Chapter 6. Extending the CLI

The plug-in loader is responsible for searching plug-in files, and checking if the plug-in provides the minimum amount of information required for it to run. Files placed in the correct location that do not provide the minimum amount of information (for example, an incomplete plugin.yaml descriptor) are ignored.

The descriptor file supports the following attributes:

name: "great-plugin"              # REQUIRED: the plug-in command name, to be invoked under 'kubectl'
shortDesc: "great-plugin plug-in" # REQUIRED: the command short description, for help
longDesc: ""                      # the command long description, for help
example: ""                       # command example(s), for help
command: "./example"              # REQUIRED: the command, binary, or script to invoke when running the plug-in
flags:                            # flags supported by the plug-in
  - name: "flag-name"             # REQUIRED for each flag: flag name
    shorthand: "f"                # short version of the flag name
    desc: "example flag"          # REQUIRED for each flag: flag description
    defValue: "extreme"           # default value of the flag
tree:                             # allows the declaration of subcommands
  - ...                           # subcommands support the same set of attributes

The preceding descriptor declares the great-plugin plug-in, which has one flag named -f | --flag-name. It could be invoked as:

$ oc plugin great-plugin -f value

When the plug-in is invoked, it calls the example binary or script, which is located in the same directory as the descriptor file, passing a number of arguments and environment variables. The Accessing Runtime Attributes section describes how the example command accesses the flag value and other runtime context.

It is recommended that each plug-in has its own subdirectory in the file system, preferably with the same name as the plug-in command. The directory must contain the plugin.yaml descriptor and any binary, script, asset, or other dependency it might require.

For example, the directory structure for the great-plugin plug-in could look like this:

~/.kube/plugins/
└── great-plugin
    ├── plugin.yaml
    └── example

In most use cases, the binary or script file you write to support the plug-in must have access to some contextual information provided by the plug-in framework. For example, if you declared flags in the descriptor file, your plug-in must have access to the user-provided flag values at runtime.

The same is true for global flags. The plug-in framework is responsible for doing that, so plug-in writers do not need to worry about parsing arguments. This also ensures the best level of consistency between plug-ins and regular oc commands.

Plug-ins have access to runtime context attributes through environment variables. To access the value provided through a flag, for example, look for the value of the proper environment variable using the appropriate function call for your binary or script.

The supported environment variables are: