Unix and Linux systems (including Mac OS X and other POSIX
compliant systems) have a (relatively) simple system for
controlling access to files and directories. This system is
defined in also known as the Single Unix Specification (SUS And
since devices such as disks, ports, etc. have file names /dev)
you control access to them the same (Note Windows systems up
through Windows ME don't support permissions, just a read only
attribute.) This systems works by assigning a user and a group
to every file. Then users of that file are put into one of three
classes: For each class of users there are three possible
permissions that can be granted: Thus there are nine permissions
you can set, in any combination. (Not all combinations make
sense however.) Any attempt to access a file's data requires
read permission. Any attempt to modify a file's data requires
write permission. Any attempt to execute a file (a program or a
script) requires execute permission. In *nix systems directories
are also files and thus use the sa