How to remove all files except the ones you want:

First, expand rm capabilities by: shopt -s extglob

Then use find and remove: find . ! -name 'file.txt' -type f -exec rm -f {} +

Rename files

It has a lot of good options but one pretty useful for removing whitespaces and set all to lowercase:

rename -c --nows <fileName>

Use umask to restrict default permissions for users outside of the group

Set umask 007 in our .bashrc. Then newly created directories will have 770 (rwxrwx—) permissions, and files will have 660 (rw-rw—-).