How can I get distribution name and version number in a simple shell script?

I'm working on a simple bash script that should be able to run on Ubuntu and CentOS distributions (support for Debian and Fedora/RHEL would be a plus) and I need to know the name and version of the distribution the script is running (in order to trigger specific actions, for instance the creation of repositories). So far what I've got is this:

OS=$(awk '/DISTRIB_ID=/' /etc/*-release | sed 's/DISTRIB_ID=//' | tr '[:upper:]' '[:lower:]')
ARCH=$(uname -m | sed 's/x86_//;s/i[3-6]86/32/')
VERSION=$(awk '/DISTRIB_RELEASE=/' /etc/*-release | sed 's/DISTRIB_RELEASE=//' | sed 's/[.]0/./')

if [ -z "$OS" ]; then
    OS=$(awk '{print $1}' /etc/*-release | tr '[:upper:]' '[:lower:]')
fi

if [ -z "$VERSION" ]; then
    VERSION=$(awk '{print $3}' /etc/*-release)
fi

echo $OS
echo $ARCH
echo $VERSION

This seems to work, returning ubuntu or centos (I haven't tried others) as the release name however, I have a feeling that there must be an easier, more reliable way of finding this out... No?

It doesn't work for RedHat. /etc/redhat-release contains : Redhat Linux Entreprise release 5.5

So, the version is not the third word, you'd better use :

OS_MAJOR_VERSION=`sed -rn 's/.*([0-9])\.[0-9].*/\1/p' /etc/redhat-release`
OS_MINOR_VERSION=`sed -rn 's/.*[0-9].([0-9]).*/\1/p' /etc/redhat-release`
echo "RedHat/CentOS $OS_MAJOR_VERSION.$OS_MINOR_VERSION"

ANSWER:-

Most recent distributions have a tool called lsb_release. Your /etc/*-release will be using/etc/lsb-release anyway, so if that file is there, running lsb_release should work too.

I think uname to get ARCH is still the best way.

e.g.
OS=$(lsb_release -si)
ARCH=$(uname -m | sed 's/x86_//;s/i[3-6]86/32/')
VER=$(lsb_release -sr)

Or you could just source /etc/lsb-release:
. /etc/lsb-release
OS=$DISTRIB_ID
ARCH=$(uname -m | sed 's/x86_//;s/i[3-6]86/32/')
VER=$DISTRIB_RELEASE

If you have to be compatible with older distributions, there is no single file you can rely on. Either fall back to the output from uname, e.g.
OS=$(uname -s)
ARCH=$(uname -m)
VER=$(uname -r)

or handle each distribution separately:
if [ -f /etc/debian_version ]; then
    OS=Debian  # XXX or Ubuntu??
    VER=$(cat /etc/debian_version)
elif [ -f /etc/redhat-release ]; then
    ...

Of course, you can combine all this:
ARCH=$(uname -m | sed 's/x86_//;s/i[3-6]86/32/')

if [ -f /etc/lsb-release ]; then
    . /etc/lsb-release
    OS=$DISTRIB_ID
    VER=$DISTRIB_RELEASE
elif [ -f /etc/debian_version ]; then
    OS=Debian  # XXX or Ubuntu??
    VER=$(cat /etc/debian_version)
elif [ -f /etc/redhat-release ]; then
    # TODO add code for Red Hat and CentOS here
    ...
else
    OS=$(uname -s)
    VER=$(uname -r)
fi


Finally, your ARCH obviously only handles Intel systems. I'd either call it BITS like this:
case $(uname -m) in
x86_64)
    BITS=64
    ;;
i*86)
    BITS=32
    ;;
*)
    BITS=?
    ;;
esac

Or change ARCH to be the more common, yet unambiguous versions: x86 and x64 or similar:
case $(uname -m) in
x86_64)
    ARCH=x64  # or AMD64 or Intel64 or whatever
    ;;
i*86)
    ARCH=x86  # or IA32 or Intel32 or whatever
    ;;
*)
    # leave ARCH as-is
    ;;
esac

but of course that's up to you.