path: root/guix/scripts/system.scm

;;; GNU Guix --- Functional package management for GNU
;;; Copyright © 2014, 2015, 2016 Ludovic Courtès <ludo@gnu.org>
;;; Copyright © 2016 Alex Kost <alezost@gmail.com>
;;;
;;; This file is part of GNU Guix.
;;;
;;; GNU Guix is free software; you can redistribute it and/or modify it
;;; under the terms of the GNU General Public License as published by
;;; the Free Software Foundation; either version 3 of the License, or (at
;;; your option) any later version.
;;;
;;; GNU Guix is distributed in the hope that it will be useful, but
;;; WITHOUT ANY WARRANTY; without even the implied warranty of
;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
;;; GNU General Public License for more details.
;;;
;;; You should have received a copy of the GNU General Public License
;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.

(define-module (guix scripts system)
  #:use-module (guix config)
  #:use-module (guix ui)
  #:use-module (guix store)
  #:use-module (guix gexp)
  #:use-module (guix derivations)
  #:use-module (guix packages)
  #:use-module (guix utils)
  #:use-module (guix monads)
  #:use-module (guix records)
  #:use-module (guix profiles)
  #:use-module (guix scripts)
  #:use-module (guix scripts build)
  #:use-module (guix graph)
  #:use-module (guix scripts graph)
  #:use-module (guix build utils)
  #:use-module (gnu build install)
  #:use-module (gnu system)
  #:use-module (gnu system file-systems)
  #:use-module (gnu system linux-container)
  #:use-module (gnu system vm)
  #:use-module (gnu system grub)
  #:use-module (gnu services)
  #:use-module (gnu services shepherd)
  #:use-module (gnu services herd)
  #:use-module (gnu packages grub)
  #:use-module (srfi srfi-1)
  #:use-module (srfi srfi-11)
  #:use-module (srfi srfi-19)
  #:use-module (srfi srfi-26)
  #:use-module (srfi srfi-34)
  #:use-module (srfi srfi-35)
  #:use-module (srfi srfi-37)
  #:use-module (ice-9 match)
  #:export (guix-system
            read-operating-system))


;;;
;;; Operating system declaration.
;;;

(define %user-module
  ;; Module in which the machine description file is loaded.
  (make-user-module '((gnu system)
                      (gnu services)
                      (gnu system shadow))))

(define (read-operating-system file)
  "Read the operating-system declaration from FILE and return it."
  (load* file %user-module))



;;;
;;; Installation.
;;;

;; TODO: Factorize.
(define references*
  (store-lift references))
(define topologically-sorted*
  (store-lift topologically-sorted))


(define* (copy-item item target
                    #:key (log-port (current-error-port)))
  "Copy ITEM to the store under root directory TARGET and register it."
  (mlet* %store-monad ((refs (references* item)))
    (let ((dest  (string-append target item))
          (state (string-append target "/var/guix")))
      (format log-port "copying '~a'...~%" item)

      ;; Remove DEST if it exists to make sure that (1) we do not fail badly
      ;; while trying to overwrite it (see <http://bugs.gnu.org/20722>), and
      ;; (2) we end up with the right contents.
      (when (file-exists? dest)
        (delete-file-recursively dest))

      (copy-recursively item dest
                        #:log (%make-void-port "w"))

      ;; Register ITEM; as a side-effect, it resets timestamps, etc.
      ;; Explicitly use "TARGET/var/guix" as the state directory, to avoid
      ;; reproducing the user's current settings; see
      ;; <http://bugs.gnu.org/18049>.
      (unless (register-path item
                             #:prefix target
                             #:state-directory state
                             #:references refs)
        (leave (_ "failed to register '~a' under '~a'~%")
               item target))

      (return #t))))

(define* (copy-closure item target
                       #:key (log-port (current-error-port)))
  "Copy ITEM and all its dependencies to the store under root directory
TARGET, and register them."
  (mlet* %store-monad ((refs    (references* item))
                       (to-copy (topologically-sorted*
                                 (delete-duplicates (cons item refs)
                                                    string=?))))
    (sequence %store-monad
              (map (cut copy-item <> target #:log-port log-port)
                   to-copy))))

(define (install-grub* grub.cfg device target)
  "This is a variant of 'install-grub' with error handling, lifted in
%STORE-MONAD"
  (let* ((gc-root      (string-append %gc-roots-directory "/grub.cfg"))
         (temp-gc-root (string-append gc-root ".new"))
         (delete-file  (lift1 delete-file %store-monad))
         (make-symlink (lift2 switch-symlinks %store-monad))
         (rename       (lift2 rename-file %store-monad)))
    (mbegin %store-monad
      ;; Prepare the symlink to GRUB.CFG to make sure that it's a GC root when
      ;; 'install-grub' completes (being a bit paranoid.)
      (make-symlink temp-gc-root grub.cfg)

      (munless (false-if-exception (install-grub grub.cfg device target))
        (delete-file temp-gc-root)
        (leave (_ "failed to install GRUB on device '~a'~%") device))

      ;; Register GRUB.CFG as a GC root so that its dependencies (background
      ;; image, font, etc.) are not reclaimed.
      (rename temp-gc-root gc-root))))

(define* (install os-drv target
                  #:key (log-port (current-output-port))
                  grub? grub.cfg device)
  "Copy the closure of GRUB.CFG, which includes the output of OS-DRV, to
directory TARGET.  TARGET must be an absolute directory name since that's what
'guix-register' expects.

When GRUB? is true, install GRUB on DEVICE, using GRUB.CFG."
  (define (maybe-copy to-copy)
    (with-monad %store-monad
      (if (string=? target "/")
          (begin
            (warning (_ "initializing the current root file system~%"))
            (return #t))
          (begin
            ;; Make sure the target store exists.
            (mkdir-p (string-append target (%store-prefix)))

            ;; Copy items to the new store.
            (copy-closure to-copy target #:log-port log-port)))))

  ;; Make sure TARGET is root-owned when running as root, but still allow
  ;; non-root uses (useful for testing.)  See
  ;; <http://lists.gnu.org/archive/html/guix-devel/2015-05/msg00452.html>.
  (if (zero? (geteuid))
      (chown target 0 0)
      (warning (_ "not running as 'root', so \
the ownership of '~a' may be incorrect!~%")
               target))

  (chmod target #o755)
  (let ((os-dir   (derivation->output-path os-drv))
        (format   (lift format %store-monad))
        (populate (lift2 populate-root-file-system %store-monad)))

    (mbegin %store-monad
      ;; Copy the closure of GRUB.CFG, which includes OS-DIR, GRUB's
      ;; background image and so on.
      (maybe-copy grub.cfg)

      ;; Create a bunch of additional files.
      (format log-port "populating '~a'...~%" target)
      (populate os-dir target)

      (mwhen grub?
        (install-grub* grub.cfg device target)))))


;;;
;;; Reconfiguration.
;;;

(define %system-profile
  ;; The system profile.
  (string-append %state-directory "/profiles/system"))

(define-syntax-rule (save-environment-excursion body ...)
  "Save the current environment variables, run BODY..., and restore them."
  (let ((env (environ)))
    (dynamic-wind
      (const #t)
      (lambda ()
        body ...)
      (lambda ()
        (environ env)))))

(define (upgrade-shepherd-services os)
  "Upgrade the Shepherd (PID 1) by unloading obsolete services and loading new
services specified in OS and not currently running.

This is currently very conservative in that it does not stop or unload any
running service.  Unloading or stopping the wrong service ('udev', say) could
bring the system down."
  (define (essential? service)
    (memq service '(root shepherd)))

  (define new-services
    (service-parameters
     (fold-services (operating-system-services os)
                    #:target-type shepherd-root-service-type)))

  (define new-service-names
    (map (compose first shepherd-service-provision)
         new-services))

  (let-values (((running stopped) (current-services)))
    (define to-load
      ;; Only load services that are either new or currently stopped.
      (remove (lambda (service)
                (memq (first (shepherd-service-provision service))
                      running))
              new-services))
    (define to-unload
      ;; Unload services that are (1) no longer required, or (2) are in
      ;; TO-LOAD.
      (remove essential?
              (append (remove (lambda (service)
                                (memq service new-service-names))
                              (append running stopped))
                      (filter (lambda (service)
                                (memq service stopped))
                              (map shepherd-service-canonical-name
                                   to-load)))))

    (for-each (lambda (unload)
                (info (_ "unloading service '~a'...~%") unload)
                (unload-service unload))
              to-unload)

    (with-monad %store-monad
      (munless (null? to-load)
        (let ((to-load-names  (map shepherd-service-canonical-name to-load))
              (to-start       (filter shepherd-service-auto-start? to-load)))
          (info (_ "loading new services:~{ ~a~}...~%") to-load-names)
          (mlet %store-monad ((files (mapm %store-monad shepherd-service-file
                                           to-load)))
            (load-services (map derivation->output-path files))

            (for-each start-service
                      (map shepherd-service-canonical-name to-start))
            (return #t)))))))

(define* (switch-to-system os
                           #:optional (profile %system-profile))
  "Make a new generation of PROFILE pointing to the directory of OS, switch to
it atomically, and then run OS's activation script."
  (mlet* %store-monad ((drv    (operating-system-derivation os))
                       (script (operating-system-activation-script os)))
    (let* ((system     (derivation->output-path drv))
           (number     (+ 1 (generation-number profile)))
           (generation (generation-file-name profile number)))
      (symlink system generation)
      (switch-symlinks profile generation)

      (format #t (_ "activating system...~%"))

      ;; The activation script may change $PATH, among others, so protect
      ;; against that.
      (save-environment-excursion
       ;; Tell 'activate-current-system' what the new system is.
       (setenv "GUIX_NEW_SYSTEM" system)

       (primitive-load (derivation->output-path script)))

      ;; Finally, try to update system services.
      (upgrade-shepherd-services os))))

(define-syntax-rule (unless-file-not-found exp)
  (catch 'system-error
    (lambda ()
      exp)
    (lambda args
      (if (= ENOENT (system-error-errno args))
          #f
          (apply throw args)))))

(define (seconds->string seconds)
  "Return a string representing the date for SECONDS."
  (let ((time (make-time time-utc 0 seconds)))
    (date->string (time-utc->date time)
                  "~Y-~m-~d ~H:~M")))

(define* (previous-grub-entries #:optional (profile %system-profile))
  "Return a list of 'menu-entry' for the generations of PROFILE."
  (define (system->grub-entry system number time)
    (unless-file-not-found
     (let* ((file             (string-append system "/parameters"))
            (params           (call-with-input-file file
                                read-boot-parameters))
            (label            (boot-parameters-label params))
            (root             (boot-parameters-root-device params))
            (kernel           (boot-parameters-kernel params))
            (kernel-arguments (boot-parameters-kernel-arguments params)))
       (menu-entry
        (label (string-append label " (#"
                              (number->string number) ", "
                              (seconds->string time) ")"))
        (linux kernel)
        (linux-arguments
         (cons* (string-append "--root=" root)
                #~(string-append "--system=" #$system)
                #~(string-append "--load=" #$system "/boot")
                kernel-arguments))
        (initrd #~(string-append #$system "/initrd"))))))

  (let* ((numbers (generation-numbers profile))
         (systems (map (cut generation-file-name profile <>)
                       numbers))
         (times   (map (lambda (system)
                         (unless-file-not-found
                          (stat:mtime (lstat system))))
                       systems)))
    (filter-map system->grub-entry systems numbers times)))


;;;
;;; Graphs.
;;;

(define (service-node-label service)
  "Return a label to represent SERVICE."
  (let ((type  (service-kind service))
        (value (service-parameters service)))
    (string-append (symbol->string (service-type-name type))
                   (cond ((or (number? value) (symbol? value))
                          (string-append " " (object->string value)))
                         ((string? value)
                          (string-append " " value))
                         ((file-system? value)
                          (string-append " " (file-system-mount-point value)))
                         (else
                          "")))))

(define (service-node-type services)
  "Return a node type for SERVICES.  Since <service> instances are not
self-contained (they express dependencies on service types, not on services),
we have to create the 'edges' procedure dynamically as a function of the full
list of services."
  (node-type
   (name "service")
   (description "the DAG of services")
   (identifier (lift1 object-address %store-monad))
   (label service-node-label)
   (edges (lift1 (service-back-edges services) %store-monad))))

(define (shepherd-service-node-label service)
  "Return a label for a node representing a <shepherd-service>."
  (string-join (map symbol->string (shepherd-service-provision service))))

(define (shepherd-service-node-type services)
  "Return a node type for SERVICES, a list of <shepherd-service>."
  (node-type
   (name "shepherd-service")
   (description "the dependency graph of shepherd services")
   (identifier (lift1 shepherd-service-node-label %store-monad))
   (label shepherd-service-node-label)
   (edges (lift1 (shepherd-service-back-edges services) %store-monad))))


;;;
;;; Generations.
;;;

(define* (display-system-generation number
                                    #:optional (profile %system-profile))
  "Display a summary of system generation NUMBER in a human-readable format."
  (unless (zero? number)
    (let* ((generation (generation-file-name profile number))
           (param-file (string-append generation "/parameters"))
           (params     (call-with-input-file param-file read-boot-parameters))
           (label      (boot-parameters-label params))
           (root       (boot-parameters-root-device params))
           (kernel     (boot-parameters-kernel params)))
      (display-generation profile number)
      (format #t (_ "  file name: ~a~%") generation)
      (format #t (_ "  canonical file name: ~a~%") (readlink* generation))
      ;; TRANSLATORS: Please preserve the two-space indentation.
      (format #t (_ "  label: ~a~%") label)
      (format #t (_ "  root device: ~a~%") root)
      (format #t (_ "  kernel: ~a~%") kernel))))

(define* (list-generations pattern #:optional (profile %system-profile))
  "Display in a human-readable format all the system generations matching
PATTERN, a string.  When PATTERN is #f, display all the system generations."
  (cond ((not (file-exists? profile))             ; XXX: race condition
         (raise (condition (&profile-not-found-error
                            (profile profile)))))
        ((string-null? pattern)
         (for-each display-system-generation (profile-generations profile)))
        ((matching-generations pattern profile)
         =>
         (lambda (numbers)
           (if (null-list? numbers)
               (exit 1)
               (leave-on-EPIPE
                (for-each display-system-generation numbers)))))
        (else
         (leave (_ "invalid syntax: ~a~%") pattern))))


;;;
;;; Action.
;;;

(define* (system-derivation-for-action os action
                                       #:key image-size full-boot? mappings)
  "Return as a monadic value the derivation for OS according to ACTION."
  (case action
    ((build init reconfigure)
     (operating-system-derivation os))
    ((container)
     (container-script os #:mappings mappings))
    ((vm-image)
     (system-qemu-image os #:disk-image-size image-size))
    ((vm)
     (system-qemu-image/shared-store-script os
                                            #:full-boot? full-boot?
                                            #:disk-image-size image-size
                                            #:mappings mappings))
    ((disk-image)
     (system-disk-image os #:disk-image-size image-size))))

(define* (perform-action action os
                         #:key grub? dry-run? derivations-only?
                         use-substitutes? device target
                         image-size full-boot?
                         (mappings '()))
  "Perform ACTION for OS.  GRUB? specifies whether to install GRUB; DEVICE is
the target devices for GRUB; TARGET is the target root directory; IMAGE-SIZE
is the size of the image to be built, for the 'vm-image' and 'disk-image'
actions.  FULL-BOOT? is used for the 'vm' action; it determines whether to
boot directly to the kernel or to the bootloader.

When DERIVATIONS-ONLY? is true, print the derivation file name(s) without
building anything."
  (define println
    (cut format #t "~a~%" <>))

  (mlet* %store-monad
      ((sys       (system-derivation-for-action os action
                                                #:image-size image-size
                                                #:full-boot? full-boot?
                                                #:mappings mappings))
       (grub      (package->derivation grub))
       (grub.cfg  (if (eq? 'container action)
                      (return #f)
                      (operating-system-grub.cfg os
                                                 (if (eq? 'init action)
                                                     '()
                                                     (previous-grub-entries)))))

       ;; For 'init' and 'reconfigure', always build GRUB.CFG, even if
       ;; --no-grub is passed, because GRUB.CFG because we then use it as a GC
       ;; root.  See <http://bugs.gnu.org/21068>.
       (drvs   -> (if (memq action '(init reconfigure))
                      (if grub?
                          (list sys grub.cfg grub)
                          (list sys grub.cfg))
                      (list sys)))
       (%         (if derivations-only?
                      (return (for-each (compose println derivation-file-name)
                                        drvs))
                      (maybe-build drvs #:dry-run? dry-run?
                                   #:use-substitutes? use-substitutes?))))

    (if (or dry-run? derivations-only?)
        (return #f)
        (begin
          (for-each (compose println derivation->output-path)
                    drvs)

          ;; Make sure GRUB is accessible.
          (when grub?
            (let ((prefix (derivation->output-path grub)))
              (setenv "PATH"
                      (string-append  prefix "/bin:" prefix "/sbin:"
                                      (getenv "PATH")))))

          (case action
            ((reconfigure)
             (mbegin %store-monad
               (switch-to-system os)
               (mwhen grub?
                 (install-grub* (derivation->output-path grub.cfg)
                                device "/"))))
            ((init)
             (newline)
             (format #t (_ "initializing operating system under '~a'...~%")
                     target)
             (install sys (canonicalize-path target)
                      #:grub? grub?
                      #:grub.cfg (derivation->output-path grub.cfg)
                      #:device device))
            (else
             ;; All we had to do was to build SYS.
             (return (derivation->output-path sys))))))))

(define (export-extension-graph os port)
  "Export the service extension graph of OS to PORT."
  (let* ((services (operating-system-services os))
         (system   (find (lambda (service)
                           (eq? (service-kind service) system-service-type))
                         services)))
    (export-graph (list system) (current-output-port)
                  #:node-type (service-node-type services)
                  #:reverse-edges? #t)))

(define (export-shepherd-graph os port)
  "Export the graph of shepherd services of OS to PORT."
  (let* ((services  (operating-system-services os))
         (pid1      (fold-services services
                                   #:target-type shepherd-root-service-type))
         (shepherds (service-parameters pid1)) ;list of <shepherd-service>
         (sinks     (filter (lambda (service)
                              (null? (shepherd-service-requirement service)))
                            shepherds)))
    (export-graph sinks (current-output-port)
                  #:node-type (shepherd-service-node-type shepherds)
                  #:reverse-edges? #t)))


;;;
;;; Options.
;;;

(define (show-help)
  (display (_ "Usage: guix system [OPTION] ACTION [FILE]
Build the operating system declared in FILE according to ACTION.\n"))
  (newline)
  (display (_ "The valid values for ACTION are:\n"))
  (newline)
  (display (_ "\
   reconfigure      switch to a new operating system configuration\n"))
  (display (_ "\
   list-generations list the system generations\n"))
  (display (_ "\
   build            build the operating system without installing anything\n"))
  (display (_ "\
   container        build a container that shares the host's store\n"))
  (display (_ "\
   vm               build a virtual machine image that shares the host's store\n"))
  (display (_ "\
   vm-image         build a freestanding virtual machine image\n"))
  (display (_ "\
   disk-image       build a disk image, suitable for a USB stick\n"))
  (display (_ "\
   init             initialize a root file system to run GNU\n"))
  (display (_ "\
   extension-graph  emit the service extension graph in Dot format\n"))
  (display (_ "\
   shepherd-graph   emit the graph of shepherd services in Dot format\n"))

  (show-build-options-help)
  (display (_ "
  -d, --derivation       return the derivation of the given system"))
  (display (_ "
      --on-error=STRATEGY
                         apply STRATEGY when an error occurs while reading FILE"))
  (display (_ "
      --image-size=SIZE  for 'vm-image', produce an image of SIZE"))
  (display (_ "
      --no-grub          for 'init', do not install GRUB"))
  (display (_ "
      --share=SPEC       for 'vm', share host file system according to SPEC"))
  (display (_ "
      --expose=SPEC      for 'vm', expose host file system according to SPEC"))
  (display (_ "
      --full-boot        for 'vm', make a full boot sequence"))
  (newline)
  (display (_ "
  -h, --help             display this help and exit"))
  (display (_ "
  -V, --version          display version information and exit"))
  (newline)
  (show-bug-report-information))

(define %options
  ;; Specifications of the command-line options.
  (cons* (option '(#\h "help") #f #f
                 (lambda args
                   (show-help)
                   (exit 0)))
         (option '(#\V "version") #f #f
                 (lambda args
                   (show-version-and-exit "guix system")))
         (option '(#\d "derivation") #f #f
                 (lambda (opt name arg result)
                   (alist-cons 'derivations-only? #t result)))
         (option '("on-error") #t #f
                 (lambda (opt name arg result)
                   (alist-cons 'on-error (string->symbol arg)
                               result)))
         (option '("image-size") #t #f
                 (lambda (opt name arg result)
                   (alist-cons 'image-size (size->number arg)
                               result)))
         (option '("no-grub") #f #f
                 (lambda (opt name arg result)
                   (alist-cons 'install-grub? #f result)))
         (option '("full-boot") #f #f
                 (lambda (opt name arg result)
                   (alist-cons 'full-boot? #t result)))

         (option '("share") #t #f
                 (lambda (opt name arg result)
                   (alist-cons 'file-system-mapping
                               (specification->file-system-mapping arg #t)
                               result)))
         (option '("expose") #t #f
                 (lambda (opt name arg result)
                   (alist-cons 'file-system-mapping
                               (specification->file-system-mapping arg #f)
                               result)))

         (option '(#\n "dry-run") #f #f
                 (lambda (opt name arg result)
                   (alist-cons 'dry-run? #t result)))
         (option '(#\s "system") #t #f
                 (lambda (opt name arg result)
                   (alist-cons 'system arg
                               (alist-delete 'system result eq?))))
         %standard-build-options))

(define %default-options
  ;; Alist of default option values.
  `((system . ,(%current-system))
    (substitutes? . #t)
    (build-hook? . #t)
    (max-silent-time . 3600)
    (verbosity . 0)
    (image-size . ,(* 900 (expt 2 20)))
    (install-grub? . #t)))


;;;
;;; Entry point.
;;;

(define (process-action action args opts)
  "Process ACTION, a sub-command, with the arguments are listed in ARGS.
ACTION must be one of the sub-commands that takes an operating system
declaration as an argument (a file name.)  OPTS is the raw alist of options
resulting from command-line parsing."
  (let* ((file     (match args
                     (() #f)
                     ((x . _) x)))
         (system   (assoc-ref opts 'system))
         (os       (if file
                       (load* file %user-module
                              #:on-error (assoc-ref opts 'on-error))
                       (leave (_ "no configuration file specified~%"))))

         (dry?     (assoc-ref opts 'dry-run?))
         (grub?    (assoc-ref opts 'install-grub?))
         (target   (match args
                     ((first second) second)
                     (_ #f)))
         (device   (and grub?
                        (grub-configuration-device
                         (operating-system-bootloader os)))))

    (with-store store
      (set-build-options-from-command-line store opts)

      (run-with-store store
        (mbegin %store-monad
          (set-guile-for-build (default-guile))
          (case action
            ((extension-graph)
             (export-extension-graph os (current-output-port)))
            ((shepherd-graph)
             (export-shepherd-graph os (current-output-port)))
            (else
             (perform-action action os
                             #:dry-run? dry?
                             #:derivations-only? (assoc-ref opts
                                                            'derivations-only?)
                             #:use-substitutes? (assoc-ref opts 'substitutes?)
                             #:image-size (assoc-ref opts 'image-size)
                             #:full-boot? (assoc-ref opts 'full-boot?)
                             #:mappings (filter-map (match-lambda
                                                      (('file-system-mapping . m)
                                                       m)
                                                      (_ #f))
                                                    opts)
                             #:grub? grub?
                             #:target target #:device device))))
        #:system system))))

(define (process-command command args opts)
  "Process COMMAND, one of the 'guix system' sub-commands.  ARGS is its
argument list and OPTS is the option alist."
  (case command
    ((list-generations)
     ;; List generations.  No need to connect to the daemon, etc.
     (let ((pattern (match args
                      (() "")
                      ((pattern) pattern)
                      (x (leave (_ "wrong number of arguments~%"))))))
       (list-generations pattern)))
    (else
     (process-action command args opts))))

(define (guix-system . args)
  (define (parse-sub-command arg result)
    ;; Parse sub-command ARG and augment RESULT accordingly.
    (if (assoc-ref result 'action)
        (alist-cons 'argument arg result)
        (let ((action (string->symbol arg)))
          (case action
            ((build container vm vm-image disk-image reconfigure init
              extension-graph shepherd-graph list-generations)
             (alist-cons 'action action result))
            (else (leave (_ "~a: unknown action~%") action))))))

  (define (match-pair car)
    ;; Return a procedure that matches a pair with CAR.
    (match-lambda
      ((head . tail)
       (and (eq? car head) tail))
      (_ #f)))

  (define (option-arguments opts)
    ;; Extract the plain arguments from OPTS.
    (let* ((args   (reverse (filter-map (match-pair 'argument) opts)))
           (count  (length args))
           (action (assoc-ref opts 'action)))
      (define (fail)
        (leave (_ "wrong number of arguments for action '~a'~%")
               action))

      (unless action
        (format (current-error-port)
                (_ "guix system: missing command name~%"))
        (format (current-error-port)
                (_ "Try 'guix system --help' for more information.~%"))
        (exit 1))

      (case action
        ((build container vm vm-image disk-image reconfigure)
         (unless (= count 1)
           (fail)))
        ((init)
         (unless (= count 2)
           (fail))))
      args))

  (with-error-handling
    (let* ((opts     (parse-command-line args %options
                                         (list %default-options)
                                         #:argument-handler
                                         parse-sub-command))
           (args     (option-arguments opts))
           (command  (assoc-ref opts 'action)))
      (process-command command args opts))))

;;; system.scm ends here