An absolute mode specification has the following format:

chmod [options] absolute-mode file . . .

n

a number from 0 to 7. An absolute mode is constructed from the OR of any of the following modes:

4000

Set user ID on execution.

20#0

Set group ID on execution if # is 7, 5, 3, or 1.

Enable mandatory locking if # is 6, 4, 2, or 0.

For directories, files are created with BSD semantics for propagation of the group ID. With this option, files and subdirectories created in the directory inherit the group ID of the directory, rather than of the current process. For directories, the set-gid bit may only be set or cleared by using symbolic mode.

1000

Turn on sticky bit. See chmod(2).

0400

Allow read by owner.

0200

Allow write by owner.

0100

Allow execute (search in directory) by owner.

0700

Allow read, write, and execute (search) by owner.

0040

Allow read by group.

0020

Allow write by group.

0010

Allow execute (search in directory) by group.

0070

Allow read, write, and execute (search) by group.

0004

Allow read by others.

0002

Allow write by others.

0001

Allow execute (search in directory) by others.

0007

Allow read, write, and execute (search) by others.

For directories, the setgid bit cannot be set (or cleared) in absolute mode; it must be set (or cleared) in symbolic mode using g+s (or g-s).

A symbolic mode specification has the following format:

chmod [options] symbolic-mode-list file . . .

[who] operator [permissions]

Operations are performed in the order given. Multiple permissions letters following a single operator cause the corresponding operations to be performed simultaneously.

who

zero or more of the characters u, g, o, and a specifying whose permissions are to be changed or assigned:

u

user's permissions

g

group's permissions

o

others' permissions

a

all permissions (user, group, and other)

If who is omitted, it defaults to a, but the setting of the file mode creation mask (see umask in sh(1) or csh(1) for more information) is taken into account. When who is omitted, chmod will not override the restrictions of your user mask.

operator

either +, -, or =, signifying how permissions are to be changed:

+

Add permissions.

If permissions is omitted, nothing is added.

If who is omitted, add the file mode bits represented by permissions, except for the those with corresponding bits in the file mode creation mask.

If who is present, add the file mode bits represented by the permissions.

-

Take away permissions.

If permissions is omitted, do nothing.

If who is omitted, clear the file mode bits represented by permissions, except for those with corresponding bits in the file mode creation mask.

If who is present, clear the file mode bits represented by permissions.

=

Assign permissions absolutely.

If who is omitted, clear all file mode bits; if who is present, clear the file mode bits represented by who.

If permissions is omitted, do nothing else.

If who is omitted, add the file mode bits represented by permissions, except for the those with corresponding bits in the file mode creation mask.

If who is present, add the file mode bits represented by permissions.

Unlike other symbolic operations, = has an absolute effect in that it resets all other bits represented by who. Omitting permissions is useful only with = to take away all permissions.

permission

any compatible combination of the following letters:

l

mandatory locking

r

read permission

s

user or group set-ID

t

sticky bit

w

write permission

x

execute permission

X

execute permission if the file is a directory or if there is execute permission for one of the other user classes

u,g,o

indicate that permission is to be taken from the current user, group or other mode respectively.

Permissions to a file may vary depending on your user identification number (UID) or group identification number (GID). Permissions are described in three sequences each having three characters:

User	Group	Other
rwx	rwx	rwx

This example (user, group, and others all have permission to read, write, and execute a given file) demonstrates two categories for granting permissions: the access class and the permissions themselves.

The letter s is only meaningful with u or g, and t only works with u.

Mandatory file and record locking (l) refers to a file's ability to have its reading or writing permissions locked while a program is accessing that file.

In a directory which has the set-group-ID bit set (reflected as either -----s--- or -----l--- in the output of 'ls -ld'), files and subdirectories are created with the group-ID of the parent directory--not that of current process.

It is not possible to permit group execution and enable a file to be locked on execution at the same time. In addition, it is not possible to turn on the set-group-ID bit and enable a file to be locked on execution at the same time. The following examples, therefore, are invalid and elicit error messages:

chmod g+x,+l file chmod g+s,+l file

Only the owner of a file or directory (or the super-user) may change that file's or directory's mode. Only the super-user may set the sticky bit on a non-directory file. If you are not super-user, chmod will mask the sticky-bit but will not return an error. In order to turn on a file's set-group-ID bit, your own group ID must correspond to the file's and group execution must be set.