Add 'guix hash'.

* guix/scripts/hash.scm: New file.
* Makefile.am (MODULES): Add it.
* po/POTFILES.in: Add it.
* doc/guix.texi (Invoking guix hash): New node.
  (Defining Packages): Add a cross-reference to the 'Invoking guix
  hash' node.

Makefile.am

@@ -30,6 +30,7 @@ MODULES =					\
   guix/scripts/import.scm			\
   guix/scripts/package.scm			\
   guix/scripts/gc.scm				\
+  guix/scripts/hash.scm				\
   guix/scripts/pull.scm				\
   guix/scripts/substitute-binary.scm		\
   guix/base32.scm				\

doc/guix.texi

@@ -814,8 +814,9 @@ the GNU mirrors defined in @code{(guix download)}.
 The @code{sha256} field specifies the expected SHA256 hash of the file
 being downloaded.  It is mandatory, and allows Guix to check the
 integrity of the file.  The @code{(base32 @dots{})} form introduces the
-base32 representation of the hash.  A convenient way to obtain this
-information is with the @code{guix download} tool.
+base32 representation of the hash.  You can obtain this information with
+the @code{guix hash} (@pxref{Invoking guix hash}) and @code{guix
+download} tools.
 
 @item
 @cindex GNU Build System
@@ -1094,6 +1095,7 @@ space.
 
 @menu
 * Invoking guix build:: Building packages from the command line.
+* Invoking guix hash:: Computing the cryptographic hash of a file.
 @end menu
 
 @node Invoking guix build
@@ -1189,6 +1191,37 @@ the @code{package-derivation} procedure of the @code{(guix packages)}
 module, and to the @code{build-derivations} procedure of the @code{(guix
 store)} module.
 
+@node Invoking guix hash
+@section Invoking @command{guix hash}
+
+The @command{guix hash} command allows to check the integrity of a file.
+It is primarily a convenience tool for anyone contributing to the
+distribution: it computes the cryptographic hash of a file, which can be
+used in the definition of a package (@pxref{Defining Packages}).
+
+The general syntax is:
+
+@example
+guix hash @var{option} @var{file}
+@end example
+
+@command{guix hash} has the following option:
+
+@table @code
+
+@item --format=@var{fmt}
+@itemx -f @var{fmt}
+Write the hash in the given format.
+
+Supported formats: @code{nix-base32}, @code{base32}, @code{base16}
+(@code{hex} and @code{hexadecimal} can be used as well).
+
+If the @option{--format} option is not specified, @command{guix hash}
+will output the hash in @code{nix-base32}.  This representation is used
+in the definitions of packages.
+
+@end table
+
 @c *********************************************************************
 @node GNU Distribution
 @chapter GNU Distribution

guix/scripts/hash.scm

@@ -0,0 +1,120 @@
+;;; GNU Guix --- Functional package management for GNU
+;;; Copyright © 2012, 2013 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2013 Nikita Karetnikov <nikita@karetnikov.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 hash)
+    #:use-module (guix base32)
+    #:use-module (guix ui)
+    #:use-module (guix utils)
+    #:use-module (rnrs io ports)
+    #:use-module (rnrs files)
+    #:use-module (ice-9 match)
+    #:use-module (srfi srfi-1)
+    #:use-module (srfi srfi-26)
+    #:use-module (srfi srfi-37)
+    #:export (guix-hash))
+
+
+;;;
+;;; Command-line options.
+;;;
+
+(define %default-options
+  ;; Alist of default option values.
+  `((format . ,bytevector->nix-base32-string)))
+
+(define (show-help)
+  (display (_ "Usage: guix hash [OPTION] FILE
+Return the cryptographic hash of FILE.
+
+Supported formats: 'nix-base32' (default), 'base32', and 'base16'
+('hex' and 'hexadecimal' can be used as well).\n"))
+  (format #t (_ "
+  -f, --format=FMT       write the hash in the given format"))
+  (newline)
+  (display (_ "
+  -h, --help             display this help and exit"))
+  (display (_ "
+  -V, --version          display version information and exit"))
+  (newline)
+  (show-bug-report-information))
+
+(define %options
+  ;; Specification of the command-line options.
+  (list (option '(#\f "format") #t #f
+                (lambda (opt name arg result)
+                  (define fmt-proc
+                    (match arg
+                      ("nix-base32"
+                       bytevector->nix-base32-string)
+                      ("base32"
+                       bytevector->base32-string)
+                      ((or "base16" "hex" "hexadecimal")
+                       bytevector->base16-string)
+                      (x
+                       (leave (_ "unsupported hash format: ~a~%")
+                              arg))))
+
+                  (alist-cons 'format fmt-proc
+                              (alist-delete 'format result))))
+
+        (option '(#\h "help") #f #f
+                (lambda args
+                  (show-help)
+                  (exit 0)))
+        (option '(#\V "version") #f #f
+                (lambda args
+                  (show-version-and-exit "guix hash")))))
+
+
+
+;;;
+;;; Entry point.
+;;;
+
+(define (guix-hash . args)
+  (define (parse-options)
+    ;; Return the alist of option values.
+    (args-fold args %options
+               (lambda (opt name arg result)
+                 (leave (_ "unrecognized option: ~a~%")
+                        name))
+               (lambda (arg result)
+                 (alist-cons 'argument arg result))
+               %default-options))
+
+    (let* ((opts (parse-options))
+           (args (filter-map (match-lambda
+                              (('argument . value)
+                               value)
+                              (_ #f))
+                             (reverse opts)))
+           (fmt  (assq-ref opts 'format)))
+
+      (match args
+        ((file)
+         (catch 'system-error
+           (lambda ()
+             (format #t "~a~%"
+                     (call-with-input-file file
+                       (compose fmt sha256 get-bytevector-all))))
+           (lambda args
+             (leave (_ "~a~%")
+                    (strerror (system-error-errno args))))))
+        (_
+         (leave (_ "wrong number of arguments~%"))))))

po/POTFILES.in

@@ -8,6 +8,7 @@ guix/scripts/build.scm
 guix/scripts/download.scm
 guix/scripts/package.scm
 guix/scripts/gc.scm
+guix/scripts/hash.scm
 guix/scripts/pull.scm
 guix/gnu-maintenance.scm
 guix/ui.scm