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Extend kubectl with plugins

FEATURE STATE: Kubernetes v1.12 alpha
This feature is currently in a alpha state, meaning:

  • The version names contain alpha (e.g. v1alpha1).
  • Might be buggy. Enabling the feature may expose bugs. Disabled by default.
  • Support for feature may be dropped at any time without notice.
  • The API may change in incompatible ways in a later software release without notice.
  • Recommended for use only in short-lived testing clusters, due to increased risk of bugs and lack of long-term support.

This guide demonstrates how to install and write extensions for kubectl. By thinking of core kubectl commands as essential building blocks for interacting with a Kubernetes cluster, a cluster administrator can think of plugins as a means of utilizing these building blocks to create more complex behavior. Plugins extend kubectl with new sub-commands, allowing for new and custom features not included in the main distribution of kubectl.

Before you begin

You need to have a working kubectl binary installed.

Note: Plugins were officially introduced as an alpha feature in the v1.8.0 release. They have been re-worked in the v1.12.0 release to support a wider range of use-cases. So, while some parts of the plugins feature were already available in previous versions, a kubectl version of 1.12.0 or later is recommended if you are following these docs.

Until a GA version is released, plugins should be considered unstable, and their underlying mechanism is prone to change.

Installing kubectl plugins

A plugin is nothing more than a standalone executable file, whose name begins with kubectl-. To install a plugin, simply move this executable file to anywhere on your PATH.

Note: Kubernetes does not provide a package manager or anything similar to install or update plugins. It is your responsibility to ensure that plugin executables have a filename that begins with kubectl-, and that they are placed somewhere on your PATH.

Discovering plugins

kubectl provides a command kubectl plugin list that searches your PATH for valid plugin executables. Executing this command causes a traversal of all files in your PATH. Any files that are executable, and begin with kubectl- will show up in the order in which they are present in your PATH in this command’s output. A warning will be included for any files beginning with kubectl- that are not executable. A warning will also be included for any valid plugin files that overlap each other’s name.

Limitations

It is currently not possible to create plugins that overwrite existing kubectl commands. For example, creating a plugin kubectl-version will cause that plugin to never be executed, as the existing kubectl version command will always take precedence over it. Due to this limitation, it is also not possible to use plugins to add new subcommands to existing kubectl commands. For example, adding a subcommand kubectl create foo by naming your plugin kubectl-create-foo will cause that plugin to be ignored. Warnings will appear under the output of kubectl plugin list for any valid plugins that attempt to do this.

Writing kubectl plugins

You can write a plugin in any programming language or script that allows you to write command-line commands.

There is no plugin installation or pre-loading required. Plugin executables receive the inherited environment from the kubectl binary. A plugin determines which command path it wishes to implement based on its name. For example, a plugin wanting to provide a new command kubectl foo, would simply be named kubectl-foo, and live somewhere in the user’s PATH.

Example plugin

#!/bin/bash

# optional argument handling
if [[ "$1" == "version" ]]
then
    echo "1.0.0"
    exit 0
fi

# optional argument handling
if [[ "$1" == "config" ]]
then
    echo $KUBECONFIG
    exit 0
fi

echo "I am a plugin named kubectl-foo"

Using a plugin

To use the above plugin, simply make it executable:

sudo chmod +x ./kubectl-foo

and place it anywhere in your PATH:

sudo mv ./kubectl-foo /usr/local/bin

You may now invoke your plugin as a kubectl command:

$ kubectl foo
I am a plugin named kubectl-foo

All args and flags are passed as-is to the executable:

$ kubectl foo version
1.0.0

All environment variables are also passed as-is to the executable:

$ export KUBECONFIG=~/.kube/config
$ kubectl foo config
/home/<user>/.kube/config

$ KUBECONFIG=/etc/kube/config kubectl foo config
/etc/kube/config

Additionally, the first argument that is passed to a plugin will always be the full path to the location where it was invoked ($0 would equal /usr/local/bin/kubectl-foo in our example above).

Naming a plugin

As seen in the example above, a plugin determines the command path that it will implement based on its filename. Every sub-command in the command path that a plugin targets, is separated by a dash (-). For example, a plugin that wishes to be invoked whenever the command kubectl foo bar baz is invoked by the user, would have the filename of kubectl-foo-bar-baz.

Flags and argument handling

Note: Unlike previous versions of kubectl, the plugin mechanism will not create any custom, plugin-specific values or environment variables to a plugin process. This means that environment variables such as KUBECTL_PLUGINS_CURRENT_NAMESPACE are no longer provided to a plugin. Plugins must parse all of the arguments passed to them by a user, and handle flag validation as part of their own implementation. For plugins written in Go, a set of utilities has been provided under k8s.io/cli-runtime to assist with this.

Taking our kubectl-foo-bar-baz plugin from the above scenario, we further explore additional cases where users invoke our plugin while providing additional flags and arguments. For example, in a situation where a user invokes the command kubectl foo bar baz arg1 --flag=value arg2, the plugin mechanism will first try to find the plugin with the longest possible name, which in this case would be kubectl-foo-bar-baz-arg1. Upon not finding that plugin, it then treats the last dash-separated value as an argument (arg1 in this case), and attempts to find the next longest possible name, kubectl-foo-bar-baz. Upon finding a plugin with this name, it then invokes that plugin, passing all args and flags after its name to the plugin executable.

Example:

# create a plugin
$ echo '#!/bin/bash\n\necho "My first command-line argument was $1"' > kubectl-foo-bar-baz
$ sudo chmod +x ./kubectl-foo-bar-baz

# "install" our plugin by placing it on our PATH
$ sudo mv ./kubectl-foo-bar-baz /usr/local/bin

# ensure our plugin is recognized by kubectl
$ kubectl plugin list
The following kubectl-compatible plugins are available:

/usr/local/bin/kubectl-foo-bar-baz

# test that calling our plugin via a "kubectl" command works
# even when additional arguments and flags are passed to our
# plugin executable by the user.
$ kubectl foo bar baz arg1 --meaningless-flag=true
My first command-line argument was arg1

As you can see, our plugin was found based on the kubectl command specified by a user, and all extra arguments and flags were passed as-is to the plugin executable once it was found.

Names with dashes and underscores

Although the kubectl plugin mechanism uses the dash (-) in plugin filenames to separate the sequence of sub-commands processed by the plugin, it is still possible to create a plugin command containing dashes in its commandline invocation by using underscores (_) in its filename.

Example:

# create a plugin containing an underscore in its filename
$ echo '#!/bin/bash\n\necho "I am a plugin with a dash in my name"' > ./kubectl-foo_bar
$ sudo chmod +x ./kubectl-foo_bar

# move the plugin into your PATH
$ sudo mv ./kubectl-foo_bar /usr/local/bin

# our plugin can now be invoked from `kubectl` like so:
$ kubectl foo-bar
I am a plugin with a dash in my name

Note that the introduction of underscores to a plugin filename does not prevent us from having commands such as kubectl foo_bar. The command from the above example, can be invoked using either a dash (-) or an underscore (_):

# our plugin can be invoked with a dash
$ kubectl foo-bar
I am a plugin with a dash in my name

# it can also be invoked using an underscore
$ kubectl foo_bar
I am a plugin with a dash in my name

Name conflicts and overshadowing

It is possible to have multiple plugins with the same filename in different locations throughout your PATH. For example, given a PATH with the following value: PATH=/usr/local/bin/plugins:/usr/local/bin/moreplugins, a copy of plugin kubectl-foo could exist in /usr/local/bin/plugins and /usr/local/bin/moreplugins, such that the output of the kubectl plugin list command is:

$ PATH=/usr/local/bin/plugins:/usr/local/bin/moreplugins kubectl plugin list
The following kubectl-compatible plugins are available:

/usr/local/bin/plugins/kubectl-foo
/usr/local/bin/moreplugins/kubectl-foo
  - warning: /usr/local/bin/moreplugins/kubectl-foo is overshadowed by a similarly named plugin: /usr/local/bin/plugins/kubectl-foo

error: one plugin warning was found

In the above scenario, the warning under /usr/local/bin/moreplugins/kubectl-foo tells us that this plugin will never be executed. Instead, the executable that appears first in our PATH, /usr/local/bin/plugins/kubectl-foo, will always be found and executed first by the kubectl plugin mechanism.

A way to resolve this issue is to ensure that the location of the plugin that you wish to use with kubectl always comes first in your PATH. For example, if we wanted to always use /usr/local/bin/moreplugins/kubectl-foo anytime that the kubectl command kubectl foo was invoked, we would simply change the value of our PATH to be PATH=/usr/local/bin/moreplugins:/usr/local/bin/plugins.

Invocation of the longest executable filename

There is another kind of overshadowing that can occur with plugin filenames. Given two plugins present in a user’s PATH kubectl-foo-bar and kubectl-foo-bar-baz, the kubectl plugin mechanism will always choose the longest possible plugin name for a given user command. Some examples below, clarify this further:

# for a given kubectl command, the plugin with the longest possible filename will always be preferred
$ kubectl foo bar baz
Plugin kubectl-foo-bar-baz is executed

$ kubectl foo bar
Plugin kubectl-foo-bar is executed

$ kubectl foo bar baz buz
Plugin kubectl-foo-bar-baz is executed, with "buz" as its first argument

$ kubectl foo bar buz
Plugin kubectl-foo-bar is executed, with "buz" as its first argument

This design choice ensures that plugin sub-commands can be implemented across multiple files, if needed, and that these sub-commands can be nested under a “parent” plugin command:

$ ls ./plugin_command_tree
kubectl-parent
kubectl-parent-subcommand
kubectl-parent-subcommand-subsubcommand

Checking for plugin warnings

You can use the aforementioned kubectl plugin list command to ensure that your plugin is visible by kubectl, and verify that there are no warnings preventing it from being called as a kubectl command.

$ kubectl plugin list
The following kubectl-compatible plugins are available:

test/fixtures/pkg/kubectl/plugins/kubectl-foo
/usr/local/bin/kubectl-foo
  - warning: /usr/local/bin/kubectl-foo is overshadowed by a similarly named plugin: test/fixtures/pkg/kubectl/plugins/kubectl-foo
plugins/kubectl-invalid
  - warning: plugins/kubectl-invalid identified as a kubectl plugin, but it is not executable

error: 2 plugin warnings were found

Using the command line runtime package

As part of the plugin mechanism update in the v1.12.0 release, an additional set of utilities have been made available to plugin authors. These utilities exist under the k8s.io/cli-runtime repository, and can be used by plugins written in Go to parse and update a user’s KUBECONFIG file, obtain REST clients to talk to the API server, and automatically bind flags associated with configuration and printing.

Plugins do not have to be written in Go in order to be recognized as valid plugins by kubectl, but they do have to use Go in order to take advantage of the tools and utilities in the CLI Runtime repository.

See the Sample CLI Plugin for an example usage of the tools provided in the CLI Runtime repo.

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