vendor/github.com/mitchellh/cli/command.go

package cli

import (
	"github.com/posener/complete"
)

const (
	// RunResultHelp is a value that can be returned from Run to signal
	// to the CLI to render the help output.
	RunResultHelp = -18511
)

// A command is a runnable sub-command of a CLI.
type Command interface {
	// Help should return long-form help text that includes the command-line
	// usage, a brief few sentences explaining the function of the command,
	// and the complete list of flags the command accepts.
	Help() string

	// Run should run the actual command with the given CLI instance and
	// command-line arguments. It should return the exit status when it is
	// finished.
	//
	// There are a handful of special exit codes this can return documented
	// above that change behavior.
	Run(args []string) int

	// Synopsis should return a one-line, short synopsis of the command.
	// This should be less than 50 characters ideally.
	Synopsis() string
}

// CommandAutocomplete is an extension of Command that enables fine-grained
// autocompletion. Subcommand autocompletion will work even if this interface
// is not implemented. By implementing this interface, more advanced
// autocompletion is enabled.
type CommandAutocomplete interface {
	// AutocompleteArgs returns the argument predictor for this command.
	// If argument completion is not supported, this should return
	// complete.PredictNothing.
	AutocompleteArgs() complete.Predictor

	// AutocompleteFlags returns a mapping of supported flags and autocomplete
	// options for this command. The map key for the Flags map should be the
	// complete flag such as "-foo" or "--foo".
	AutocompleteFlags() complete.Flags
}

// CommandHelpTemplate is an extension of Command that also has a function
// for returning a template for the help rather than the help itself. In
// this scenario, both Help and HelpTemplate should be implemented.
//
// If CommandHelpTemplate isn't implemented, the Help is output as-is.
type CommandHelpTemplate interface {
	// HelpTemplate is the template in text/template format to use for
	// displaying the Help. The keys available are:
	//
	//   * ".Help" - The help text itself
	//   * ".Subcommands"
	//
	HelpTemplate() string
}

// CommandFactory is a type of function that is a factory for commands.
// We need a factory because we may need to setup some state on the
// struct that implements the command itself.
type CommandFactory func() (Command, error)