Add the shift built-in

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

Makefile.am

@@ -54,6 +54,7 @@ MODULES =                                       \
   geesh/built-ins/read.scm                      \
   geesh/built-ins/readonly.scm                  \
   geesh/built-ins/set.scm                       \
+  geesh/built-ins/shift.scm                     \
   geesh/built-ins/true.scm                      \
   geesh/built-ins/unset.scm                     \
   geesh/built-ins/utils.scm                     \

geesh/built-ins.scm

@@ -43,7 +43,7 @@
     ("readonly" . ,(@@ (geesh built-ins readonly) main))
     ("return" . ,undefined)
     ("set" . ,(@@ (geesh built-ins set) main))
-    ("shift" . ,undefined)
+    ("shift" . ,(@@ (geesh built-ins shift) main))
     ("times" . ,undefined)
     ("trap" . ,undefined)
     ("unset" . ,(@@ (geesh built-ins unset) main))))

geesh/built-ins/shift.scm

@@ -0,0 +1,50 @@
+;;; 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 shift)
+  #:use-module (geesh environment)
+  #:use-module (ice-9 match)
+  #:use-module (srfi srfi-1))
+
+;;; Commentary:
+;;;
+;;; The 'shift' utility.
+;;;
+;;; Code:
+
+(define (main . args)
+  (match args
+    (() (main "1"))
+    ((n-string)
+     (let ((n (string->number n-string)))
+       (cond
+        ((and n (exact? n) (>= n 0)
+              (<= n (length (cdr (program-arguments)))))
+         (set-program-arguments
+          (cons (car (program-arguments))
+                (drop (cdr (program-arguments)) n)))
+         EXIT_SUCCESS)
+        (else
+         (format (current-error-port)
+                 "~a: shift: Invalid option ~s.~%"
+                 (car (program-arguments)) n-string)
+         EXIT_FAILURE))))
+    (_ (format (current-error-port)
+               "~a: shift: Invalid options ~s.~%"
+               (car (program-arguments)) args)
+       EXIT_FAILURE)))