Add 'guix system'.

* guix/scripts/system.scm: New file.
* Makefile.am (MODULES): Add it.
* po/POTFILES.in: Likewise.
* doc/guix.texi (Using the Configuration System): Link to "Invoking guix
  system".
  (Invoking guix system): New node.

Makefile.am

@@ -77,6 +77,7 @@ MODULES =					\
   guix/scripts/substitute-binary.scm		\
   guix/scripts/authenticate.scm			\
   guix/scripts/refresh.scm			\
+  guix/scripts/system.scm			\
   guix.scm					\
   $(GNU_SYSTEM_MODULES)
 

doc/guix.texi

@@ -2547,8 +2547,9 @@ instantiated.  Then we show how this mechanism can be extended, for
 instance to support new system services.
 
 @menu
-* Using the Configuration System::      Customizing your GNU system.
-* Defining Services::                   Adding new service definitions.
+* Using the Configuration System::  Customizing your GNU system.
+* Invoking guix system::        Instantiating a system configuration.
+* Defining Services::           Adding new service definitions.
 @end menu
 
 @node Using the Configuration System
@@ -2614,13 +2615,12 @@ daemon listening on port 2222, and allowing remote @code{root} logins
 right command-line options, possibly with supporting configuration files
 generated as needed (@pxref{Defining Services}).
 
-@c TODO: update when that command exists
 Assuming the above snippet is stored in the @file{my-system-config.scm}
-file, the (yet unwritten!) @command{guix system --boot
-my-system-config.scm} command instantiates that configuration, and makes
-it the default GRUB boot entry.  The normal way to change the system's
-configuration is by updating this file and re-running the @command{guix
-system} command.
+file, the @command{guix system boot my-system-config.scm} command
+instantiates that configuration, and makes it the default GRUB boot
+entry (@pxref{Invoking guix system}).  The normal way to change the
+system's configuration is by updating this file and re-running the
+@command{guix system} command.
 
 At the Scheme level, the bulk of an @code{operating-system} declaration
 is instantiated with the following monadic procedure (@pxref{The Store
@@ -2635,6 +2635,33 @@ the packages, configuration files, and other supporting files needed to
 instantiate @var{os}.
 @end deffn
 
+@node Invoking guix system
+@subsection Invoking @code{guix system}
+
+Once you have written an operating system declaration, as seen in the
+previous section, it can be @dfn{instantiated} using the @command{guix
+system} command.  The synopsis is:
+
+@example
+guix system @var{options}@dots{} @var{action} @var{file}
+@end example
+
+@var{file} must be the name of a file containing an
+@code{operating-system} declaration.  @var{action} specifies how the
+operating system is instantiate.  Currently only one value is supported:
+
+@table @code
+@item vm
+@cindex virtual machine
+Build a virtual machine that contain the operating system declared in
+@var{file}, and return a script to run that virtual machine (VM).
+
+The VM shares its store with the host system.
+@end table
+
+@var{options} can contain any of the common build options provided by
+@command{guix build} (@pxref{Invoking guix build}).
+
 
 @node Defining Services
 @subsection Defining Services

guix/scripts/system.scm

@@ -0,0 +1,148 @@
+;;; GNU Guix --- Functional package management for GNU
+;;; Copyright © 2014 Ludovic Courtès <ludo@gnu.org>
+;;;
+;;; 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 ui)
+  #:use-module (guix store)
+  #:use-module (guix derivations)
+  #:use-module (guix packages)
+  #:use-module (guix utils)
+  #:use-module (guix monads)
+  #:use-module (guix scripts build)
+  #:use-module (gnu system vm)
+  #:use-module (srfi srfi-1)
+  #:use-module (srfi srfi-37)
+  #:use-module (ice-9 match)
+  #:export (guix-system))
+
+(define %user-module
+  ;; Module in which the machine description file is loaded.
+  (let ((module (make-fresh-user-module)))
+    (for-each (lambda (iface)
+                (module-use! module (resolve-interface iface)))
+              '((gnu system)
+                (gnu system dmd)
+                (gnu system shadow)))
+    module))
+
+(define (read-operating-system file)
+  "Read the operating-system declaration from FILE and return it."
+  ;; TODO: Factorize.
+  (catch #t
+    (lambda ()
+      ;; Avoid ABI incompatibility with the <operating-system> record.
+      (set! %fresh-auto-compile #t)
+
+      (save-module-excursion
+       (lambda ()
+         (set-current-module %user-module)
+         (primitive-load file))))
+    (lambda args
+      (match args
+        (('system-error . _)
+         (let ((err (system-error-errno args)))
+           (leave (_ "failed to open operating system file '~a': ~a~%")
+                  file (strerror err))))
+        (_
+         (leave (_ "failed to load machine file '~a': ~s~%")
+                file args))))))
+
+
+;;;
+;;; Options.
+;;;
+
+(define (show-help)
+  (display (_ "Usage: guix system [OPTION] ACTION FILE
+Build the operating system declared in FILE according to ACTION.\n"))
+  (display (_ "Currently the only valid value for ACTION is 'vm', which builds
+a virtual machine of the given operating system.\n"))
+  (show-build-options-help)
+  (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 '(#\n "dry-run") #f #f
+                 (lambda (opt name arg result)
+                   (alist-cons 'dry-run? #t result)))
+         %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)))
+
+
+;;;
+;;; Entry point.
+;;;
+
+(define (guix-system . args)
+  (define (parse-options)
+    ;; Return the alist of option values.
+    (args-fold* args %options
+                (lambda (opt name arg result)
+                  (leave (_ "~A: unrecognized option~%") name))
+                (lambda (arg result)
+                  (if (assoc-ref result 'action)
+                      (let ((previous (assoc-ref result 'argument)))
+                        (if previous
+                            (leave (_ "~a: extraneous argument~%") previous)
+                            (alist-cons 'argument arg result)))
+                      (let ((action (string->symbol arg)))
+                        (case action
+                          ((vm) (alist-cons 'action action result))
+                          (else (leave (_ "~a: unknown action~%")
+                                       action))))))
+                %default-options))
+
+  (with-error-handling
+    (let* ((opts  (parse-options))
+           (file  (assoc-ref opts 'argument))
+           (os    (if file
+                      (read-operating-system file)
+                      (leave (_ "no configuration file specified~%"))))
+           (mdrv  (system-qemu-image/shared-store-script os))
+           (store (open-connection))
+           (dry?  (assoc-ref opts 'dry-run?))
+           (drv   (run-with-store store mdrv)))
+      (set-build-options-from-command-line store opts)
+      (show-what-to-build store (list drv)
+                          #:dry-run? dry?
+                          #:use-substitutes? (assoc-ref opts 'substitutes?))
+
+      (unless dry?
+        (build-derivations store (list drv))
+        (display (derivation->output-path drv))
+        (newline)))))

po/POTFILES.in

@@ -12,6 +12,7 @@ guix/scripts/hash.scm
 guix/scripts/pull.scm
 guix/scripts/substitute-binary.scm
 guix/scripts/authenticate.scm
+guix/scripts/system.scm
 guix/gnu-maintenance.scm
 guix/ui.scm
 guix/http-client.scm