Inherited Environment and Multi-attach

[John Davidorff Pell]

Re: screen and ssh agents + agent forwarding (+ $DISPLAY)
I'm replying to this old thread because it is something that I have  
been working on for a while and have a good solution for.

Here's how I do things:

A)

I have a script [1] that starts screen in the background somewhere  
(detached). This runs when I log in to my GUI (or when I run it  
arbitrarily). :-)

My .screenrc starts at least one window, preferably one that I will  
not ever close, like a tail on my console.log. This ensures that  
screen doesn't ever exit. :-)

** Would it be a good idea to add an option to screen so that it does  
*not* exit after the last command completes? **

B)

When I open a terminal window (xterm, Eterm, &c.) I have it set to  
run an attach script [2] instead of a shell.

This script saves various variables to a file in my temp directory (I  
use ~/.temp, I don't like storing this stuff in /tmp). It also adds  
the path to this new file to the end of a file in ~/.screen, which I  
call my stack.

It then attaches to screen.

Once screen exits (detaches), it removes the path to the environment  
file from the stack. It does NOT just remove the last file in the  
stack, but specifically the file that was created during this  
particular attach. This creates a race condition, but is unlikely  
ever to fail. (The race condition is if I log in and accesses the  
stack at the same time, possibley damaging it by concurrent writes;  
also happens with two simultaneous logins, though less so because I  
just append to the file instead of overwriting it.)

C)

Another script [3] is sourced before every prompt (PROMPT_COMMAND).  
This reads the most-recent environment file (saved by [2]), as  
determined by the last line in the stack file.


D) That's it!





This means that if I log in at home, and leave my screen attached,  
then log in at work, my work variables (like DISPLAY or  
SSH_AUTH_SOCK) are set properly. When I log out from work, my home  
variables are at the top of the stack, and so they also get restored  
as soon as I press enter at home.

Even better: If I log in at home, then log in at work, then log in on  
my laptop, then log out from work, my home variables will be properly  
restored when I get home (after I have logged out from my laptop).

BUG: If I log in at home, then at work, then on my laptop, then go  
home my laptop variables are still set. If I then logout from my  
laptop, my work variables become set.
Solution: Re-attach at home, and it becomes the most recent session  
on the stack. :-)



Comments and Suggestions wanted!!

JP


P.S. My scripts make assumptions about the system they're on.  
Portability fixes definitely welcome!


[1] http://homepage.mac.com/johnpell/2005March26/Screen.sh
[2] http://homepage.mac.com/johnpell/2005March26/screenAppAttach.sh
[3] http://homepage.mac.com/johnpell/2005March26/screenenv.sh