autotmux.fish

function autotmux --on-variable TMUX_SESSION_NAME
  if test -n "$TMUX_SESSION_NAME" #only if set
    if test -z $TMUX #not if in TMUX
      tmux new-session -t $TMUX_SESSION_NAME; or tmux new-session -s $TMUX_SESSION_NAME
    end
  end
end

I use fish, tmux, direnv and autojump, so this lets me:

~> j proj

and always get put into a named Tmux session - linked to an existing one if there is one.