File-system permissions

Most current file systems have methods of administering permissions or access rights to specific users and groups of users. These systems control the ability of the users to view or make changes to the contents of the filesystem.

Differences between operating systems

Unix-like and otherwise POSIX-compliant systems, including Linux-based systems and all Mac OS X versions, have a simple system for managing individual file permissions, which in this article are called "traditional Unix permissions". Most of these systems also support some kind of access control lists, either proprietary (old HP-UX ACLs, for example), or POSIX.1e ACLs, based on an early POSIX draft that was abandoned, or NFSv4 ACLs, which are part of the NFSv4 standard.

DOS variants (including MS-DOS, Windows 95, Windows 98, and Windows Me) do not have permissions, only file attributes. There is a read-only attribute (R), which can be set or unset on a file by any user or program, and therefore does not prevent him/her from changing/deleting the file. There is no permission in these systems which would keep a user from reading a file.

OpenVMS (a.k.a. VMS), as well as Microsoft Windows NT and its derivatives (including Windows 2000 and Windows XP), use access control lists (ACLs) to administer a more complex and varied set of permissions. OpenVMS also uses a permission scheme similar to that of Unix, but more complex. There are four categories(System, Owner, Group, and World) and four types of access permissions (Read, Write, Execute, and Delete). The categories are not mutually disjoint: World includes Group which in turn includes Owner. The System category independently includes system users (similar to superusers in Unix).

Classic Mac OSes are similar to DOS variants and DOS-based Windows: they do not support permissions, but only a "Protected" file attribute.

The AmigaOS Filesystem, AmigaDOS supports a relatively advanced permissions system, for a single-user OS. In AmigaOS 1.x, files had Archive, Read, Write, Execute and Delete (collectively known as ARWED) permissions/flags. In AmigaOS 2.x and higher, additional Hold, Script, and Pure permissions/flags were added.

Mac OS X versions 10.3 ("Panther") and prior use POSIX-compliant permissions. Mac OS X, beginning with version 10.4 ("Tiger"), also support the use of NFSv4 ACLs. They still support "traditional Unix permissions" as used in previous versions of Mac OS X, and the Apple Mac OS X Server version 10.4+ File Services Administration Manual recommends using only traditional Unix permissions if possible. It also still supports the Mac OS Classic's "Protected" attribute.

Solaris ACL support depends on the filesystem being used; older UFS filesystem supports POSIX.1e ACLs, while ZFS supports only NFSv4 ACLs.

Linux supports POSIX.1e ACLs. There is experimental support for NFSv4 ACLs for ext3 filesystem.

FreeBSD supports POSIX.1e ACLs on UFS, and NFSv4 ACLs on UFS and ZFS.

IBM z/OS implements file security via RACF (Resource Access Control Facility)

Traditional Unix permissions

Permissions on Unix-like systems are managed in three distinct classes. These classes are known as user, group, and others. In effect, Unix permissions are a simplified form of access control lists (ACLs).

When a new file is created on a Unix-like system, its permissions are determined from the umask of the process that created it.

Classes

Files and directories are owned by a user. The owner determines the file's owner class. Distinct permissions apply to the owner.

Files and directories are assigned a group, which define the file's group class. Distinct permissions apply to members of the file's group members. The owner doesn't need to be a member of the file's group.

Users who are not the owner, nor a member of the group, comprise a file's others class. Distinct permissions apply to others.

The effective permissions are determined based on the user's class. For example, the user who is the owner of the file will have the permissions given to the owner class regardless of the permissions assigned to the group class or others class.

Permissions

There are three specific permissions on Unix-like systems that apply to each class:

• The read permission, which grants the ability to read a file. When set for a directory, this permission grants the ability to read the names of files in the directory (but not to find out any further information about them such as contents, file type, size, ownership, permissions, etc.)
• The write permission, which grants the ability to modify a file. When set for a directory, this permission grants the ability to modify entries in the directory. This includes creating files, deleting files, and renaming files.
• The execute permission, which grants the ability to execute a file. This permission must be set for executable binaries (for example, a compiled C++ program) or shell scripts (for example, a Perl program) in order to allow the operating system to run them. When set for a directory, this permission grants the ability to traverse its tree in order to access files or subdirectories, but not see the content of files inside the directory (unless read is set).

The effect of setting the permissions on a directory (rather than a file) is "one of the most frequently misunderstood file permission issues".

When a permission is not set, the rights it would grant are denied. Unlike ACL-based systems, permissions on a Unix-like system are not inherited. Files created within a directory will not necessarily have the same permissions as that directory.

Changing permission behavior with setuid, setgid, and sticky bits

Unix-like systems typically employ three additional modes. These are actually attributes but are referred to as permissions or modes. These special modes are for a file or directory overall, not by a class.

• The set user ID, setuid, or SUID mode. When a file with setuid is executed, the resulting process will assume the effective user ID given to the owner class. This enables users to be treated temporarily as root (or another user).
• The set group ID, setgid, or SGID permission. When a file with setgid is executed, the resulting process will assume the group ID given to the group class. When setgid is applied to a directory, new files and directories created under that directory will inherit the group from that directory. (Default behaviour is to use the primary group of the effective user when setting the group of new files and directories.)
• The sticky mode. (Also known as the Text mode.) The typical behaviour of the sticky bit on executable files encourages the kernel to retain the resulting process image in memory beyond termination. On a directory, the sticky permission prevents users from renaming, moving or deleting contained files owned by users other than themselves, even if they have write permission to the directory. Only the directory owner and superuser are exempt from this.

These additional modes are also referred to as setuid bit, setgid bit, and sticky bit, due to the fact that they each occupy only one bit.

Notation of traditional Unix permissions

Symbolic notation

There are many ways by which Unix permission schemes are represented. The most common form is symbolic notation.

Three permission triads
first triad	what the owner can do
second triad	what the group members can do
third triad	what other users can do
Each triad
first character	r: readable
second character	w: writable
third character	x: executable s or t: executable and setuid/setgid/sticky S or T: setuid/setgid or sticky, but not executable

The first character indicates the file type:

• - denotes a regular file
• d denotes a directory
• b denotes a block special file
• c denotes a character special file
• l denotes a symbolic link
• p denotes a named pipe
• s denotes a domain socket

Each class of permissions is represented by three characters. The first set of characters represents the user class. The second set represents the group class. The third and final set of three characters represents the others class.

Each of the three characters represent the read, write, and execute permissions respectively:

• r if the read bit is set, - if it is not.
• w if the write bit is set, - if it is not.
• x if the execute bit is set, - if it is not.

The following are some examples of symbolic notation:

• -rwxr-xr-x for a regular file whose user class has full permissions and whose group and others classes have only the read and execute permissions.
• crw-rw-r-- for a character special file whose user and group classes have the read and write permissions and whose others class has only the read permission.
• dr-x------ for a directory whose user class has read and execute permissions and whose group and others classes have no permissions.

Octal notation

Another common method for representing Unix permissions is octal notation. Octal notation consists of a three- or four-digit base-8 value.

With three-digit octal notation, each numeral represents a different component of the permission set: user class, group class, and "others" class respectively.

Each of these digits is the sum of its component bits (see also Binary numeral system). As a result, specific bits add to the sum as it is represented by a numeral:

• The read bit adds 4 to its total (in binary 100),
• The write bit adds 2 to its total (in binary 010), and
• The execute bit adds 1 to its total (in binary 001).

These values never produce ambiguous combinations; each sum represents a specific set of permissions.

These are the examples from the Symbolic notation section given in octal notation:

• "-rwxr-xr-x" would be represented as 755 in three-digit octal.
• "-rw-rw-r--" would be represented as 664 in three-digit octal.
• "-r-x------" would be represented as 500 in three-digit octal.

Here is a summary of the meanings for individual octal digit values:

0 --- no permission
1 --x execute
2 -w- write
3 -wx write and execute
4 r-- read
5 r-x read and execute
6 rw- read and write
7 rwx read, write and execute

Note the similarity to binary counting (starting at the right and going left):

0: 000
1: 001
2: 010
3: 011
4: 100
5: 101
6: 110
7: 111

Octal digit values can be added together to make Symbolic Notations:
(4=r)+(1=x) == (5=r-x)
(4=r)+(2=w) == (6=rw-)
(4=r)+(2=w)+(1=x) == (7=rwx)

Here is a summary showing which octal digits affect permissions for user, group, and other:

• UGO = User, Group, Other
• 777 = "-rwxrwxrwx" = rwx for all
• 754 = "-rwxr-xr--" = rwx for owner, r-x for group, r-- for other
• 124 = "--x-w-r--" = x for owner, w for group, r for other

Listing Permissions

ls -l prints symbolic notation for files and directories.

stat -c '%n %a %A' can be used to print out octal notation.

User private group

Some systems diverge from the traditional POSIX-model of users and groups, by creating a new group – a "user private group" – for each user. The "user private group" scheme can be preferred for a variety of reasons including using a umask of 002 and not having every "user" able to write to newly created files. In this case however, no other users must be added to the "user private group" or they will have write-permission on all files.

See also

• Access control list
• chattr
• chmod, the command used to set permissions on Unix-like systems
• File system
• Group identifier (Unix)
• lsattr
• POSIX
• User identifier (Unix)

External links

• Apple Mac OS X Server version 10.4+ File Services Administration Manual (see pages 16-26)
• "Linux File Permission Confusion" by Brian Hatch 2003.
• "Linux File Permission Confusion pt 2" by Brian Hatch 2003.
• The Linux Cookbook: Groups and How to Work in Them by Michael Stutz 2004