Add 'guix describe'.

* guix/scripts/describe.scm: New file.
* Makefile.am (MODULES): Add it.
(SH_TESTS): Add tests/guix-describe.sh.
* po/guix/POTFILES.in: Add it.
* guix/scripts/pull.scm (display-profile-content): Export.
* guix/describe.scm (current-profile, current-profile-entries): Export.
* tests/guix-describe.sh: New file.
* doc/guix.texi (Features): Mention 'guix pull' and provenance tracking.
(Invoking guix pull): Link to 'guix describe'.
(Channels): Likewise.
(Invoking guix describe): New node.

Makefile.am

@@ -204,6 +204,7 @@ MODULES =					\
   guix/scripts/authenticate.scm			\
   guix/scripts/refresh.scm			\
   guix/scripts/repl.scm				\
+  guix/scripts/describe.scm			\
   guix/scripts/system.scm			\
   guix/scripts/system/search.scm		\
   guix/scripts/lint.scm				\
@@ -409,6 +410,7 @@ SH_TESTS =					\
   tests/guix-environment.sh			\
   tests/guix-environment-container.sh		\
   tests/guix-graph.sh				\
+  tests/guix-describe.sh			\
   tests/guix-lint.sh
 
 TESTS = $(SCM_TESTS) $(SH_TESTS)

doc/guix.texi

@@ -147,6 +147,7 @@ Package Management
 * Invoking guix gc::            Running the garbage collector.
 * Invoking guix pull::          Fetching the latest Guix and distribution.
 * Channels::                    Customizing the package collection.
+* Invoking guix describe::      Display information about your Guix revision.
 * Invoking guix pack::          Creating software bundles.
 * Invoking guix archive::       Exporting and importing store files.
 
@@ -1698,6 +1699,7 @@ guix package -i emacs-guix
 * Invoking guix gc::            Running the garbage collector.
 * Invoking guix pull::          Fetching the latest Guix and distribution.
 * Channels::                    Customizing the package collection.
+* Invoking guix describe::      Display information about your Guix revision.
 * Invoking guix pack::          Creating software bundles.
 * Invoking guix archive::       Exporting and importing store files.
 @end menu
@@ -1751,7 +1753,7 @@ collected.
 
 @cindex reproducibility
 @cindex reproducible builds
-Finally, Guix takes a @dfn{purely functional} approach to package
+Guix takes a @dfn{purely functional} approach to package
 management, as described in the introduction (@pxref{Introduction}).
 Each @file{/gnu/store} package directory name contains a hash of all the
 inputs that were used to build that package---compiler, libraries, build
@@ -1779,6 +1781,15 @@ a package to quickly set up the right development environment for their
 package, without having to manually install the dependencies of the
 package into their profile (@pxref{Invoking guix environment}).
 
+@cindex replication, of software environments
+@cindex provenance tracking, of software artifacts
+All of Guix and its package definitions is version-controlled, and
+@command{guix pull} allows you to ``travel in time'' on the history of Guix
+itself (@pxref{Invoking guix pull}).  This makes it possible to replicate a
+Guix instance on a different machine or at a later point in time, which in
+turn allows you to @emph{replicate complete software environments}, while
+retaining precise @dfn{provenance tracking} of the software.
+
 @node Invoking guix package
 @section Invoking @command{guix package}
 
@@ -2806,6 +2817,9 @@ Generation 3	Jun 13 2018 23:31:07	(current)
   69 packages upgraded: borg@@1.1.6, cheese@@3.28.0, @dots{}
 @end example
 
+@ref{Invoking guix describe, @command{guix describe}}, for other ways to
+describe the current status of Guix.
+
 This @code{~/.config/guix/current} profile works like any other profile
 created by @command{guix package} (@pxref{Invoking guix package}).  That
 is, you can list generations, roll back to the previous
@@ -2851,6 +2865,9 @@ is provided, the subset of generations that match @var{pattern}.
 The syntax of @var{pattern} is the same as with @code{guix package
 --list-generations} (@pxref{Invoking guix package}).
 
+@ref{Invoking guix describe}, for a way to display information about the
+current generation only.
+
 @item --profile=@var{profile}
 @itemx -p @var{profile}
 Use @var{profile} instead of @file{~/.config/guix/current}.
@@ -3023,6 +3040,9 @@ say, on another machine, by providing a channel specification in
        (branch "dd3df5e2c8818760a8fc0bd699e55d3b69fef2bb")))
 @end lisp
 
+The @command{guix describe --format=channels} command can even generate this
+list of channels directly (@pxref{Invoking guix describe}).
+
 At this point the two machines run the @emph{exact same Guix}, with access to
 the @emph{exact same packages}.  The output of @command{guix build gimp} on
 one machine will be exactly the same, bit for bit, as the output of the same
@@ -3034,6 +3054,78 @@ This gives you super powers, allowing you to track the provenance of binary
 artifacts with very fine grain, and to reproduce software environments at
 will---some sort of ``meta reproducibility'' capabilities, if you will.
 
+@node Invoking guix describe
+@section Invoking @command{guix describe}
+
+@cindex reproducibility
+@cindex replicating Guix
+Often you may want to answer questions like: ``Which revision of Guix am I
+using?'' or ``Which channels am I using?''  This is useful information in many
+situations: if you want to @emph{replicate} an environment on a different
+machine or user account, if you want to report a bug or to determine what
+change in the channels you are using caused it, or if you want to record your
+system state for reproducibility purposes.  The @command{guix describe}
+command answers these questions.
+
+When run from a @command{guix pull}ed @command{guix}, @command{guix describe}
+displays the channel(s) that it was built from, including their repository URL
+and commit IDs (@pxref{Channels}):
+
+@example
+$ guix describe
+Generation 10	Sep 03 2018 17:32:44	(current)
+  guix e0fa68c
+    repository URL: https://git.savannah.gnu.org/git/guix.git
+    branch: master
+    commit: e0fa68c7718fffd33d81af415279d6ddb518f727
+@end example
+
+If you're familiar with the Git version control system, this is similar in
+spirit to @command{git describe}; the output is also similar to that of
+@command{guix pull --list-generations}, but limited to the current generation
+(@pxref{Invoking guix pull, the @option{--list-generations} option}).  Because
+the Git commit ID shown above unambiguously refers to a snapshot of Guix, this
+information is all it takes to describe the revision of Guix you're using, and
+also to replicate it.
+
+To make it easier to replicate Guix, @command{guix describe} can also be asked
+to return a list of channels instead of the human-readable description above:
+
+@example
+$ guix describe -f channels
+(list (channel
+        (name 'guix)
+        (url "https://git.savannah.gnu.org/git/guix.git")
+        (commit
+          "e0fa68c7718fffd33d81af415279d6ddb518f727")))
+@end example
+
+@noindent
+You can save this to a file and feed it to @command{guix pull -C} on some
+other machine or at a later point in time, which will instantiate @emph{this
+exact Guix revision} (@pxref{Invoking guix pull, the @option{-C} option}).
+From there on, since you're able to deploy the same revision of Guix, you can
+just as well @emph{replicate a complete software environment}.  We humbly
+think that this is @emph{awesome}, and we hope you'll like it too!
+
+The details of the options supported by @command{guix describe} are as
+follows:
+
+@table @code
+@item --format=@var{format}
+@itemx -f @var{format}
+Produce output in the specified @var{format}, one of:
+
+@table @code
+@item human
+produce human-readable output;
+@item channels
+produce a list of channel specifications that can be passed to @command{guix
+pull -C} or installed as @file{~/.config/guix/channels.scm} (@pxref{Invoking
+guix pull}).
+@end table
+@end table
+
 @node Invoking guix pack
 @section Invoking @command{guix pack}
 

guix/describe.scm

@@ -21,7 +21,9 @@
   #:use-module (guix profiles)
   #:use-module (srfi srfi-1)
   #:use-module (ice-9 match)
-  #:export (package-path-entries))
+  #:export (current-profile
+            current-profile-entries
+            package-path-entries))
 
 ;;; Commentary:
 ;;;

guix/scripts/describe.scm

@@ -0,0 +1,160 @@
+;;; GNU Guix --- Functional package management for GNU
+;;; Copyright © 2018 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 describe)
+  #:use-module ((guix ui) #:hide (display-profile-content))
+  #:use-module (guix scripts)
+  #:use-module (guix describe)
+  #:use-module (guix profiles)
+  #:use-module ((guix scripts pull) #:select (display-profile-content))
+  #:use-module (git)
+  #:use-module (srfi srfi-1)
+  #:use-module (srfi srfi-37)
+  #:use-module (ice-9 match)
+  #:autoload   (ice-9 pretty-print) (pretty-print)
+  #:export (guix-describe))
+
+
+;;;
+;;; Command-line options.
+;;;
+
+(define %options
+  ;; Specifications of the command-line options.
+  (list (option '(#\f "format") #t #f
+                (lambda (opt name arg result)
+                  (unless (member arg '("human" "channels"))
+                    (leave (G_ "~a: unsupported output format~%") arg))
+                  (alist-cons 'format 'channels result)))
+        (option '(#\h "help") #f #f
+                (lambda args
+                  (show-help)
+                  (exit 0)))
+        (option '(#\V "version") #f #f
+                (lambda args
+                  (show-version-and-exit "guix describe")))))
+
+(define %default-options
+  ;; Alist of default option values.
+  '((format . human)))
+
+(define (show-help)
+  (display (G_ "Usage: guix describe [OPTION]...
+Display information about the channels currently in use.\n"))
+  (display (G_ "
+  -f, --format=FORMAT    display information in the given FORMAT"))
+  (newline)
+  (display (G_ "
+  -h, --help             display this help and exit"))
+  (display (G_ "
+  -V, --version          display version information and exit"))
+  (newline)
+  (show-bug-report-information))
+
+(define (display-package-search-path fmt)
+  "Display GUIX_PACKAGE_PATH, if it is set, according to FMT."
+  (match (getenv "GUIX_PACKAGE_PATH")
+    (#f #t)
+    (string
+     (match fmt
+       ('human
+        (format #t "~%GUIX_PACKAGE_PATH=\"~a\"~%" string))
+       ('channels
+        (format #t (G_ "~%;; warning: GUIX_PACKAGE_PATH=\"~a\"~%")
+                string))))))
+
+(define (display-checkout-info fmt)
+  "Display information about the current checkout according to FMT, a symbol
+denoting the requested format.  Exit if the current directory does not lie
+within a Git checkout."
+  (let* ((program    (car (command-line)))
+         (directory  (catch 'git-error
+                       (lambda ()
+                         (repository-discover (dirname program)))
+                       (lambda (key err)
+                         (leave (G_ "failed to determine origin~%")))))
+         (repository (repository-open directory))
+         (head       (repository-head repository))
+         (commit     (oid->string (reference-target head))))
+    (match fmt
+      ('human
+       (format #t (G_ "Git checkout:~%"))
+       (format #t (G_ "  repository: ~a~%") (dirname directory))
+       (format #t (G_ "  branch: ~a~%") (reference-shorthand head))
+       (format #t (G_ "  commit: ~a~%") commit))
+      ('channels
+       (pretty-print `(list (channel
+                             (name 'guix)
+                             (url ,(dirname directory))
+                             (commit ,commit))))))
+    (display-package-search-path fmt)))
+
+(define (display-profile-info profile fmt)
+  "Display information about PROFILE, a profile as created by (guix channels),
+in the format specified by FMT."
+  (define number
+    (match (profile-generations profile)
+      ((_ ... last) last)))
+
+  (match fmt
+    ('human
+     (display-profile-content profile number))
+    ('channels
+     (pretty-print
+      `(list ,@(map (lambda (entry)
+                      (match (assq 'source (manifest-entry-properties entry))
+                        (('source ('repository ('version 0)
+                                               ('url url)
+                                               ('branch branch)
+                                               ('commit commit)
+                                               _ ...))
+                         `(channel (name ',(string->symbol
+                                            (manifest-entry-name entry)))
+                                   (url ,url)
+                                   (commit ,commit)))
+
+                        ;; Pre-0.15.0 Guix does not provide that information,
+                        ;; so there's not much we can do in that case.
+                        (_ '???)))
+
+                    ;; Show most recently installed packages last.
+                    (reverse
+                     (manifest-entries
+                      (profile-manifest (generation-file-name profile
+                                                              number)))))))))
+  (display-package-search-path fmt))
+
+
+;;;
+;;; Entry point.
+;;;
+
+(define (guix-describe . args)
+  (let* ((opts   (args-fold* args %options
+                             (lambda (opt name arg result)
+                               (leave (G_ "~A: unrecognized option~%")
+                                      name))
+                             cons
+                             %default-options))
+         (format (assq-ref opts 'format)))
+    (with-error-handling
+      (match (current-profile)
+        (#f
+         (display-checkout-info format))
+        (profile
+         (display-profile-info profile format))))))

guix/scripts/pull.scm

@@ -48,7 +48,8 @@
   #:use-module (srfi srfi-37)
   #:use-module (ice-9 match)
   #:use-module (ice-9 vlist)
-  #:export (guix-pull))
+  #:export (display-profile-content
+            guix-pull))
 
 
 ;;;

po/guix/POTFILES.in

@@ -31,6 +31,7 @@ guix/scripts/challenge.scm
 guix/scripts/copy.scm
 guix/scripts/pack.scm
 guix/scripts/weather.scm
+guix/scripts/describe.scm
 guix/gnu-maintenance.scm
 guix/scripts/container.scm
 guix/scripts/container/exec.scm

tests/guix-describe.sh

@@ -0,0 +1,47 @@
+# GNU Guix --- Functional package management for GNU
+# Copyright © 2018 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/>.
+
+#
+# Test 'guix describe'.
+#
+
+guix describe --version
+
+tmpfile="t-guix-describe-$$"
+trap "rm -f $tmpfile" EXIT
+rm -f "$tmpfile"
+
+if [ -d "$abs_top_srcdir/.git" ]
+then
+    # Since we're in a Git checkout, we can at least check that these things
+    # work.
+    guix describe | grep -i "checkout"
+    if git --version > /dev/null 2>&1
+    then
+	result="`guix describe | grep commit: | cut -d : -f 2-`"
+	commit="`git log | head -1 | cut -c 7-`"
+	test "x$result" = "x$commit"
+    fi
+    guix describe -f channels
+    case "`guix describe -f channels | grep url`" in
+	*"(url \"$abs_top_srcdir\")") true;;
+	*) false;;
+    esac
+else
+    exit 77
+fi