where
    options are comma delimited and selected from [rw|ro] and [z|Z].
           If 'host-dir' is missing, then docker creates a new volume.
		   If neither 'rw' or 'ro' is specified then the volume is mounted
		   in read-write mode.
    --volumes-from="": Mount all volumes from the given container(s)

> **Note**:
> The auto-creation of the host path has been [*deprecated*](../misc/deprecated.md#auto-creating-missing-host-paths-for-bind-mounts).

The volumes commands are complex enough to have their own documentation
in section [*Managing data in
containers*](../userguide/dockervolumes.md). 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).

The `container-dir` must always be an absolute path such as `/src/docs`.
The `host-dir` can either be an absolute path or a `name` value. If you
supply an absolute path for the `host-dir`, Docker bind-mounts to the path
you specify. If you supply a `name`, Docker creates a named volume by that `name`.

A `name` value must start with start with an alphanumeric character,
followed by `a-z0-9`, `_` (underscore), `.` (period) or `-` (hyphen).
An absolute path starts with a `/` (forward slash).

For example, you can specify either `/foo` or `foo` for a `host-dir` value.
If you supply the `/foo` value, Docker creates a bind-mount. If you supply
the `foo` specification, Docker creates a named volume.

### USER

`root` (id = 0) is the default user within a container. The image developer can
create additional users. Those users are accessible by name.  When passing a numeric
ID, the user does not have to exist in the container.

The developer can set a default user to run the first process with the
Dockerfile `USER` instruction. When starting a container, the operator can override
the `USER` instruction by passing the `-u` option.

    -u="": Username or UID

> **Note:** if you pass a numeric uid, it must be in the range of 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