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diff --git a/src/mcron/environment.scm b/src/mcron/environment.scm
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+;;   Copyright (C) 2015, 2016 Mathieu Lirzin
+;;   Copyright (C) 2003 Dale Mellor
+;; 
+;;   This file is part of GNU mcron.
+;;
+;;   GNU mcron 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.
+;;
+;;   GNU mcron 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 mcron.  If not, see <http://www.gnu.org/licenses/>.
+
+
+
+;; This file defines the variable current-environment-mods, and the procedures
+;; append-environment-mods (which is available to user configuration files),
+;; clear-environment-mods and modify-environment. The idea is that the
+;; current-environment-mods is a list of pairs of environment names and values,
+;; and represents the cumulated environment settings in a configuration
+;; file. When a job definition is seen in a configuration file, the
+;; current-environment-mods are copied into the internal job description, and
+;; when the job actually runs these environment modifications are applied to
+;; the UNIX environment in which the job runs.
+
+
+
+
+(define-module (mcron environment)
+  #:export (modify-environment
+            clear-environment-mods
+            append-environment-mods
+            get-current-environment-mods-copy))
+            
+
+
+;; As we parse configuration files, we build up an alist of environment
+;; variables here.
+
+(define current-environment-mods '())
+
+
+
+;; Each time a job is added to the system, we take a snapshot of the current
+;; set of environment modifiers.
+
+(define (get-current-environment-mods-copy)
+  (list-copy current-environment-mods))
+
+
+
+;; When we start to parse a new configuration file, we want to start with a
+;; fresh environment (actually an umodified version of the pervading mcron
+;; environment).
+
+(define (clear-environment-mods)
+  (set! current-environment-mods '()))
+
+
+
+;; Procedure to add another environment setting to the alist above. This is
+;; used both implicitly by the Vixie parser, and can be used directly by users
+;; in scheme configuration files. The return value is purely for the
+;; convenience of the parse-vixie-environment in the vixie-specification module
+;; (yuk).
+
+(define (append-environment-mods name value)
+  (set! current-environment-mods (append current-environment-mods
+                                         (list (cons name value))))
+  #t)
+
+(define (modify-environment env passwd-entry)
+  "Modify the environment (in the UNIX sense) by setting the variables from
+ENV and some default ones which are modulated by PASSWD-ENTRY.  \"LOGNAME\"
+and \"USER\" environment variables can't be overided by ENV.  ENV must be an
+alist which associate environment variables to their value.  PASSWD-ENTRY must
+be an object representing user information which corresponds to a valid entry
+in /etc/passwd.  The return value is not specified."
+  (for-each (lambda (pair) (setenv (car pair) (cdr pair)))
+            (let ((home-dir  (passwd:dir passwd-entry))
+                  (user-name (passwd:name passwd-entry)))
+              (append
+               ;; Default environment variables which can be overided by ENV.
+               `(("HOME"    . ,home-dir)
+                 ("CWD"     . ,home-dir)
+                 ("SHELL"   . ,(passwd:shell passwd-entry))
+                 ("TERM"    . #f)
+                 ("TERMCAP" . #f))
+               env
+               ;; Environment variables with imposed values.
+               `(("LOGNAME" . ,user-name)
+                 ("USER"    . ,user-name))))))