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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