guix system: Add "describe" action.

* guix/scripts/system.scm (show-help): Add "describe".
(process-command): Handle it.
(guix-system): Likewise.
* doc/guix.texi (Invoking guix system): Document it.

doc/guix.texi

@@ -26301,6 +26301,10 @@ bootloader boot menu:
 @table @code
+@item describe
+Describe the current system generation: its file name, the kernel and
+bootloader used, etc., as well as provenance information when available.
+
 @item list-generations
 List a summary of each generation of the operating system available on
 disk, in a human-readable way.  This is similar to the

guix/scripts/system.scm

@@ -921,6 +921,8 @@ (define (show-help)
    reconfigure      switch to a new operating system configuration\n"))
   (display (G_ "\
    roll-back        switch to the previous operating system configuration\n"))
+  (display (G_ "\
+   describe         describe the current system\n"))
   (display (G_ "\
    list-generations list the system generations\n"))
   (display (G_ "\
@@ -1196,6 +1198,12 @@ (define (process-command command args opts)
                       ((pattern) pattern)
                       (x (leave (G_ "wrong number of arguments~%"))))))
        (list-generations pattern)))
+    ((describe)
+     (match (generation-number %system-profile)
+       (0
+        (error (G_ "no system generation, nothing to describe~%")))
+       (generation
+        (display-system-generation generation))))
     ((search)
      (apply (resolve-subcommand "search") args))
     ;; The following commands need to use the store, but they do not need an
@@ -1235,7 +1243,8 @@ (define (parse-sub-command arg result)
           (case action
             ((build container vm vm-image disk-image reconfigure init
               extension-graph shepherd-graph
-              list-generations delete-generations roll-back
+              list-generations describe
+              delete-generations roll-back
               switch-generation search docker-image)
              (alist-cons 'action action result))
             (else (leave (G_ "~a: unknown action~%") action))))))