Display env variable, tmux and zsh over ssh


I have been using zsh within tmux, and found upon reattaching tmux X forwarding wasn’t working. For example when trying to launch gvim I’d get the error:

E233: cannot open display

The problem, a quick google determined, is that each time I ssh into my sever a new $DISPLAY environment variable is set. When I run ’tmux attach’ the new $DISPLAY variable is passed through (see so any new windows within tmux will have the correct environment. However the environment of any existing windows can’t be changed, causing the problem.

The best solution I found was proposed by Alex Teichman here: However I had two problems:

  1. It doesn’t seem to work with zsh rather than bash. I guess this is due to the behaviour of preexec() being different, but I couldn’t quickly work this out from the zsh manual.
  2. It maybe felt slightly inelegant to update $DISPLAY every single time a command is run

My solution is pretty similar. I add the following to ~/.zshrc:

echo $DISPLAY > ~/.display.txt
alias up_disp='export DISPLAY=\`cat ~/.display.txt\`'

This writes the correct $DISPLAY variable to a hidden file when a session is started (i.e. when I connect to the server). When I find forwarding isn’t working, I just run up_disp in that window. Not the perfect solution, but it works ok for me