How to process sub-command style command line arguments is a question that arises more and more. Many of the basic option handling libraries can not handle this at all, or they make it very difficult to do so.

One of the newer libraries in the option processing field is Adopt by Steve Losh. It was not designed to handle sub-commands but it is in fact very capable to do this without having to jump through too many hoops.

In a Reddit thread someone asked if Adopt can handle sub-command processing and Steve answered with the following example:

(eval-when (:compile-toplevel :load-toplevel :execute)
  (ql:quickload '(:adopt) :silent t))

(defpackage :subex
  (:use :cl)
  (:export :toplevel *ui*))

(in-package :subex)

;;;; Global Options and UI ----------------------------------------------------
(defparameter *o/help*
  (adopt:make-option 'help :long "help" :help "display help and exit" :reduce (constantly t)))

(defparameter *o/version*
  (adopt:make-option 'version :long "version" :help "display version and exit" :reduce (constantly t)))

(defparameter *ui/main*
    :name "subex"
    :usage "[subcommand] [options]"
    :help "subcommand example program"
    :summary "an example program that uses subcommands"
    :contents (list *o/help* *o/version*)))

(defparameter *ui* *ui/main*)

;;;; Subcommand Foo -----------------------------------------------------------
(defparameter *o/foo/a*
  (adopt:make-option 'a :result-key 'mode :short #\a :help "run foo in mode A" :reduce (constantly :a)))

(defparameter *o/foo/b*
  (adopt:make-option 'b :result-key 'mode :short #\b :help "run foo in mode B" :reduce (constantly :b)))

(defparameter *ui/foo*
    :name "subex foo"
    :usage "foo [-a|-b]"
    :summary "foo some things"
    :help "foo some things"
    :contents (list *o/foo/a* *o/foo/b*)))

(defun run/foo (mode)
  (format t "Running foo in ~A mode.~%" mode))

;;;; Subcommand Bar -----------------------------------------------------------
(defparameter *o/bar/meow*
  (adopt:make-option 'meow :long "meow" :help "meow loudly after each step" :reduce (constantly t)))

(defparameter *ui/bar*
    :name "subex bar"
    :usage "bar [--meow] FILE..."
    :summary "bar some files"
    :help "bar some files"
    :contents (list *o/bar/meow*)))

(defun run/bar (paths meow?)
  (dolist (p paths)
    (format t "Bar-ing ~A.~%" p)
    (when meow?
      (write-line "meow."))))

;;;; Toplevel -----------------------------------------------------------------
(defun toplevel/foo (args)
  (multiple-value-bind (arguments options) (adopt:parse-options-or-exit *ui/foo* args)
    (unless (null arguments)
      (error "Foo does not take arguments, got ~S" arguments))
    (run/foo (gethash 'mode options))))

(defun toplevel/bar (args)
  (multiple-value-bind (arguments options) (adopt:parse-options-or-exit *ui/bar* args)
    (when (null arguments)
      (error "Bar requires arguments, got none."))
    (run/bar arguments (gethash 'meow options))))

(defun lookup-subcommand (string)
    ((null string) (values nil *ui/main*))
    ((string= string "foo") (values #'toplevel/foo *ui/foo*))
    ((string= string "bar") (values #'toplevel/bar *ui/bar*))
    (t (error "Unknown subcommand ~S" string))))

(defun toplevel ()
  (multiple-value-bind (arguments global-options)
      (handler-bind ((adopt:unrecognized-option 'adopt:treat-as-argument))
        (adopt:parse-options *ui/main*))
    (when (gethash 'version global-options)
      (write-line "1.0.0")
    (multiple-value-bind (subtoplevel ui) (lookup-subcommand (first arguments))
      (when (or (null subtoplevel)
                (gethash 'help global-options))
        (adopt:print-help-and-exit ui))
      (funcall subtoplevel (rest arguments)))))