A version that deals with the empty line at the end so that it doesn't spew the result of a bare "export". It also splits on newlines instead of commas, so embedded commas in the envvar value won't be misinterpreted.

tmup

# adapted from https://gist.github.com/simonjbeaumont/4672606
function tmup() {
    local IFS 
    echo -n "Updating to latest tmux environment...";
    export IFS=$'\n';
    for line in $(tmux showenv -t $(tmux display -p "#S"));
    do  
        if [[ "$line" == -* ]]; then
            unset $(echo $line | cut -c2-);
        elif [[ -n "$line" ]] ; then
            export "$line";
        fi  
    done
    echo "Done"
}