File permissions in Interix

In Interix, each file has three sets of permissions: one for the owner, one for the file's group members, and one for everyone else. Each set of permissions consists of a read permission, a write permission, and an execute permission.

You can display file permissions with the ls -l command, which is the long-file listing of ls(1). The following example illustrates output of the ls -l command:

drwxr-xr-x 1  Jimbo  Accounting	 0   Aug 24  14:35 lib
-rwxr-xr-x 1  Jimbo  Accounting   342   Aug 27  00:20 stuff

In a permission list, such as drwxr-xr-x, the first character represents the file type (d or - in this example). The next nine characters represent permission bits. The list is divided into three parts: the owner permission set (rwx in this example), the group permission set (r-x in this example), and the permission set for everyone else (r-x). Each set can contain read (r), write (w), and execute (x) bits.

The following list describes the first character and each part of the permission list. For more information about permission lists and bits, see ls(1).

• The first bit indicates the file type. For example, the letter d represents a directory, the dash (-) denotes a regular file, and a letter l marks a symbolic link.
• The next three bits are the read, write, and execute bits for the owner of the file. A dash (-) indicates that the permission is not granted. For a directory, "execute" indicates the ability to search the directory. The third position could have letters other than x. An s in the third position indicates that the file has both set user identifier (setuid) and execute permissions. An S denotes setuid without execute permission.
• The next three bits are the read, write, and execute bits for the file owner's group. In the example, members of the group other than the owner can read and execute the file called stuff, but cannot write to the file. An s in the third position indicates that the set group identifier (setgid) bit is set with execute permission, and an S indicates that the setgid bit is set without execute permission.
• The last three bits are the read, write, and execute bits for everyone other than the owner and the owner's group; that is, everyone else. A t in the third position indicates a sticky bit with execute permission; a T is a sticky bit without execute permission.

If you own a file or have appropriate privileges, you can change the permissions for the file with the chmod(1) command.

Files created in the Interix environment

When you create a file in Interix and view it with ls -l, the following permissions and attributes apply:

• The file owner is the user who created it.
• The file inherits its group from the group of the directory.

  Group names can contain spaces. For example, "domain users" is a valid group name. Although it is possible to create group names with spaces, if you do so, shell and awk scripts might behave unexpectedly because these types of scripts parse a group name as a single token.

• File permissions are dictated by the user mask. For more information on the user mask, see umask(1).

POSIX files are given three access control entries (ACEs) in Windows: one for the owner, one for the group, and one for the Everyone group, which represents everyone else. For more information on ACEs, see Windows object security. POSIX permissions work as follows:

• The POSIX read permission is represented by the Windows read (R) permission.
• The POSIX write permission is represented by the Windows write (W) permission. If the file's read-only attribute is set, Interix does not assign the write permission, regardless of the content of the ACEs. If you use the chmod command to assign a write permission to a file that has the read-only attribute set, the read-only attribute is removed.
• The POSIX execute permission is represented by the Windows execute (X) permission.
• You can deny access in cases where the Windows permission is too broad. For example, a file with a mode of 077 (---rwxrwx) should not be readable by the owner. The Windows Everyone group gives the owner access to the file, however. In this case, Interix adds a deny permission for the owner to accurately represent the permissions.
• The owner of a file can change permissions on a file.
• The owner of a file can grant or deny the permission to others to take ownership of a file.
• Some special permissions, such as the setuid bit, are not represented through ACEs and are not visible through standard Win32 tools.

Files created in the Win32 subsystem

A file created through the Win32 subsystem can have a number of ACEs associated with it. In addition, those ACEs might not fit neatly into the categories of user, group, and everyone else. The Interix tools will "assemble" permission from the available ACEs:

• The file is owned by the user unless the user is a member of the Windows Administrators group, in which case the file is owned by the Administrators group.
• The file's group will probably be Windows Domain Users on a server, or None on a workstation.
• If an owner has no specific ACE associated with it, the owner-permission bits are empty.
• If the owner is a group, the group permissions are transferred to the owner permissions, and the group permissions are made empty.

If the ACE used to determine the owner's permissions does not have a change permission (P) or a take ownership (O) permission, the chown(1), chgrp(1), and chmod(1) commands might not work as expected.

All objects on a file allocation table (FAT) file system or high-performance file system (HPFS) are owned by the Everyone user and the Everyone group.