forwarding ssh and X through screen

I have an update to my [previous article]{screen-ssh-agent} on forwarding [ssh-agent]{tag/ssh} through [screen]{tag/screen}. I’ve since switched to [zsh]{tag/zsh} and am now forwarding the X DISPLAY environment variable through to the screen shell.

You can grab my ~/.zsh.d/S51_screen, ~/.zsh.d/S60_prompt, and ~/.screenrc or read below.

( I assume you already have ssh-agent working; see this if you need help )

Here is the zsh setup; note that I use HOSTNAME to differentiate between multiple hosts that share the same /home/ directory mounted over NFS.

    # need the host name set sometimes
    [ -z "$HOSTNAME" ] && export HOSTNAME=$(hostname)

    # preserve the X environment variables
    store_display() {
            export | grep '\(DISPLAY\|XAUTHORITY\)=' > ~/.display.${HOSTNAME}
    }

    # read out the X environment variables
    update_display() {
            [ -r ~/.display.${HOSTNAME} ] && source ~/.display.${HOSTNAME}
    }

    # WINDOW is set when we are in a screen session
    if [ -n "$WINDOW" ] ; then 
            # update the display variables right away
            update_display

            # setup the preexec function to update the variables before each command
            preexec () {
                    update_display
            }
    fi

    # this will reset the ssh-auth-sock link and screen display file before we run screen
    _screen_prep() {
            if [ "$SSH_AUTH_SOCK" != "$HOME/.screen/ssh-auth-sock.$HOSTNAME" ] ; then
                    ln -fs "$SSH_AUTH_SOCK" "$HOME/.screen/ssh-auth-sock.$HOSTNAME"
            fi
            store_display
    }
    alias screen='_screen_prep ; screen'

We also teach screen to override the SSH_AUTH_SOCK variable when starting up.

    unsetenv SSH_AUTH_SOCK
    setenv SSH_AUTH_SOCK "$HOME/.screen/ssh-auth-sock.$HOSTNAME"

And that’s it.