guix build: Add '--quiet'.

Fixes <http://bugs.gnu.org/19772>.
Reported by Andrei Osipov <andrspv@gmail.com>.

* guix/scripts/build.scm (show-help, %options): Add --quiet.
(guix-build): Parameterize 'current-build-output-port' accordingly.
* doc/guix.texi (Invoking guix build): Use it in example.
(Additional Build Options): Document it.

doc/guix.texi

@@ -3836,7 +3836,7 @@ guix build emacs guile
 Similarly, the following command builds all the available packages:
 
 @example
-guix build --keep-going \
+guix build --quiet --keep-going \
   `guix package -A | cut -f1,2 --output-delimiter=@@`
 @end example
 
@@ -4070,6 +4070,12 @@ build}.
 
 @table @code
 
+@item --quiet
+@itemx -q
+Build quietly, without displaying the build log.  Upon completion, the
+build log is kept in @file{/var} (or similar) and can always be
+retrieved using the @option{--log-file} option.
+
 @item --file=@var{file}
 @itemx -f @var{file}
 

guix/scripts/build.scm

@@ -466,6 +466,8 @@ Build the given PACKAGE-OR-DERIVATION and return their output paths.\n"))
   (display (_ "
   -r, --root=FILE        make FILE a symlink to the result, and register it
                          as a garbage collector root"))
+  (display (_ "
+  -q, --quiet            do not show the build log"))
   (display (_ "
       --log-file         return the log file names for the given derivations"))
   (newline)
@@ -534,6 +536,9 @@ must be one of 'package', 'all', or 'transitive'~%")
          (option '(#\r "root") #t #f
                  (lambda (opt name arg result)
                    (alist-cons 'gc-root arg result)))
+         (option '(#\q "quiet") #f #f
+                 (lambda (opt name arg result)
+                   (alist-cons 'quiet? #t result)))
          (option '("log-file") #f #f
                  (lambda (opt name arg result)
                    (alist-cons 'log-file? #t result)))
@@ -638,6 +643,9 @@ needed."
     (parse-command-line args %options
                         (list %default-options)))
 
+  (define quiet?
+    (assoc-ref opts 'quiet?))
+
   (with-error-handling
     ;; Ask for absolute file names so that .drv file names passed from the
     ;; user to 'read-derivation' are absolute when it returns.
@@ -646,6 +654,9 @@ needed."
         ;; Set the build options before we do anything else.
         (set-build-options-from-command-line store opts)
 
+        (parameterize ((current-build-output-port (if quiet?
+                                                      (%make-void-port "w")
+                                                      (current-error-port))))
           (let* ((mode  (assoc-ref opts 'build-mode))
                  (drv   (options->derivations store opts))
                  (urls  (map (cut string-append <> "/log")
@@ -667,7 +678,8 @@ needed."
 
             (unless (assoc-ref opts 'log-file?)
               (show-what-to-build store drv
-                                #:use-substitutes? (assoc-ref opts 'substitutes?)
+                                  #:use-substitutes?
+                                  (assoc-ref opts 'substitutes?)
                                   #:dry-run? (assoc-ref opts 'dry-run?)
                                   #:mode mode))
 
@@ -689,4 +701,4 @@ needed."
                                          (map cdr
                                               (derivation->output-paths drv)))
                                        drv)
-                                roots)))))))))
+                                  roots))))))))))