idempotent hosts file configuration with sed

hosts_sed

ip4="$( dig +short "$( hostname -f )" | tail -n 1 )"

sed     -nEi                                                                                                                                            \
        -e      '/^[[:space:]]*'"$ip4"'[[:space:]].*|[[:space:]0-9.]*('"$(hostname -s)|$(hostname -f | sed 's/\./\\./g')"')([[:space:]#]|$).*/ {'       \
        -e      '  s/.*/'"${ip4} $(hostname -f) $(hostname -s)/"                                                                                        \
        -e      '  p'                                                                                                                                   \
        -e      '  :noappend'                                                                                                                           \
        -e      '  n'                                                                                                                                   \
        -e      '  /[[:space:]]*'"$ip4"'[[:space:]].*$/n'                                                                                               \
        -e      '  p'                                                                                                                                   \
        -e      '  b noappend'                                                                                                                          \
        -e      '}'                                                                                                                                     \
        -e      'p'                                                                                                                                     \
        -e      '$a\
'"${ip4} $( hostname -f ) $( hostname -s )"                                                                                                             \
        /etc/hosts