posix-compliant (ash-compliant) test for root

[David Coe <david@someotherplace.org>]

In a shell script that tests for root like this: 

        if [ $UID != 0 ] ; then ...

it fails if the shell is ash, because $UID is undefined.

After looking at dh_testroot, I figured out that it could instead do:

        perl -we 'exit($< == 0)'
        if [ $? ] ; then ...

but it's relatively slow.

Is there a better way?  Thanks.