Why does sh display an error when bash does not?

Question

Why does the following script display an "unexpected operator" message and fail when run with SH, but not with BASH.

!/bin/sh
if [ $UID -ne 0 ]
then
        echo "You must be root."
        exit 1
else
        echo "Open sesame."
        exit 0
fi

Answer 1

Not all shells define a variable UID. This is a bash and zsh feature only. In other shells, the UID variable is not defined, so your test command expands to [ -ne 0 ] which is a syntax error.

A portable way to obtain the user ID is with the id utility.

if [ "$(id -u)" -ne 0 ]; then …