This is because it is looking for a variable name NOW_website_files
which does not exist, and thus the resulting file name evaluates to .tar.gz
.
To solve it, do:
#!/bin/sh
NOW=$(date +"%F")
FILE="${NOW}_website_files.tar.gz"
^ ^
instead of
FILE="$NOW_website_files.tar.gz"
This way it will concatenate the variable $NEW
to the _website_files.tar.gz
text.