How to auto-update SSH agent environment variables when attaching to existing tmux sessions?

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There’s an excellent gist by Martijn Vermaat, which addresses your problem in great depth, although it is intended for screen users, so I’m adjusting it for tmux here.

To summarize:

  1. create ~/.ssh/rc if it doesn’t exist yet, and add the following content:

    #!/bin/bash

# Fix SSH auth socket location so agent forwarding works with tmux.
if test "$SSH_AUTH_SOCK" ; then
  ln -sf $SSH_AUTH_SOCK ~/.ssh/ssh_auth_sock
fi

  2. Make it work in tmux, add this to your ~/.tmux.conf:

    # fix ssh agent when tmux is detached
setenv -g SSH_AUTH_SOCK $HOME/.ssh/ssh_auth_sock

Extra work is required if you want to enable X11 forwarding, see the gist.

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