- change the permissions mode of a file
chmod [-fR] absolute-mode file...
chmod [-fR] symbolic-mode-list file...
chmod [-fR] acl_operation file...
The chmod utility changes or assigns the mode of a file.
chmod can also be used to modify Access Control Lists (ACLs) on files and directories.
An absolute mode specification has the following format:
chmod [options] absolute-mode file . . .
where absolute-mode is specified using octal numbers nnnn defined as follows:
a number from 0 to 7. An absolute mode is constructed from the OR of any of the following modes:
Set user ID on execution.
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 can only be set or cleared by using symbolic mode.
Turn on sticky bit. See chmod(2).
Allow read by owner.
Allow write by owner.
Allow execute (search in directory) by owner.
Allow read, write, and execute (search) by owner.
Allow read by group.
Allow write by group.
Allow execute (search in directory) by group.
Allow read, write, and execute (search) by group.
Allow read by others.
Allow write by others.
Allow execute (search in directory) by others.
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 . . .
where symbolic-mode-list is a comma-separated list (with no intervening whitespace) of symbolic mode expressions of the form:
[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.
zero or more of the characters u, g, o, and a specifying whose permissions are to be changed or assigned:
user's permissions
group's permissions
others' permissions
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 does not override the restrictions of your user mask.
either +, -, or =, signifying how permissions are to be changed:
Add permissions.
If permissions are 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 are 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 are 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.
any compatible combination of the following letters:
mandatory locking
read permission
user or group set-ID
sticky bit
write permission
execute permission
execute permission if the file is a directory or if there is execute permission for one of the other user classes
indicate that permission is to be taken from the current user, group or other mode respectively.
Permissions to a file can vary depending on your user identification number (UID) or group identification number (GID). Permissions are described in three sequences each having three characters:
|
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) can change that file's or directory's mode. Only the super-user can set the sticky bit on a non-directory file. If you are not super-user, chmod masks the sticky-bit but does 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.
An ACL Operation specification has the following format:
chmod [options] A[number]- file ... chmod [options] A-acl_specification file ... chmod [options] A[index]{+|=}acl_specification file ...
Where acl_specification is a comma-separated list (with no intervening whitespace) of an ACL specification of the form:
Prepends the access control entries (ACE) specified in acl_specification to the beginning of the file's ACL. Depending on the file system, the ACL can be reordered when applied to the file. If “optional” number is specified then new ACEs are inserted before specified number.
Removes all ACEs for current ACL on file and replaces current ACL with new ACL that represents only the current mode of the file.
Removes ACE specified by index number.
Removes ACEs specified by acl_specification, if they exist in current file's ACL.
Replaces a files entire ACL with acl_specification.
Replaces ACEs starting at a specific index number in the current ACL on the file. If multiple ACEs are specified, then each subsequent ACE in acl_specification replaces the corresponding ACE in the current ACL.
POSIX-draft ACL Specification (as supported by UFS)
POSIX-draft ACLs (as supported by UFS) are specified as colon (:) separated fields of the following.
File owner permissions.
Permissions for a specific user.
File group owner permissions.
Permissions for a specific group.
Permissions for user other than the file owner or members of file group owner.
The ACL mask. The mask entry specifies the maximum permissions allowed for user (other than that the owner) and for groups.
Default file owner permissions.
Default permissions for a specific user.
Default file group owner permissions.
Default permissions for a specific group.
Default permissions for user other than the file owner or members of the file group owner.
Default ACL mask.
The above specification allows for ACLs to be specified such as:
user:tom:rw-,mask:rwx,group:staff:r-x
NFSv4 ACL Specification (as supported by NFSv4 and ZFS)
NFSv4 ACLs provide richer ACL semantics. They provide both allow and deny entries, finer grained permissions, and enhanced inheritance control.
NFSv4 ACLs are specified as colon (:) separated fields of the following.
Permissions for file owner.
Permissions for file group owner.
Permissions for everyone, including file owner and group owner.
Permissions for a specific user.
Permissions for a specific group.
Permissions for a specific group, but group is specified by SID.
Permissions for a specific SID, but it doesn't matter if it is a user or a group.
Permissions can be specified in three different chmod ACL formats: verbose, compact, or positional. The verbose format uses words to indicate that the permissions are separated with a forward slash (/) character. Compact format uses the permission letters and positional format uses the permission letters or the hyphen (-) to identify no permissions.
The permissions for verbose mode and their abbreviated form in parentheses for compact and positional mode are described as follows:
Permission to read the data of a file.
Permission to list the contents of a directory.
Permission to modify a file's data. anywhere in the file's offset range.
Permission to add a new file to a directory.
The ability to modify a file's data, but only starting at EOF.
Currently, this permission is not supported.
Permission to create a subdirectory to a directory.
Ability to read the extended attributes of a file.
Ability to create extended attributes or write to the extended attribute directory.
Permission to execute a file.
The ability to read basic attributes (non-ACLs) of a file.
Permission to change the times associated with a file or directory to an arbitrary value.
Permission to delete a file.
Permission to delete a file within a directory.
Permission to read the ACL of a file.
Permission to write the ACL of a file.
Permission to change the owner of a file.
Permission to access file locally at server with synchronize reads and writes.
Currently, this permission is not supported.
Using the compact ACL format, permissions are specified by using 14 unique letters to indicate permissions.
Using the positional ACL format, permissions are specified as positional arguments similar to the ls -V format. The hyphen (-), which indicates that no permission is granted at that position, can be omitted and only the required letters have to be specified.
The letters above are listed in the order they would be specified in positional notation.
Permissions can be specified with these letters in the following way:
rwx--D--------
The hyphens can be removed to compact the string as follows:
rwxD
The optional inheritance flags can be specified in the three formats. The first format uses words to indicate the various inheritance flags separated with a forward slash (/) character.
Inherit to all newly created files.
Inherit to all newly created directories.
When placed on a directory, do not apply to the directory, only to newly created files and directories. This flag requires that either file_inherit and or dir_inherit is also specified.
Indicates that ACL entries should be inherited to objects in a directory, but inheritance should stop after descending one level. This flag is dependent upon either file_inherit and or dir_inherit also being specified.
The inheritance flags listed can also be specified in the compact format or as positional arguments similar to the ls -V format. A hyphen character indicates that the inheritance flag at that postion is not specified in the positional ACL format.
The inheritance flags can be specified with these letters in any of the following equivalent ways.
file_inherit/dir_inherit/no_propagate
fd-n--
fdn
With this inheritance model, an ACL entry can be specified such as:
user:tom:read_data/write_data/read_attributes:file_inherit:allow user:fred:read_data:file_inherit/dir_inherit:deny user:bob:read_data:allow
The following options are supported:
Force. chmod does not complain if it fails to change the mode of a file.
Recursively descends through directory arguments, setting the mode for each file as described above. When symbolic links are encountered, the mode of the target file is changed, but no recursion takes place.
The following operands are supported:
Represents the change to be made to the file mode bits of each file named by one of the file operands. See Absolute Mode and Symbolic Mode above in the Description section for more information.
Represents the modification to be performed on the file's ACL. See ACL Operation in the DESCRIPTION section for more information.
acl_operation is one of the following:
A[number] - A-acl_specification A[index]{+|=}acl_specification
A path name of a file whose file mode bits are to be modified.
See largefile(5) for the description of the behavior of chmod when encountering files greater than or equal to 2 Gbyte ( 231 bytes).
Example 1 Denying execute Permission
The following example denies execute permission to everyone:
example% chmod a-x file
Example 2 Allowing read-only Permission
The following example allows only read permission to everyone:
example% chmod 444 file
Example 3 Making a File readable and writable
The following example makes a file readable and writable by the group and others:
example% chmod go+rw file example% chmod 066 file
Example 4 Locking a File From Access
The following example locks a file from access:
example% chmod +l file
Example 5 Granting read, write, execute, and set group-ID Permission on a File
The following example grants everyone read, write, and execute permissions on the file, and turns on the set group-ID:
example% chmod a=rwx,g+s file example% chmod 2777 file
Example 6 Prepending a New ACL Entry on a ZFS File
The following example prepends a new ACL entry on a ZFS file.
First, display the current ACL:
example% ls -v file.3 -rw-r--r-- 1 marks staff 0 Oct 9 15:49 file.3 0:owner@:execute:deny 1:owner@:read_data/write_data/append_data/write_xattr/ write_attributes/write_acl/write_owner:allow 2:group@:write_data/append_data/execute:deny 3:group@:read_data:allow 4:everyone@:write_data/append_data/write_xattr/execute/ write_attributes/write_acl/write_owner:deny 5:everyone@:read_data/read_xattr/read_attributes/read_acl/ synchronize:allow
Issue the following command:
example% chmod A+user:lp:read_data:deny file.3
Display the new ACL:
example% ls -v file.3 -rw-r--r--+ 1 marks staff 0 Oct 9 15:49 file.3 0:user:lp:read_data:deny 1:owner@:execute:deny 2:owner@:read_data/write_data/append_data/write_xattr/ write_attributes/write_acl/write_owner:allow 3:group@:write_data/append_data/execute:deny 4:group@:read_data:allow 5:everyone@:write_data/append_data/write_xattr/execute/ write_attributes/write_acl/write_owner:deny 6:everyone@:read_data/read_xattr/read_attributes/read_acl/ synchronize:allow
Example 7 Prepending a New POSIX-draft ACL Entry on a UFS File
The following example prepends a new POSIX-draft ACL entry on a UFS file.
First, display the current ACL:
example% ls -v file.2 -rw-r--r-- 1 marks staff 0 Oct 9 15:52 file.2 0:user::rw- 1:group::r-- #effective:r-- 2:mask:r-- 3:other:r--
Issue the following command:
example% chmod A+user:lp:-wx file.2
Display the new ACL:
example% ls -v file.2 -rw-r--r--+ 1 marks staff 0 Oct 9 15:52 file.2 0:user::rw- 1:user:lp:-wx #effective:--- 2:group::r-- #effective:r-- 3:mask:r-- 4:other:r--
Example 8 Inserting an ACL Entry in a Specific Position on a ZFS file
The following example inserts an ACL entry in a specific position on a ZFS file system. It also illustrates the compact ACL format.
First, display the ACL to pick a location to insert a new ACE.
example% ls -V file.1 -rw-r--r--+ 1 root root 0 Oct 6 12:16 file.1 user:lp:rw------------:------:allow owner@:--x-----------:------:deny owner@:rw-p---A-W-Co-:------:allow group@:-wxp----------:------:deny group@:r-------------:------:allow everyone@:-wxp---A-W-Co-:------:deny everyone@:r-----a-R-c--s:------:allow
Next, insert a new entry in location 3. This causes the entries that are currently in position 3 - 6 to be pushed down.
Issue the following command:
example% chmod A3+user:marks:r:deny file.1
Display the new ACL:
example% ls -V file.1 -rw-r--r--+ 1 root staff 0 Feb 3 14:13 file.1 user:lp:rw------------:------:allow owner@:--x-----------:------:deny owner@:rw-p---A-W-Co-:------:allow user:marks:r-------------:------:deny group@:-wxp----------:------:deny group@:r-------------:------:allow everyone@:-wxp---A-W-Co-:------:deny everyone@:r-----a-R-c--s:------:allow
Example 9 Inserting a POSIX-draft ACL in a Specific Position on a UFS File
The file system reorders ACLs when they are stored in the file system. The following example illustrates this behavior.
example% ls -v file.1 -rw-r--r--+ 1 root root 0 Sep 29 16:10 file.1 0:user::rw- 1:user:lp:rw- #effective:r-- 2:group::r-- #effective:r-- 3:mask:r-- 4:other:r--
Now, insert an entry at index position 3. The command works, but the file system reorders the ACL.
example% chmod A3+user:marks:rw- file.1 example% ls -v file.1 -rw-r--r--+ 1 root root 0 Sep 29 16:10 file.1 0:user::rw- 1:user:lp:rw- #effective:r-- 2:user:marks:rw- #effective:r-- 3:group::r-- #effective:r-- 4:mask:r-- 5:other:r--
Rather than inserting the ACL entry in position 3 as requested, it actually ends up in position 2.
Example 10 Removing an ACL Entry on a ZFS File
The following example removes the lp entry from an ACL:
example% ls -v file.3 -rw-r--r--+ 1 marks staff 0 Oct 9 15:49 file.3 0:user:lp:read_data:deny 1:owner@:execute:deny 2:owner@:read_data/write_data/append_data/write_xattr/ write_attributes/write_acl/write_owner:allow 3:group@:write_data/append_data/execute:deny 4:group@:read_data:allow 5:everyone@:write_data/append_data/write_xattr/execute/ write_attributes/write_acl/write_owner:deny 6:everyone@:read_data/read_xattr/read_attributes/read_acl/ synchronize:allow
example% chmod A-user:lp:read_data:deny file.3 example% ls -v file.3 -rw-r--r-- 1 marks staff 0 Oct 9 15:49 file.3 0:owner@:execute:deny 1:owner@:read_data/write_data/append_data/write_xattr/ write_attributes/write_acl/write_owner:allow 2:group@:write_data/append_data/execute:deny 3:group@:read_data:allow 4:everyone@:write_data/append_data/write_xattr/execute/ write_attributes/write_acl/write_owner:deny 5:everyone@:read_data/read_xattr/read_attributes/read_acl/ synchronize:allow
Example 11 Removing a POSIX-draft ACL on a UFS File
The following example removes the lp entry from an ACL:
example% ls -v file.2 -rw-r--r--+ 1 marks staff 0 Oct 9 15:52 file.2 0:user::rw- 1:user:lp:-wx #effective:--- 2:group::r-- #effective:r-- 3:mask:r-- 4:other:r--
example% chmod A-user:lp:-wx file.2 example% ls -v file.2 -rw-r--r-- 1 marks staff 0 Oct 9 15:52 file.2 0:user::rw- 1:group::r-- #effective:r-- 2:mask:r-- 3:other:r--
Example 12 Removing a Specific ACL Entry by Index Number on a ZFS File
Consider the following ACL:
example% ls -v file 0:group:staff:read_data/write_data/execute/read_acl:allow 1:user:bin:read_data:deny 2:user:bin:read_data:allow 3:owner@:write_data/append_data:deny 4:owner@:read_data/write_xattr/execute/write_attributes/write_acl /write_owner:allow 5:group@:write_data/append_data:deny 6:group@:read_data/execute:allow 7:everyone@:write_data/append_data/write_xattr/write_attributes /write_acl/write_owner:deny 8:everyone@:read_data/read_xattr/execute/read_attributes/read_acl /synchronize:allow
Remove the second user entry for bin.
example% chmod A2- file example% ls -v file 0:group:staff:read_data/write_data/execute/read_acl:allow 1:user:bin:read_data:deny 2:owner@:write_data/append_data:deny 3:owner@:read_data/write_xattr/execute/write_attributes/write_acl /write_owner:allow 4:group@:write_data/append_data:deny 5:group@:read_data/execute:allow 6:everyone@:write_data/append_data/write_xattr/write_attributes /write_acl/write_owner:deny 7:everyone@:read_data/read_xattr/execute/read_attributes/read_acl /synchronize:allow
Example 13 Removing a Specific POSIX-draft ACL Entry on a UFS File
The following example removes the lp entry by index number from the following ACL:
example% ls -v file.1 -rw-r--r--+ 1 root root 0 Sep 29 16:10 file.1 0:user::rw- 1:user:lp:rw- #effective:r-- 2:group::r-- #effective:r-- 3:mask:r-- 4:other:r-- example% chmod A1- file.1 example% ls -v -rw-r--r--+ 1 root root 0 Sep 29 16:10 file.1 0:user::rw- 1:group::r-- #effective:r-- 2:mask:r-- 3:other:r--
Example 14 Removing All ACLs From a File
The following command works with either NFSv4/ZFS or POSIX-draft ACLs.
Consider the following ACL:
example% ls -v file.3 -rw-r--r--+ 1 marks staff 0 Oct 9 15:49 file.3 0:user:lp:read_data/write_data:allow 1:user:marks:read_acl:allow 2:owner@:execute:deny 3:owner@:read_data/write_data/append_data/write_xattr/ write_attributes/write_acl/write_owner:allow 4:group@:write_data/append_data/execute:deny 5:group@:read_data:allow 6:everyone@:write_data/append_data/write_xattr/execute/ write_attributes/write_acl/write_owner:deny 7:everyone@:read_data/read_xattr/read_attributes/read_acl/ synchronize:allow
The existing ACL is effectively removed and is replaced with an ACL that represents the permission bits of the file.
example% chmod A- file.3 example% ls -v file.3 -rw-r--r-- 1 marks staff 0 Oct 9 15:49 file.3 0:owner@:execute:deny 1:owner@:read_data/write_data/append_data/write_xattr/ write_attributes/write_acl/write_owner:allow 2:group@:write_data/append_data/execute:deny 3:group@:read_data:allow 4:everyone@:write_data/append_data/write_xattr/execute/ write_attributes/write_acl/write_owner:deny 5:everyone@:read_data/read_xattr/read_attributes/read_acl/ synchronize:allow
Example 15 Replacing an Entire ACL Entry on a ZFS File
Use the following chmod syntax if you want to replace an ACL in its entirety:
example% chmod A=owner@:read_data/write_data:allow,group@:read_data/ write_data:allow,user:lp:read_data:allow file.4 example% ls -v file.4 -rw-rw----+ 1 marks staff 0 Oct 9 16:12 file.4 0:owner@:read_data/write_data:allow 1:group@:read_data/write_data:allow 2:user:lp:read_data:allow
Example 16 Replacing an Entire POSIX-draft ACL on a UFS File
This operation is a little more complicated. The replacement ACL needs the necessary entries to represent the file owner, file group owner, other, mask and any additional entries you wish to set.
example% chmod A=user::rw-,group::rw-,other::---,mask:r--, user:lp:r-- file.3 example% ls -v file.3 -rw-r-----+ 1 root root 0 Oct 9 16:14 file.3 0:user::rw- 1:user:lp:r-- #effective:r-- 2:group::rw- #effective:r-- 3:mask:r-- 4:other:---
Example 17 Replacing a Specific Entry on a ZFS File
Consider the following ACL.
example% ls -v file.5 -rw-r--r--+ 1 marks staff 0 Oct 9 16:18 file.5 0:user:marks:read_data:allow 1:owner@:execute:deny 2:owner@:read_data/write_data/append_data/write_xattr/ write_attributes/write_acl/write_owner:allow 3:group@:write_data/append_data/execute:deny 4:group@:read_data:allow 5:everyone@:write_data/append_data/write_xattr/execute/ write_attributes/write_acl/write_owner:deny 6:everyone@:read_data/read_xattr/read_attributes/read_acl/ synchronize:allow
Now, change the allow access to a deny for user marks:
example% chmod A0=user:marks:read_data:deny file.5 example% ls -v file.5 -rw-r--r--+ 1 marks staff 0 Aug 23 09:11 file.5 0:user:marks:read_data:deny 1:owner@:read_data/write_data/append_data/write_xattr/write_attributes /write_acl/write_owner:allow 2:group@:write_data/append_data/execute:deny 3:group@:read_data:allow 4:everyone@:write_data/append_data/write_xattr/execute/write_attributes /write_acl/write_owner:deny 5:everyone@:read_data/read_xattr/read_attributes/read_acl/synchronize :allow
Example 18 Replacing a Specific POSIX-draft ACL on a UFS File
Consider the following ACL.
example% ls -v file.4 -rw-r--r--+ 1 marks staff 0 Oct 9 16:21 file.4 0:user::rw- 1:user:lp:rwx #effective:r-- 2:group::r-- #effective:r-- 3:mask:r-- 4:other:r--
Now, change the permission on lp from rwx to r--:
example% chmod A1=user:lp:r-- file.4 example% ls -v file -rw-r--r--+ 1 marks staff 0 Oct 9 16:21 file.4 0:user::rw- 1:user:lp:r-- #effective:r-- 2:group::r-- #effective:r-- 3:mask:r-- 4:other:r--
Example 19 Setting ACL Inheritance Flags on a ZFS File
You can only set inheritance flags on ZFS files. When setting ACLs on directories, several inheritance flags can be optionally set.
Suppose you have an ACL entry for user lp that you want to be inherited to newly created files in a directory. First, you need to create an inheritable ACL entry on the directory:
example% chmod A+user:lp:read_data:file_inherit:allow test.dir example% ls -dv test.dir drwxr-xr-x+ 2 marks staff 2 Aug 23 09:08 test.dir/ 0:user:lp:read_data:file_inherit:allow 1:owner@::deny 2:owner@:list_directory/read_data/add_file/write_data/add_subdirectory /append_data/write_xattr/execute/write_attributes/write_acl /write_owner:allow 3:group@:add_file/write_data/add_subdirectory/append_data:deny 4:group@:list_directory/read_data/execute:allow 5:everyone@:add_file/write_data/add_subdirectory/append_data/write_xattr /write_attributes/write_acl/write_owner:deny 6:everyone@:list_directory/read_data/read_xattr/execute/read_attributes /read_acl/synchronize:allow
The lp entry is inherited to newly created files in the directory test.dir.
example% touch test.dir/file.test example% ls -v test.dir/file.test -rw-r--r--+ 1 marks staff 0 Oct 9 16:29 test.dir/file.test 0:user:lp::deny 1:user:lp:read_data:allow 2:owner@:execute:deny 3:owner@:read_data/write_data/append_data/write_xattr/ write_attributes/write_acl/write_owner:allow 4:group@:write_data/append_data/execute:deny 5:group@:read_data:allow 6:everyone@:write_data/append_data/write_xattr/execute/ write_attributes/write_acl/write_owner:deny 7:everyone@:read_data/read_xattr/read_attributes/read_acl/ synchronize:allow
The user lp entry is inherited to the newly created file. Multiple combinations of the inheritance flags can be specified. For example, if you wanted the lp entry to also be inherited to directories, then the following command can be used:
example% chmod A+user:lp:read_data:file_inherit/\ dir_inherit:allow test.dir
See environ(5) for descriptions of the following environment variables that affect the execution of chmod: LANG, LC_ALL, LC_CTYPE, LC_MESSAGES, and NLSPATH.
The following exit values are returned:
Successful completion.
An error occurred.
See attributes(5) for descriptions of the following attributes:
|
getfacl(1), ls(1), setfacl(1), chmod(2), acl(5), attributes(5), environ(5), largefile(5), standards(5)
Absolute changes do not work for the set-group-ID bit of a directory. You must use g+s or g-s.
chmod permits you to produce useless modes so long as they are not illegal (for instance, making a text file executable). chmod does not check the file type to see if mandatory locking is meaningful.
If the filesystem is mounted with the nosuid option, setuid execution is not allowed.
If you use chmod to change the file group owner permissions on a file with ACL entries, both the file group owner permissions and the ACL mask are changed to the new permissions. Be aware that the new ACL mask permissions can change the effective permissions for additional users and groups who have ACL entries on the file. Use the getfacl(1) or ls(1)command to make sure the appropriate permissions are set for all ACL entries.