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+;;; env.el --- functions to manipulate environment variables.
+
+;; Copyright (C) 1991, 1994 Free Software Foundation, Inc.
+
+;; Maintainer: FSF
+;; Keywords: processes, unix
+
+;; This file is part of GNU Emacs.
+
+;; GNU Emacs 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 2, or (at your option)
+;; any later version.
+
+;; GNU Emacs 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 GNU Emacs; see the file COPYING.  If not, write to the
+;; Free Software Foundation, Inc., 59 Temple Place - Suite 330,
+;; Boston, MA 02111-1307, USA.
+
+;;; Commentary:
+
+;; UNIX processes inherit a list of name-to-string associations from their
+;; parents called their `environment'; these are commonly used to control
+;; program options.  This package permits you to set environment variables
+;; to be passed to any sub-process run under Emacs.
+
+;;; Code:
+
+;; History list for environment variable names.
+(defvar read-envvar-name-history nil)
+
+(defun read-envvar-name (prompt &optional mustmatch)
+  "Read environment variable name, prompting with PROMPT.
+Optional second arg MUSTMATCH, if non-nil, means require existing envvar name.
+If it is also not t, RET does not exit if it does non-null completion."
+  (completing-read prompt
+                  (mapcar (function
+                           (lambda (enventry)
+                             (list (substring enventry 0
+                                              (string-match "=" enventry)))))
+                          process-environment)
+                  nil mustmatch nil 'read-envvar-name-history))
+
+;; History list for VALUE argument to setenv.
+(defvar setenv-history nil)
+
+;;;###autoload
+(defun setenv (variable &optional value unset)
+  "Set the value of the environment variable named VARIABLE to VALUE.
+VARIABLE should be a string.  VALUE is optional; if not provided or is
+`nil', the environment variable VARIABLE will be removed.  
+
+Interactively, a prefix argument means to unset the variable.
+Interactively, the current value (if any) of the variable
+appears at the front of the history list when you type in the new value.
+
+This function works by modifying `process-environment'."
+  (interactive
+   (if current-prefix-arg
+       (list (read-envvar-name "Clear environment variable: " 'exact) nil t)
+     (let* ((var (read-envvar-name "Set environment variable: " nil))
+           (oldval (getenv var))
+           newval
+           oldhist)
+       ;; Don't put the current value on the history
+       ;; if it is already there.
+       (if (equal oldval (car setenv-history))
+          (setq oldval nil))
+       ;; Now if OLDVAL is non-nil, we should add it to the history.
+       (if oldval
+          (setq setenv-history (cons oldval setenv-history)))
+       (setq oldhist setenv-history)
+       (setq newval (read-from-minibuffer (format "Set %s to value: " var)
+                                         nil nil nil 'setenv-history))
+       ;; If we added the current value to the history, remove it.
+       ;; Note that read-from-minibuffer may have added the new value.
+       ;; Don't remove that!
+       (if oldval
+          (if (eq oldhist setenv-history)
+              (setq setenv-history (cdr setenv-history))
+            (setcdr setenv-history (cdr (cdr setenv-history)))))
+       ;; Here finally we specify the args to give call setenv with.
+       (list var newval))))
+  (if unset (setq value nil))
+  (if (string-match "=" variable)
+      (error "Environment variable name `%s' contains `='" variable)
+    (let ((pattern (concat "\\`" (regexp-quote (concat variable "="))))
+         (case-fold-search nil)
+         (scan process-environment)
+         found)
+      (if (string-equal "TZ" variable)
+         (set-time-zone-rule value))
+      (while scan
+       (cond ((string-match pattern (car scan))
+              (setq found t)
+              (if (eq nil value)
+                  (setq process-environment (delq (car scan) process-environment))
+                (setcar scan (concat variable "=" value)))
+              (setq scan nil)))
+       (setq scan (cdr scan)))
+      (or found
+         (if value
+             (setq process-environment
+                   (cons (concat variable "=" value)
+                         process-environment)))))))
+
+(provide 'env)
+
+;;; env.el ends here