The Helm Plugins Guide

A Helm plugin is a tool that can be accessed through the helm CLI, but which is not part of the built-in Helm codebase.

Existing plugins can be found on related section or by searching GitHub.

This guide explains how to use and create plugins.

An Overview

Helm plugins are add-on tools that integrate seamlessly with Helm. They provide a way to extend the core feature set of Helm, but without requiring every new feature to be written in Go and added to the core tool.

Helm plugins have the following features:

  • They can be added and removed from a Helm installation without impacting the core Helm tool.
  • They can be written in any programming language.
  • They integrate with Helm, and will show up in helm help and other places.

Helm plugins live in $HELM_PLUGINS. You can find the current value of this, including the default value when not set in the environment, using the helm env command.

The Helm plugin model is partially modeled on Git's plugin model. To that end, you may sometimes hear helm referred to as the porcelain layer, with plugins being the plumbing. This is a shorthand way of suggesting that Helm provides the user experience and top level processing logic, while the plugins do the "detail work" of performing a desired action.

Installing a Plugin

Plugins are installed using the $ helm plugin install <path|url> command. You can pass in a path to a plugin on your local file system or a url of a remote VCS repo. The helm plugin install command clones or copies the plugin at the path/url given into $HELM_PLUGINS

$ helm plugin install https://github.com/adamreese/helm-env

If you have a plugin tar distribution, simply untar the plugin into the $HELM_PLUGINS directory. You can also install tarball plugins directly from url by issuing helm plugin install https://domain/path/to/plugin.tar.gz

Building Plugins

In many ways, a plugin is similar to a chart. Each plugin has a top-level directory, and then a plugin.yaml file.

$HELM_PLUGINS/
  |- last/
      |
      |- plugin.yaml
      |- last.sh

In the example above, the last plugin is contained inside of a directory named last. It has two files: plugin.yaml (required) and an executable script, last.sh (optional).

The core of a plugin is a simple YAML file named plugin.yaml. Here is a plugin YAML for a plugin that helps get the last release name:

name: "last"
version: "0.1.0"
usage: "get the last release name"
description: "get the last release name"
ignoreFlags: false
command: "$HELM_BIN --host $TILLER_HOST list --short --max 1 --date -r"
platformCommand:
  - os: linux
    arch: i386
    command: "$HELM_BIN list --short --max 1 --date -r"
  - os: linux
    arch: amd64
    command: "$HELM_BIN list --short --max 1 --date -r"
  - os: windows
    arch: amd64
    command: "$HELM_BIN list --short --max 1 --date -r"

The name is the name of the plugin. When Helm executes this plugin, this is the name it will use (e.g. helm NAME will invoke this plugin).

name should match the directory name. In our example above, that means the plugin with name: last should be contained in a directory named last.

Restrictions on name:

  • name cannot duplicate one of the existing helm top-level commands.
  • name must be restricted to the characters ASCII a-z, A-Z, 0-9, _ and -.

version is the SemVer 2 version of the plugin. usage and description are both used to generate the help text of a command.

The ignoreFlags switch tells Helm to not pass flags to the plugin. So if a plugin is called with helm myplugin --foo and ignoreFlags: true, then --foo is silently discarded.

Finally, and most importantly, platformCommand or command is the command that this plugin will execute when it is called. The platformCommand section defines the OS/Architecture specific variations of a command. The following rules will apply in deciding which command to use:

  • If platformCommand is present, it will be searched first.
  • If both os and arch match the current platform, search will stop and the command will be used.
  • If os matches and there is no more specific arch match, the command will be used.
  • If no platformCommand match is found, the default command will be used.
  • If no matches are found in platformCommand and no command is present, Helm will exit with an error.

Environment variables are interpolated before the plugin is executed. The pattern above illustrates the preferred way to indicate where the plugin program lives.

There are some strategies for working with plugin commands:

  • If a plugin includes an executable, the executable for a platformCommand: or a command: should be packaged in the plugin directory.
  • The platformCommand: or command: line will have any environment variables expanded before execution. $HELM_PLUGIN_DIR will point to the plugin directory.
  • The command itself is not executed in a shell. So you can't oneline a shell script.
  • Helm injects lots of configuration into environment variables. Take a look at the environment to see what information is available.
  • Helm makes no assumptions about the language of the plugin. You can write it in whatever you prefer.
  • Commands are responsible for implementing specific help text for -h and --help. Helm will use usage and description for helm help and helm help myplugin, but will not handle helm myplugin --help.

Downloader Plugins

By default, Helm is able to pull Charts using HTTP/S. As of Helm 2.4.0, plugins can have a special capability to download Charts from arbitrary sources.

Plugins shall declare this special capability in the plugin.yaml file (top level):

downloaders:
- command: "bin/mydownloader"
  protocols:
  - "myprotocol"
  - "myprotocols"

If such plugin is installed, Helm can interact with the repository using the specified protocol scheme by invoking the command. The special repository shall be added similarly to the regular ones: helm repo add favorite myprotocol://example.com/ The rules for the special repos are the same to the regular ones: Helm must be able to download the index.yaml file in order to discover and cache the list of available Charts.

The defined command will be invoked with the following scheme: command certFile keyFile caFile full-URL. The SSL credentials are coming from the repo definition, stored in $HELM_REPOSITORY_CONFIG (i.e., $HELM_CONFIG_HOME/repositories.yaml). A Downloader plugin is expected to dump the raw content to stdout and report errors on stderr.

The downloader command also supports sub-commands or arguments, allowing you to specify for example bin/mydownloader subcommand -d in the plugin.yaml. This is useful if you want to use the same executable for the main plugin command and the downloader command, but with a different sub-command for each.

Environment Variables

When Helm executes a plugin, it passes the outer environment to the plugin, and also injects some additional environment variables.

Variables like KUBECONFIG are set for the plugin if they are set in the outer environment.

The following variables are guaranteed to be set:

  • HELM_PLUGINS: The path to the plugins directory.
  • HELM_PLUGIN_NAME: The name of the plugin, as invoked by helm. So helm myplug will have the short name myplug.
  • HELM_PLUGIN_DIR: The directory that contains the plugin.
  • HELM_BIN: The path to the helm command (as executed by the user).
  • HELM_DEBUG: Indicates if the debug flag was set by helm.
  • HELM_REGISTRY_CONFIG: The location for the registry configuration (if using). Note that the use of Helm with registries is an experimental feature.
  • HELM_REPOSITORY_CACHE: The path to the repository cache files.
  • HELM_REPOSITORY_CONFIG: The path to the repository configuration file.
  • HELM_NAMESPACE: The namespace given to the helm command (generally using the -n flag).
  • HELM_KUBECONTEXT: The name of the Kubernetes config context given to the helm command.

Additionally, if a Kubernetes configuration file was explicitly specified, it will be set as the KUBECONFIG variable

A Note on Flag Parsing

When executing a plugin, Helm will parse global flags for its own use. None of these flags are passed on to the plugin.

  • --debug: If this is specified, $HELM_DEBUG is set to 1
  • --registry-config: This is converted to $HELM_REGISTRY_CONFIG
  • --repository-cache: This is converted to $HELM_REPOSITORY_CACHE
  • --repository-config: This is converted to $HELM_REPOSITORY_CONFIG
  • --namespace and -n: This is converted to $HELM_NAMESPACE
  • --kube-context: This is converted to $HELM_KUBECONTEXT
  • --kubeconfig: This is converted to $KUBECONFIG

Plugins should display help text and then exit for -h and --help. In all other cases, plugins may use flags as appropriate.

Providing shell auto-completion

As of Helm 3.2, a plugin can optionally provide support for shell auto-completion as part of Helm's existing auto-completion mechanism.

Static auto-completion

If a plugin provides its own flags and/or sub-commands, it can inform Helm of them by having a completion.yaml file located in the plugin's root directory. The completion.yaml file has the form:

name: <pluginName>
flags:
- <flag 1>
- <flag 2>
validArgs:
- <arg value 1>
- <arg value 2>
commands:
  name: <commandName>
  flags:
  - <flag 1>
  - <flag 2>
  validArgs:
  - <arg value 1>
  - <arg value 2>
  commands:
     <and so on, recursively>

Notes:

  1. All sections are optional but should be provided if applicable.
  2. Flags should not include the - or -- prefix.
  3. Both short and long flags can and should be specified. A short flag need not be associated with its corresponding long form, but both forms should be listed.
  4. Flags need not be ordered in any way, but need to be listed at the correct point in the sub-command hierarchy of the file.
  5. Helm's existing global flags are already handled by Helm's auto-completion mechanism, therefore plugins need not specify the following flags --debug, --namespace or -n, --kube-context, and --kubeconfig, or any other global flag.
  6. The validArgs list provides a static list of possible completions for the first parameter following a sub-command. It is not always possible to provide such a list in advance (see the Dynamic Completion section below), in which case the validArgs section can be omitted.

The completion.yaml file is entirely optional. If it is not provided, Helm will simply not provide shell auto-completion for the plugin (unless Dynamic Completion is supported by the plugin). Also, adding a completion.yaml file is backwards-compatible and will not impact the behavior of the plugin when using older helm versions.

As an example, for the fullstatus plugin which has no sub-commands but accepts the same flags as the helm status command, the completion.yaml file is:

name: fullstatus
flags:
- o
- output
- revision

A more intricate example for the 2to3 plugin, has a completion.yaml file of:

name: 2to3
commands:
- name: cleanup
  flags:
  - config-cleanup
  - dry-run
  - l
  - label
  - release-cleanup
  - s
  - release-storage
  - tiller-cleanup
  - t
  - tiller-ns
  - tiller-out-cluster
- name: convert
  flags:
  - delete-v2-releases
  - dry-run
  - l
  - label
  - s
  - release-storage
  - release-versions-max
  - t
  - tiller-ns
  - tiller-out-cluster
- name: move
  commands:
  - name: config
    flags:
    - dry-run

Dynamic completion

Also starting with Helm 3.2, plugins can provide their own dynamic shell auto-completion. Dynamic shell auto-completion is the completion of parameter values or flag values that cannot be defined in advance. For example, completion of the names of helm releases currently available on the cluster.

For the plugin to support dynamic auto-completion, it must provide an executable file called plugin.complete in its root directory. When the Helm completion script requires dynamic completions for the plugin, it will execute the plugin.complete file, passing it the command-line that needs to be completed. The plugin.complete executable will need to have the logic to determine what the proper completion choices are and output them to standard output to be consumed by the Helm completion script.

The plugin.complete file is entirely optional. If it is not provided, Helm will simply not provide dynamic auto-completion for the plugin. Also, adding a plugin.complete file is backwards-compatible and will not impact the behavior of the plugin when using older helm versions.

The output of the plugin.complete script should be a new-line separated list such as:

rel1
rel2
rel3

When plugin.complete is called, the plugin environment is set just like when the plugin's main script is called. Therefore, the variables $HELM_NAMESPACE, $HELM_KUBECONTEXT, and all other plugin variables will already be set, and their corresponding global flags will be removed.

The plugin.complete file can be in any executable form; it can be a shell script, a Go program, or any other type of program that Helm can execute. The plugin.complete file must have executable permissions for the user. The plugin.complete file must exit with a success code (value 0).

In some cases, dynamic completion will require to obtain information from the Kubernetes cluster. For example, the helm fullstatus plugin requires a release name as input. In the fullstatus plugin, for its plugin.complete script to provide completion for current release names, it can simply run helm list -q and output the result.

If it is desired to use the same executable for plugin execution and for plugin completion, the plugin.complete script can be made to call the main plugin executable with some special parameter or flag; when the main plugin executable detects the special parameter or flag, it will know to run the completion. In our example, plugin.complete could be implemented like this:

#!/usr/bin/env sh

# "$@" is the entire command-line that requires completion.
# It is important to double-quote the "$@" variable to preserve a possibly empty last parameter.
$HELM_PLUGIN_DIR/status.sh --complete "$@"

The fullstatus plugin's real script (status.sh) must then look for the --complete flag and if found, printout the proper completions.

Tips and tricks

  1. The shell will automatically filter out completion choices that don't match user input. A plugin can therefore return all relevant completions without removing the ones that don't match the user input. For example, if the command-line is helm fullstatus ngin<TAB>, the plugin.complete script can print all release names (of the default namespace), not just the ones starting with ngin; the shell will only retain the ones starting with ngin.
  2. To simplify dynamic completion support, especially if you have a complex plugin, you can have your plugin.complete script call your main plugin script and request completion choices. See the Dynamic Completion section above for an example.
  3. To debug dynamic completion and the plugin.complete file, one can run the following to see the completion results :
    • helm __complete <pluginName> <arguments to complete>. For example:
    • helm __complete fullstatus --output js<ENTER>,
    • helm __complete fullstatus -o json ""<ENTER>
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