Re: setenv and find-file

[Peter Dyballa]

Am 17.05.2013 um 23:07 schrieb Doug Lewan:

> I see a problem when M-x find-file is expanding ${VARIABLE} but not taking it 
> from the newly defined environment variable. Everything else picks it up (I 
> presume just via fork/exec).
> 
> I hope this script is clearer.
> 
> In shell:
>    $ echo ${VARIABLE}
>    /a/very/long/path
>    $ emacs &
> Inside emacs:
>    C-x C-f ${VARIABLE}/to/work                <== Finds 
> "/a/very/long/path/to/work".
>    (setenv "VARIABLE" "/shorter/path")
>        => "/shorter/path"
>    M-! echo ${VARIABLE}
>    "/shorter/path"
>    M-x shell
> In shell inside emacs:
>    $ echo ${VARIABLE}
>    "/shorter/path"
> Back in emacs:
>    C-x C-f ${VARIABLE}/to/Grandmothers/house
> This finds the file "/a/very/long/path/to/Grandmothers/house", where I sort 
> of expect it to find "/shorter/path/to/Grandmothers/house".

No, this cannot work in GNU Emacs with a regular *shell* buffer – maybe it 
works in VIM?

GNU Emacs is a host (or a PC) that allows other programmes to run inside it. 
Every variable the external (whatsoever: Korn, Bourne, C, …) shell interpreter 
is setting, is a private variable of and only inside this shell interpreter. 
And since it's just a shell interpreter it has no access to the variables of 
GNU Emacs. No shell function exists for this purpose.


The situation is a bit different when some shell variables are defined in a 
shell from which GNU Emacs is launched. These environment variables also exist 
in the GNU Emacs process, they are inherited by it, so they can be accessed:

        (getenv "PATH")

With this Elisp statement GNU Emacs has read an environment variable which the 
shell inside its *shell* buffer also has (the shell's $PATH can be different, 
because shell interpreter init files can set their own values), but GNU Emacs 
cannot use the shell interpreter's private value because there is no 
specialised function (yet) that creates automatically a GNU Emacs variable when 
you try to access either a (private) shell or an environment variable. In your 
init file you would need to create "normal" GNU Emacs variables and set their 
values with what you get from reading environment variables (not: private own 
shell variables).


Another different situation is when you use eshell instead of an external shell 
interpreter. This in Emacs Lisp written interpreter has access to all of GNU 
Emacs variables and functions and vice versa – presumingly! I'm not using it on 
a regular basis…


Since GNU Emacs and inferior processes are cleanly separated processes, there 
is no way that both can easily share variables and their values. They would 
need "neutral" external locations like regular files, pipes, sockets, FIFOs, 
LIFOs, shared memory areas, servers or services for data exchange (and extra 
programme code to make use of these external things). Since eshell and GNU 
Emacs are not separated they probably can share what you are thinking of. But 
used eshell users should comment on my opinion!

--
Greetings

  Pete     === -Q
             ==<__/% >>
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