Using chown $USER:$USER inside bash script

If, for some reason, $USER is not set, you can use the id command to obtain the identity of the real user. So the first time you use the $USER variable, you can use the shell expansion to supply a default value. Change the chown line in your script to:

sudo chown ${USER:=$(/usr/bin/id -run)}:$USER /var/www/$sitename

If USER is empty or unset when this is run, bash will set the USER variable to the output of /usr/bin/id -run

When I calling my script with sudo it would set $USER to root.

$ sudo ./myscript.sh

I tried the chown ${USER:=$(/usr/bin/id -run)}:$USER /var/www/$sitename but it would still return root.

I found if I used who with awk I was able to get the current user that called the script with sudo.

currentuser=$(who | awk '{print $1}')}
chown -R $currentuser:$currentuser /var/www/$sitename`

In order to simplify the problem and since your are getting the variable sitename, why don't you read a username variable?

With that you'd make sure that the script execution is not dependent on the environmental variables made available the way the script is executed.