Here's something which takes a different twist on
/etc/os-release, which has a standardized format based around setting shell variables, meaning you can
source it and use them. First have a look at the actual file to get the idea, then consider:
case $VERSION_ID in
echo "Raspbian 7 is wheezy."
echo "Raspbian 8 is jessie."
echo "Raspbian 9 is stretch."
echo "This is $NAME, not Raspbian."
It doesn't include anything about the kernel for which parsing
uname -r (or
/proc/version) is more appropriate. Also, it could be made more robust by actually checking
$NAME to make sure this isn't some other Debian/Raspbian derived distro using the same version numbering (although what you are doing will likely work with them anyway as they are parallel, which is why there's a Debian 6 but no Raspbian 6).
BTW stay away from Raspbian 9, it is still considered "testing".