Add the umask built-in

* geesh/built-ins/umask.scm: New file.
* Makefile.am: Add it.
* geesh/built-ins.scm (*built-ins*): Add umask.

Makefile.am

@@ -56,6 +56,7 @@ MODULES =                                       \
   geesh/built-ins/set.scm                       \
   geesh/built-ins/shift.scm                     \
   geesh/built-ins/true.scm                      \
+  geesh/built-ins/umask.scm                     \
   geesh/built-ins/unset.scm                     \
   geesh/built-ins/utils.scm                     \
   geesh/built-ins.scm                           \

geesh/built-ins.scm

@@ -67,7 +67,7 @@
     ("pwd" . ,(@@ (geesh built-ins pwd) main))
     ("read" . ,(@@ (geesh built-ins read) main))
     ("true" . ,(@@ (geesh built-ins true) main))
-    ("umask" . ,undefined)
+    ("umask" . ,(@@ (geesh built-ins umask) main))
     ("unalias" . ,undefined)
     ("wait" . ,undefined)
     ;; Other built-ins.

geesh/built-ins/umask.scm

@@ -0,0 +1,44 @@
+;;; The Geesh Shell Interpreter
+;;; Copyright 2018 Timothy Sample <samplet@ngyro.com>
+;;;
+;;; This file is part of Geesh.
+;;;
+;;; Geesh 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.
+;;;
+;;; Geesh 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 Geesh.  If not, see <http://www.gnu.org/licenses/>.
+
+(define-module (geesh built-ins umask)
+  #:use-module (ice-9 match))
+
+;;; Commentary:
+;;;
+;;; The 'umask' utility.
+;;;
+;;; Code:
+
+(define (main . args)
+  (match args
+    ((mask)
+     (let ((n (string->number mask 8)))
+       (cond
+        ((and n (integer? n) (>= n 0) (< n 512))
+         (umask n)
+         EXIT_SUCCESS)
+        (else
+         (format (current-error-port)
+                 "~a: umask: Invalid option ~s.~%"
+                 (car (program-arguments)) mask)
+         EXIT_FAILURE))))
+    (_ (format (current-error-port)
+               "~a: umask: Invalid options ~s.~%"
+               (car (program-arguments)) args)
+       EXIT_FAILURE)))