;;; env.el --- functions to manipulate environment variables.

;;; Copyright 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, 675 Mass Ave, Cambridge, MA 02139, 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."
  (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: " t) 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)
      (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
