A snippet to make sure you are always running a tmux session in your terminal. If you have an detached session, this will attach to it, otherwise, if there are no sessions or only ones which are attached, a new session will be started. Insert into your .bashrc or .bash_profile at the top to call every time you open a terminal session.

semper-tmux.sh

#############################################
# Attach tmux to the first detached session 
#############################################
 
if [[ -z $TMUX ]]; then
    TMUX_SESSIONS=`tmux list-sessions -F "#{session_name}ABC#{?session_attached,attached,not_attached}" 2>&1`
    TMUX_FOUND=0
    for i in $TMUX_SESSIONS
    do
        IS_ATTACHED=`echo $i | awk '{split($0,array,"ABC"); print array[2]}'`
        if [[ $IS_ATTACHED == "not_attached" ]]; then
            TMUX_FOUND=1
            tmux attach -t `echo $i | awk '{split($0,array,"ABC"); print array[1]}'`
            break
        fi
    done
    
    if [[ $TMUX_FOUND -eq 0 ]];then
        tmux new-session
    fi
fi