Add the exec built-in

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

Makefile.am

@@ -48,6 +48,7 @@ MODULES =                                       \
   geesh/built-ins/dot.scm                       \
   geesh/built-ins/echo.scm                      \
   geesh/built-ins/eval.scm                      \
+  geesh/built-ins/exec.scm                      \
   geesh/built-ins/export.scm                    \
   geesh/built-ins/false.scm                     \
   geesh/built-ins/pwd.scm                       \

geesh/built-ins.scm

@@ -37,7 +37,7 @@
     ("break" . ,(@@ (geesh built-ins break) main))
     ("continue" . ,(@@ (geesh built-ins continue) main))
     ("eval" . ,(@@ (geesh built-ins eval) main))
-    ("exec" . ,undefined)
+    ("exec" . ,(@@ (geesh built-ins exec) main))
     ("exit" . ,undefined)
     ("export" . ,(@@ (geesh built-ins export) main))
     ("readonly" . ,(@@ (geesh built-ins readonly) main))

geesh/built-ins/exec.scm

@@ -0,0 +1,49 @@
+;;; 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 exec)
+  #:use-module (geesh environment)
+  #:use-module (ice-9 match)
+  #:use-module (srfi srfi-1))
+
+;;; Commentary:
+;;;
+;;; The 'exec' utility.
+;;;
+;;; Code:
+
+(define (main . args)
+  (match args
+    ((name . args)
+     (flush-all-ports)
+     (with-environ (get-environ)
+       (lambda ()
+         (catch 'system-error
+           (lambda ()
+             (apply execlp name name args)
+             EXIT_SUCCESS)
+           (lambda args
+             (format (current-error-port)
+                     "~a: exec: ~a: ~a.~%"
+                     (car (program-arguments)) name
+                     (strerror (system-error-errno args)))
+             EXIT_FAILURE)))))
+    (_ (format (current-error-port)
+               "~a: exec: Invalid options ~s.~%"
+               (car (program-arguments)) args)
+       EXIT_FAILURE)))