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From: merlyn@iwarp.intel.com (Randal Schwartz)
Newsgroups: comp.unix.questions
Subject: Re: How Does One Change Variables That Are Set From A Sub-Shell
Keywords: csh, programs, lunch, variables, set
Message-ID: <5253@omepd.UUCP>
Date: 23 Nov 89 00:51:37 GMT
References: <1989Nov20.190903.27550@wucs1.wustl.edu> <1989Nov21.052026.11968@athena.mit.edu>
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Reply-To: merlyn@iwarp.intel.com (Randal Schwartz)
Organization: Stonehenge; netaccess via Intel, Hillsboro, Oregon, USA
Lines: 35
In-reply-to: jik@athena.mit.edu (Jonathan I. Kamens)

In article <1989Nov21.052026.11968@athena.mit.edu>, jik@athena (Jonathan I. Kamens) writes:
| In article <1989Nov20.190903.27550@wucs1.wustl.edu> flan@ai.wustl.edu (Ian "No
| Excuse" Flanigan) writes:
| >	How does one pass a csh variable back to the old csh -or- grab said
| >	variable from a currently running shell and then feed it back.
| 
|   You cannot pass environment variables or shell variables from a
| sub-shell back to its parent, and you cannot (without reading the
| memory image of the running sub-process in order to find the value of
| the variable) "grab" a variable from a sub-process in order to set its
| value in another process' environment.
| 
|   The only way this would be possible is if the parent and child
| defined some special protocol (using a pipe, socket, whatever) for
| transferring variable names and values between themselves.  None of
| the "standard" shells (sh, csh, tcsh, ksh, bash, whatever) have any
| facility for doing this, at least not that I know of.

However, reaching into my bag of nifty /bin/sh tricks, I could do
something like:

trap '. $HOME/.doit; /bin/rm -f $HOME/.doit' 4
(/my/background/process $$)&

where "/my/background/process" is responsible for putting some
/bin/sh commands, like:

PS1="${PS1L}YOU HAVE MAIL ${PS1R}"

into $HOME/.doit, and sending the shell (as passed by the first arg, a
signal 4).

The detailed implementation is left as an exercise to the reader. :-)

Just another shell wizard,