File permissions have already been introduced in the File System article. USE them. If you have a file, or a directory which contains sensitive information, close it to everyone but yourself (using chmod 700 file). If a file is to be run, but the contents should not be seen by any user, do not allow read permissions to anyone (use chmod 700 file). Many users completely close their home directory to other users, using the chmod command. To system administrators: most files outside of /tmp, /var/tmp and the user areas should be write protected.