What I want is a quick, reliable command to ascertain that the Linux I'm running in is Debian-based. The following command seems to work. It also returns the Debian version:
dpkg -L base-files 2>/dev/null | grep '^/etc/debian_version$' >/dev/null 2>&1 && cat /etc/debian_version
My previous idea was to use dpkg -S /etc/debian_version to see if it belonged to some package, regardless of name. But dpkg -S is slow.
I only have a lingering doubt: are there older Debian-derivated distros where my assumption doesn't hold? Is there a better way?
A better approach is to test that what you want to do is possible rather than relying on what the distribution is. For example my Ubuntu box claims it is "squeeze/sid". If you want to install something via dpkg, check dpkg is where you expect it an is executable. If you want to modify the networking setup? Check that /etc/networking/interfaces exists. Check for tools like update-rc.d etc.
You should also use /etc/lsb-release rather than /etc/debian_version.
I don't believe you can make that assumption no.
Please see: