The file mode bits have two parts: the file permission bits, which control ordinary access to the file, and special mode bits, which affect only some files.
There are three kinds of permissions that a user can have for a file:
There are three categories of users who may have different permissions to perform any of the above operations on a file:
Files are given an owner and group when they are created. Usually the
owner is the current user and the group is the group of the directory
the file is in, but this varies with the operating system, the
file system the file is created on, and the way the file is created. You
can change the owner and group of a file by using the
In addition to the three sets of three permissions listed above, the file mode bits have three special components, which affect only executable files (programs) and, on most systems, directories:
On execution, set the process’s effective user ID to that of the file. For directories on a few systems, give files created in the directory the same owner as the directory, no matter who creates them, and set the set-user-ID bit of newly-created subdirectories.
On execution, set the process’s effective group ID to that of the file. For directories on most systems, give files created in the directory the same group as the directory, no matter what group the user who creates them is in, and set the set-group-ID bit of newly-created subdirectories.
Prevent unprivileged users from removing or renaming a file in a directory unless they own the file or the directory; this is commonly found on world-writable directories like /tmp. For regular files on some older systems, save the program’s text image on the swap device so it will load more quickly when run, so that the image is “sticky”.
In addition to the file mode bits listed above, there may be file attributes specific to the file system, e.g., access control lists (ACLs), whether a file is compressed, whether a file can be modified (immutability), and whether a file can be dumped. These are usually set using programs specific to the file system. For example:
On GNU and GNU/Linux the file attributes specific to
the ext2 file system are set using
On FreeBSD the file flags specific to the FFS
file system are set using
Even if a file’s mode bits allow an operation on that file, that operation may still fail, because:
For example, if the immutable attribute is set on a file,
it cannot be modified, regardless of the fact that you
may have just run
chmod a+w FILE.