About chmod...

1. set it readable only for outsiders:

Before:	-rwxr-xr-x  archive.sh
Command:	chmod o=r archive.sh
After:	-rwxr-xr--  archive.sh

2. Take away all permissions for the group for topsecret.inf

Before:	-rw-r-----  topsecret.inf
Command:	chmod g= topsecret.inf
After:	-rw-------  topsecret.inf

3. Open up publicity.html for reading and writing by anyone.

Before:	-rw-r--r--  publicity.html
Command:	chmod og=rw publicity.html
After:	-rw-rw-rw-  publicity.html