File Protection by Owner

It has been suggested that the system protects users from interfering with

files that aren't theirs. This is done using the permission bits and ownership

information that is kept by the system for every file in the file system. These

permission bits determine who may read, write, and execute the file. These

permissions are divided into owner, group, and others. If only the owner

permission bits are set, then only the owner may perform those functions

on the file. Debian creates user accounts, each with their own group, and

defaults the umask value to 002, which causes all files created, to have default

permissions that match the user only, thus protecting any files created by this

user account from inadvertent tampering.

Only the owner of a file, or root, may change the ownership of a file. This

is done with the program chown which modifies the ownership to be that of

another user account. If the file is permitted for read by others, then when

Joe copies such a file from one of Fred's publicly readable directories, the file

copied into Joe's directory will now be owned by Joe's user account. This

give Fred the ability to let others use his material without letting them change

any of the original files. To prevent such  theft  of a file, for material that is

private, the permissions on the file need to be set to restrict read permission

for others.

The command chmod is used to change the permissions of files in the file

system. Only the owner (and root) may change the permissions on a file. If

Fred wants to keep the file private.txt from being read by others, he would

enter the following command:

chmod go wxr private.txt

This removes all permissions from either group or others, to either write,

execute, or read the file private.txt. Now only the fred account can read,

write, or execute the file private.txt.

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