chmod(C)
chmod --
change the access permissions of a file or directory
Syntax
Symbolic mode
chmod [ -R ] [ who ] [
+|-|= ] [ mode ... ]
file ...
Absolute mode
chmod [ -R ] mode
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 super
user. mode may be an expression composed of letters and
operators (called ``symbolic mode''), or a number (called
``absolute mode'').
chmod takes the following option:
-R-
Recursively change file mode bits. For each specified file that
names a directory, chmod will change the file mode bits of
the directory and all files in the file hierarchy below it.
Symbolic mode
In symbolic mode, the who operand is any one, or a
combination, of the following letters:
a-
Stands for ``all users''. If who is not indicated on the
command line, a is the default.
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.
If who is not indicated on the command line, ``all users''
is taken as the default, but the file creation mask
(umask) is used when modifying the permissions. Otherwise
the umask is not used. The operators are:
+-
Adds permission.
--
Removes permission.
=-
Assigns the indicated permission and removes all other permissions
(if any) for that variable. If no permission is assigned, existing
permissions are removed.
Permissions are set using any combination of the following letters
for mode:
g-
Set the permissions for who as are currently set for
``group''.
l-
Mandatory locking will occur during access.
o-
Set the permissions for who as are currently set for
``owner''.
r-
Read.
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 subsequently
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. Only the super user can set the sticky bit. If the
file is a directory, the following users can delete files in it:
-
root
-
the owner of the directory
-
the owner of the files if they have write permissions on the directory
-
anyone with write permissions to both file and directory
If the file is not a
directory, the sticky bit has no effect. (If the sticky bit is set
on an executable file, the system attempts to keep the text segment
in core after execution ceases.)
u-
Set the permissions for who as are currently set for
``user''.
w-
Write.
x-
Execute (sets search permission for directories).
X-
Represents the execute/search permissions if the file is a
directory, or if the current file mode bits have at least one of the
execute bits set.
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
Absolute mode
In absolute mode, the mode operand is an octal number
constructed by combining the following flags using logical
OR:
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
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
To apply a combination of permissions to a single file using
absolute mode, combine the permissions by adding the digits in each
character position. For example, to combine Read by owner permission
(0400) and Write by owner permission (0200), add the digits in each
column to obtain the combined absolute mode 0600. (This is
equivalent to the logical OR operation.)
Exit values
chmod returns the following values:
0-
the command executed successfully, and all reqyested changes were
made
>0-
an error occurred
Examples
Symbolic mode
The following command gives all users execute permission for
file:
chmod a+x file
Either of the following commands removes execute permissions that
would be allowed if file was created with the same
umask:
chmod -- -x file
chmod -x file
The first version of the command conforms to POSIX.2 and
will be portable to all systems that also conform to this standard;
it may not be available on non-POSIX.2 conformant
systems. The second version of the command is provided by the Santa
Cruz Operation for historical compatibility; it may not be available
on systems that conform to POSIX.2. See the ``Warning''
section for more details.
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 others:
chmod g+r,o+r file
The following example causes the mandatory locking of file
on access:
chmod +l 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
Warning
POSIX.2 requires that a ``--'' be used to delimit the end
of the options if you specify an operand of the form
-mode. This is to prevent such an operand being rejected
as an illegal option. However, existing scripts that use the
historical form of chmod will break if they are not
amended.
The version of chmod provided by the Santa Cruz Operation
has been extended to provide historical compatibility for the
-mode settings: -r, -w, and
-x. This functionality is in addition to that required by
POSIX.2. You should note that some other systems that are
POSIX.2 compliant may not provide historical
compatibility, and non-POSIX.2 systems may not allow the
use of the ``--'' delimiter.
Limitations
The setuid, setgid and sticky bit settings have no effect on shell
scripts.
When used to change the permissions of a symbolic link,
chmod automatically follows the link and tries to change
the permissions of the linked file. It makes no sense for symbolic
links to have their own permissions and ownerships.
See also
chmod(S),
ls(C),
umask(C)
Standards conformance
chmod is conformant with:
ISO/IEC DIS 9945
2:1992, Information technology Portable Operating System Interface (POSIX) Part 2: Shell and Utilities (IEEE Std 1003.21992);
AT&T SVID Issue 2;
X/Open CAE Specification, Commands and Utilities, Issue 4, 1992.
© 2003 Caldera International, Inc. All rights reserved.
SCO OpenServer Release 5.0.7 -- 11 February 2003