I always want to use tmux. So I put the following in my bashrc (just before the functions).
# If not running interactively, do not do anything # If you don't do this, your (graphical) login sessions will hang [[ $- != *i* ]] && return # If we are not yet in a screen session if [[ $TERM != screen* ]]; then # Start tmux if there is no panicfile and tmux actually exists. [ ! -f /tmp/panic -a -x /usr/bin/tmux ] && exec tmux fi