if [ "$(id -u)" = "1000" ]; then
do stuff for user 1000
do stuff for other users
However, it would probably be more effective to chown the script to that user and then chmod 700, since it appears from your example that you are attempting to stop execution of the script for other users. This would also give you more control i.e. allowing you to use groups (w/ chmod 750 or whatever) to manage who can perform the action. If only part of your script should be accessible to user 1000 you can separate that part into its own script and apply permissions appropriately.