CHMOD(C) UNIX System V
Name
chmod - changes the access permissions of a file or
directory
Syntax
chmod mode file
chmod [who] [+|-|=] [permission ...] file ...
Description
The chmod command changes the access permissions (or mode)
of a specified file or directory. It is used to control file
and directory access by users other than the owner and
super-user. The mode may be an expression composed of
letters and operators (called symbolic mode), or a number
(called absolute mode).
A chmod command using symbolic mode has the form:
chmod [who] [+|-|=] [permission ...] filename
In place of who you can use one or any combination of the
following letters:
a Stands for ``all users''. If who is not indicated on
the command line, a is the default. The definition of
``all users'' depends on the user's umask. See
umask(C).
g Stands for ``group'', all users who have the same group
ID as the owner of the file or directory.
o Stands for ``others'', all users on the system.
u Stands for ``user'', the owner of the file or
directory.
The operators are:
+ Adds permission
- Removes permission
= Assigns the indicated permission and removes all other
permissions (if any) for that who. If no permission is
assigned, existing permissions are removed.
Permissions can be any combination of the following letters:
x Execute (search permission for directories)
r Read
w Write
s Sets owner or group ID on execution of the file to that
of the owner of the file. The mode ``u+s'' sets the
user ID bit for the file. The mode ``g+s'' sets the
group ID bit. Other combinations have no effect. When
the group ID bit is set on a directory, all files
created under it thereafter receive the group ID of
that directory. When the group ID bit is not set,
files are created with the group ID of the creating
process/user.
t This is known as the ``sticky bit.'' (see chmod(S)).
Only the mode ``u+t'' sets the sticky bit. All other
combinations have no effect. When this bit is set on a
directory, files within the directory cannot be removed
by anyone but the owner or the super-user. The owner
can set or remove the sticky bit.
l Mandatory locking will occur during access
Multiple symbolic modes may be given, separated by commas,
on a single command line. See the following Examples section
for sample permission settings.
Mandatory file and record locking refers to a file having
locked reading or writing permissions while a program is
accessing that file. A file cannot have group execution
permission and be able to be locked on execution. In
addition, it is not possible to turn on the set-group-ID and
enable a file to be locked on execution at the same time.
The following examples show illegal uses of chmod and will
generate error messages:
chmod g+x,+l filename
chmod g+s,+l filename
A chmod command using absolute mode has the form:
chmod mode filename
where mode is an octal number constructed by performing
logical OR on the following:
4000 Set user ID on execution
20#0 Set group ID on execution if ``#'' is 7, 5, 3, or
1 and enable mandatory locking if ``#'' is 6, 4,
2, or 0.
1000 Sets the sticky bit (see chmod(S))
0400 Read by owner
0200 Write by owner
0100 Execute (search in directory) by owner
0040 Read by group
0020 Write by group
0010 Execute (search in directory) by group
0004 Read by others
0002 Write by others
0001 Execute (search in directory) by others
0000 No permissions
Examples
Symbolic Mode
The following command gives all users execute permission for
file:
chmod +x file
The following command removes read and write permission for
group and others from file:
chmod go-rw file
The following command gives other users read and write
permission for file:
chmod o+rw file
The following command gives read permission to group and
other:
chmod g+r,o+r file
Absolute Mode
The following command gives all users read, write and
execute permission for file:
chmod 0777 file
The following command gives read and write permission to all
users for file:
chmod 0666 file
The following command gives read and write permission to the
owner of file only:
chmod 0600 file
The following example causes the file to be locked on
access:
chmod +l file
See Also
ls(C), chmod(S)
Notes
The setuid, setgid and sticky bit settings have no effect on
shell scripts.
Standards Conformance
chmod is conformant with:
AT&T SVID Issue 2, Select Code 307-127;
and The X/Open Portability Guide II of January 1987.
(printed 2/15/90) CHMOD(C)