A File is an abstraction of any file object accessible by the
program and is closely associated with class IO
File includes the methods of module FileTest as
class methods, allowing you to write (for example)
File.exist?("foo").
In the description of File methods, permission bits are a platform-specific set of bits that indicate permissions of a file. On Unix-based systems, permissions are viewed as a set of three octets, for the owner, the group, and the rest of the world. For each of these entities, permissions may be set to read, write, or execute the file:
The permission bits 0644 (in octal) would thus be interpreted
as read/write for owner, and read-only for group and other. Higher-order
bits may also be used to indicate the type of file (plain, directory, pipe,
socket, and so on) and various other special features. If the permissions
are for a directory, the meaning of the execute bit changes; when set the
directory can be searched.
On non-Posix operating systems, there may be only the ability to make a
file read-only or read-write. In this case, the remaining permission bits
will be synthesized to resemble typical values. For instance, on Windows NT
the default permission bits are 0644, which means read/write
for owner, read-only for all others. The only change that can be made is to
make the file read-only, which is reported as 0444.