Path: utzoo!utgpu!attcan!uunet!lll-winken!lll-tis!ames!mailrus!cornell!uw-beaver!mit-eddie!bbn!spdcc!dyer
From: dyer@spdcc.COM (Steve Dyer)
Newsgroups: comp.unix.xenix
Subject: Re: Changing the environment in sh
Keywords: sh export shell
Message-ID: <1778@spdcc.COM>
Date: 2 Sep 88 17:37:15 GMT
References: <59@csnz.nz>
Reply-To: dyer@spdcc.COM (Steve Dyer)
Organization: S.P. Dyer Computer Consulting, Cambridge MA
Lines: 23

There is no way to change a shell environment variable directly
from a child process, because the environment is an attribute of
each process, and is not a global resource.  A child process inherits
the environment of its parent (at least if it's invoked from a shell
or via the system call execve), but it then owns a separate copy of
the environment in its own address space--you can change things all
day and it won't make any different to the parent process and any siblings
which might be around.

The only way to have a subprocess change the parent's environment
is to set up some form of convention to communicate this.  For
example, if you had a program which wrote out environment variable
assignments (in some shell's syntax) to the standard output, you
could invoke the command from the shell in the form:

eval `command`

The parent shell would be reading the shell variable reassignments
and changing them in its own context.  If your shell supported it
(csh, ksh), you could make this more convenient by using aliases.
-- 
Steve Dyer
dyer@harvard.harvard.edu
dyer@spdcc.COM aka {harvard,husc6,linus,ima,bbn,m2c,mipseast}!spdcc!dyer