POSTIN

/bin/sh

if [ ! -f /etc/shells ] ; then
    echo "/usr/bin/tmux" > /etc/shells
else
    grep -q "^/usr/bin/tmux$" /etc/shells || echo "/usr/bin/tmux" >> /etc/shells
fi

POSTUN

/bin/sh

if [ $1 -eq 0 ] && [ -f /etc/shells ]; then
    sed -i '\!^/usr/bin/tmux$!d' /etc/shells
fi