Move Commands and Capabilities out of User Guide.
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steveperry-53
@@ -4,113 +4,7 @@ assignees:
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title: Commands and Capabilities
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---
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* TOC
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{:toc}
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{% include redirection-note.md %}
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## Containers and commands
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So far the Pods we've seen have all used the `image` field to indicate what process Kubernetes
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should run in a container. In this case, Kubernetes runs the image's default command. If we want
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to run a particular command or override the image's defaults, there are two additional fields that
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we can use:
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1. `command`: Controls the actual command run by the image
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2. `args`: Controls the arguments passed to the command
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### How docker handles command and arguments
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Docker images have metadata associated with them that is used to store information about the image.
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The image author may use this to define defaults for the command and arguments to run a container
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when the user does not supply values. Docker calls the fields for commands and arguments
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`Entrypoint` and `Cmd` respectively. The full details for this feature are too complicated to
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describe here, mostly due to the fact that the docker API allows users to specify both of these
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fields as either a string array or a string and there are subtle differences in how those cases are
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handled. We encourage the curious to check out Docker's documentation for this feature.
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Kubernetes allows you to override both the image's default command (docker `Entrypoint`) and args
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(docker `Cmd`) with the `command` and `args` fields of `container`. The rules are:
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1. If you do not supply a `command` or `args` for a container, the defaults defined by the image
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will be used.
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2. If you supply a `command` but no `args` for a container, only the supplied `command` will be
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used; the image's default arguments are ignored.
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3. If you supply only `args`, the image's default command will be used with the arguments you
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supply.
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4. If you supply a `command` **and** `args`, the image's defaults will be ignored and the values
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you supply will be used.
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Here are examples for these rules in table format
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| Image `Entrypoint` | Image `Cmd` | Container `command` | Container `args` | Command Run |
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|--------------------|------------------|---------------------|--------------------|------------------|
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| `[/ep-1]` | `[foo bar]` | <not set> | <not set> | `[ep-1 foo bar]` |
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| `[/ep-1]` | `[foo bar]` | `[/ep-2]` | <not set> | `[ep-2]` |
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| `[/ep-1]` | `[foo bar]` | <not set> | `[zoo boo]` | `[ep-1 zoo boo]` |
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| `[/ep-1]` | `[foo bar]` | `[/ep-2]` | `[zoo boo]` | `[ep-2 zoo boo]` |
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## Capabilities
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By default, Docker containers are "unprivileged" and cannot, for example, run a Docker daemon inside a Docker container. We can have fine grain control over the capabilities using cap-add and cap-drop. More details [here](https://docs.docker.com/engine/reference/run/#/runtime-privilege-and-linux-capabilities).
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The relationship between Docker's capabilities and [Linux capabilities](http://man7.org/linux/man-pages/man7/capabilities.7.html)
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| Docker's capabilities | Linux capabilities |
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| ---- | ---- |
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| SETPCAP | CAP_SETPCAP |
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| SYS_MODULE | CAP_SYS_MODULE |
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| SYS_RAWIO | CAP_SYS_RAWIO |
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| SYS_PACCT | CAP_SYS_PACCT |
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| SYS_ADMIN | CAP_SYS_ADMIN |
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| SYS_NICE | CAP_SYS_NICE |
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| SYS_RESOURCE | CAP_SYS_RESOURCE |
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| SYS_TIME | CAP_SYS_TIME |
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| SYS_TTY_CONFIG | CAP_SYS_TTY_CONFIG |
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| MKNOD | CAP_MKNOD |
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| AUDIT_WRITE | CAP_AUDIT_WRITE |
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| AUDIT_CONTROL | CAP_AUDIT_CONTROL |
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| MAC_OVERRIDE | CAP_MAC_OVERRIDE |
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| MAC_ADMIN | CAP_MAC_ADMIN |
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| NET_ADMIN | CAP_NET_ADMIN |
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| SYSLOG | CAP_SYSLOG |
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| CHOWN | CAP_CHOWN |
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| NET_RAW | CAP_NET_RAW |
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| DAC_OVERRIDE | CAP_DAC_OVERRIDE |
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| FOWNER | CAP_FOWNER |
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| DAC_READ_SEARCH | CAP_DAC_READ_SEARCH |
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| FSETID | CAP_FSETID |
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| KILL | CAP_KILL |
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| SETGID | CAP_SETGID |
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| SETUID | CAP_SETUID |
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| LINUX_IMMUTABLE | CAP_LINUX_IMMUTABLE |
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| NET_BIND_SERVICE | CAP_NET_BIND_SERVICE |
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| NET_BROADCAST | CAP_NET_BROADCAST |
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| IPC_LOCK | CAP_IPC_LOCK |
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| IPC_OWNER | CAP_IPC_OWNER |
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| SYS_CHROOT | CAP_SYS_CHROOT |
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| SYS_PTRACE | CAP_SYS_PTRACE |
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| SYS_BOOT | CAP_SYS_BOOT |
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| LEASE | CAP_LEASE |
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| SETFCAP | CAP_SETFCAP |
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| WAKE_ALARM | CAP_WAKE_ALARM |
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| BLOCK_SUSPEND | CAP_BLOCK_SUSPEND |
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You can add or drop capabilities in the [`SecurityContext`](http://kubernetes.io/docs/api-reference/v1/definitions/#_v1_securitycontext), e.g.:
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```yaml
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apiVersion: v1
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kind: Pod
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metadata:
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name: hello-world
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spec:
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containers:
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- name: friendly-container
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image: "alpine:3.4"
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command: ["/bin/echo", "hello", "world"]
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securityContext:
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capabilities:
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add:
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- SYS_NICE
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drop:
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- KILL
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```
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* [Container Command and Arguments](/docs/concepts/configuration/container-command-args/)
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* [Container Capabilities](/docs/concepts/policy/container-capabilities/)
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