Re: reading an environment variable from screen

[Thomas Köhler]

Hi Michael,

On Wed, Dec 16, 2020 at 03:08:45PM +0000, Michael Grant <mgrant@grant.org> 
wrote:
> When I reattach to screen, the SSH_AUTH_SOCK is wrong.
> 
> I often ssh in and run screen simultaneously on a laptop and desktop.  So I 
> can’t use the trick of symlinking something to /tmp/ssh-xxxxxxxxxx.
> 
> Is there some way to interrogate screen to tell me what the contents of an 
> environment variable are?
> 
> Running this in a shell inside of screen:
> 
>        screen -S $STY -X colon 'exec |echo $SSH_AUTH_SOCK\r'
> 
> gives me the info I’m looking for, but it pushes it out to the display.  I’m 
> so close!  How can I capture this output?
> 
> My goal here is to create a sort of ssh/scp pre-command as a shell alias that 
> probes screen each time and sets the SSH_AUTH_SOCK variable correctly.

I solved this whole issue as follows:

- during login, my zshrc runs this code:
  touch $HOME/.ssh/agent_var ; chmod 600 $HOME/.ssh/agent_var
  echo "export SSH_AGENT_PID=$SSH_AGENT_PID" > $HOME/.ssh/agent_var
  echo "export SSH_AUTH_SOCK=$SSH_AUTH_SOCK" >> $HOME/.ssh/agent_var
- in my session, zsh's precmd() (that would maybe be
  prompt_command in bash) can now ". $HOME/.ssh/agent_var" so the
  variables are always set to something that works.
- I actually almost never worry about $SSH_AUTH_SOCK again. Only
  case where this fails would be in the situation that I
  a) login to the machine and reattach screen
  b) login to the same machine a second time and logout of this
     second session
  c) now this machine has $HOME/.ssh/agent_var setup wrongly.
  d) Fix is to logout of the first session and login again, then
     reattach screen
  I only end up in such a situation when actually testing some
  changes to my shell configuration that requires me to login
  again, as I otherwise work from my existing screen session, so
  it's not so bad.

Ciao,
Thomas

-- 
 Thomas Köhler       Email:       jean-luc@picard.franken.de
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