installer: Add new utils.

* gnu/installer/utils.scm (nearest-exact-integer): New exported procedure,
(read-percentage): ditto,
(run-shell-command): ditto.

gnu/installer/utils.scm

@@ -17,10 +17,16 @@
 ;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
 
 (define-module (gnu installer utils)
+  #:use-module (guix utils)
+  #:use-module (guix build utils)
   #:use-module (ice-9 rdelim)
+  #:use-module (ice-9 regex)
   #:use-module (ice-9 textual-ports)
   #:export (read-lines
-            read-all))
+            read-all
+            nearest-exact-integer
+            read-percentage
+            run-shell-command))
 
 (define* (read-lines #:optional (port (current-input-port)))
   "Read lines from PORT and return them as a list."
@@ -35,3 +41,23 @@
   "Return the content of the given FILE as a string."
   (call-with-input-file file
     get-string-all))
+
+(define (nearest-exact-integer x)
+  "Given a real number X, return the nearest exact integer, with ties going to
+the nearest exact even integer."
+  (inexact->exact (round x)))
+
+(define (read-percentage percentage)
+  "Read PERCENTAGE string and return the corresponding percentage as a
+number. If no percentage is found, return #f"
+  (let ((result (string-match "^([0-9]+)%$" percentage)))
+    (and result
+         (string->number (match:substring result 1)))))
+
+(define (run-shell-command command)
+  (call-with-temporary-output-file
+   (lambda (file port)
+     (format port "~a~%" command)
+     ;; (format port "exit~%")
+     (close port)
+     (invoke "bash" "--init-file" file))))