chmod

In Unix-like operating systems, chmod is the name of a Unix shell command and a system call, both used to change the access to file system objects (including files and directories), as well as specifying special flags. The name is an abbreviation of change mode.

History

A chmod command first appeared in AT&T Unix version 1.

Command syntax

chmod [options] mode[,mode] file1 [file2 ...]

Usual implemented options include:

• -R recursive, i.e. include objects in subdirectories
• -f force, forge ahead with all objects even if errors occur
• -v verbose, show objects processed

If a symbolic link is specified, the target object is affected. File modes directly associated with symbolic links themselves are typically never used.

To view the file mode, the ls or stat commands may be used:

$ ls -l phoneNumbers
-rwxr-xr--  1 dgerman  staff  823 Dec 16 15:03 phoneNumbers
$ stat -c %a phoneNumbers
754

The r, w, and x specify the read, write, and execute access. This file can be read, written to, and executed by the user, read and executed by other members of the staff group and can also be read by others.

Octal modes

See also: Octal notation of file system permissions

The chmod numeric format accepts up to four octal digits. The rightmost three refer to permissions for the file owner, the group and other users. The next digit (fourth from the right) specifies special setuid, setgid and sticky flags.

Numerical permissions

#	Permisssion	rwx
7	full	111
6	read and write	110
5	read and execute	101
4	read only	100
3	write and execute	011
2	write only	010
1	execute only	001
0	none	000

Numeric example

In order to permit all users who are members of the programmers group to update a file

$ ls -l sharedFile
-rw-r--r--  1 jsmith programmers 57 Jul  3 10:13  sharedFile
$ chmod 664 sharedFile
$ ls -l sharedFile
-rw-rw-r--  1 jsmith programmers 57 Jul  3 10:13  sharedFile

Since the setuid, setgid and sticky bits are not specified, this is equivalent to:

$ chmod 0664 sharedFile

Symbolic modes

See also: Symbolic notation of file system permissions

The chmod command also accepts a finer-grained symbolic notation, which allows modifying specific modes while leaving other modes untouched. The symbolic mode is composed of three components, which are combined to form a single string of text:

$ chmod [references][operator][modes] file ...

The references (or classes) are used to distinguish the users to whom the permissions apply. If no references are specified it defaults to “all” but modifies only the permissions allowed by the umask. The references are represented by one or more of the following letters:

Reference	Class	Description
u	user	the owner of the file
g	group	users who are members of the file's group
o	others	users who are not the owner of the file or members of the group
a	all	all three of the above, is the same as ugo

The chmod program uses an operator to specify how the modes of a file should be adjusted. The following operators are accepted:

Operator	Description
+	adds the specified modes to the specified classes
-	removes the specified modes from the specified classes
=	the modes specified are to be made the exact modes for the specified classes

The modes indicate which permissions are to be granted or removed from the specified classes. There are three basic modes which correspond to the basic permissions:

Mode	Name	Description
r	read	read a file or list a directory's contents
w	write	write to a file or directory
x	execute	execute a file or recurse a directory tree
X	special execute	which is not a permission in itself but rather can be used instead of x. It applies execute permissions to directories regardless of their current permissions and applies execute permissions to a file which already has at least 1 execute permission bit already set (either user, group or other). It is only really useful when used with '+' and usually in combination with the -R option for giving group or other access to a big directory tree without setting execute permission on normal files (such as text files), which would normally happen if you just used "chmod -R a+rx .", whereas with 'X' you can do "chmod -R a+rX ." instead
s	setuid/gid	details in Special modes section
t	sticky	details in Special modes section

Multiple changes can be specified by separating multiple symbolic modes with commas.

Symbolic examples

Add write permission (w) to the group's(g) access modes of a directory,
allowing users in the same group to add files:

$ ls -ld shared_dir # show access modes before chmod
drwxr-xr-x   2 teamleader  usguys 96 Apr 8 12:53 shared_dir
$ chmod  g+w shared_dir
$ ls -ld shared_dir  # show access modes after chmod
drwxrwxr-x   2 teamleader  usguys 96 Apr 8 12:53 shared_dir

Remove write permissions (w) for all classes a)
preventing anyone from writing to the file:

$ ls -l ourBlestReferencefile
-rw-rw-r--   2 teamleader  usguys 96 Apr 8 12:53 ourBlestReferencefile
$ chmod a-w ourBlestReferencefile
$ ls -l ourBlestReferencefile
-r--r--r--   2 teamleader  usguys 96 Apr 8 12:53 ourBlestReferencefile

Set the permissions for the user and the group (ug) to read and execute (rx) only (no write permission) on referenceLib
Preventing anyone other than the owner to add files.

$ ls -ld referenceLib
drwxr-----   2 teamleader  usguys 96 Apr 8 12:53 referenceLib
$ chmod ug=rx referenceLib
$ ls -ld mydir
drwxr-x---   2 teamleader  usguys 96 Apr 8 12:53 referenceLib

Special modes

See also: File system permissions

The chmod command is also capable of changing the additional permissions or special modes of a file or directory. The symbolic modes use s to represent the setuid and setgid modes, and t to represent the sticky mode. The modes are only applied to the appropriate classes, regardless of whether or not other classes are specified.

Most operating systems support the specification of special modes using octal modes, but some do not. On these systems, only the symbolic modes can be used.

Command line examples

command	explanation
chmod a+r file	read is added for all
chmod a-x file	execute permission is removed for all
chmod a+rw file	change the permissions of the file file to read and write for all.
chmod +rwx file	On some UNIX platforms such as BSD, this will restore the permission of the file file to default: -rwxr-xr-x.
chmod u=rw,go= file	read and write is set for the owner, all permissions are cleared for the group and others
chmod -R u+w,go-w docs	change the permissions of the directory docs and all its contents to add write access for the user, and deny write access for everybody else.
chmod file	removes all privileges for all.
chmod 777 file	change the permissions of the file file to read, write, and execute for all.
chmod 664 file	sets read and write and no execution access for the owner and group, and read, no write, no execute for all others.
chmod 0755 file	equivalent to u=rwx (4+2+1),go=rx (4+1 & 4+1). The 0 specifies no special modes.
chmod 4755 file	the 4 specifies set user ID and the rest is equivalent to u=rwx (4+2+1),go=rx (4+1 & 4+1).
chmod 2755 file	the 2 specifies set group ID and the rest is equivalent to u=rwx (4+2+1),go=rx (4+1 & 4+1).
chmod -R u+rwX,g-rwx,o-rwx directory	set a directory tree to rwx for owner directories, rw for owner files, --- for group and others.
chmod -R a-x+X directory	remove the execute permission on all files in a directory tree, while allowing for directory browsing.

System call

The POSIX standard defines the following function prototype:

int chmod(const char *path, mode_t mode);

The mode parameter is a bitfield composed of various flags:

Flag	Octal value	Purpose
S_ISUID	04000	Set user ID on execution
S_ISGID	02000	Set group ID on execution
S_ISVTX	01000	Sticky bit
S_IRUSR, S_IREAD	00400	Read by owner
S_IWUSR, S_IWRITE	00200	Write by owner
S_IXUSR, S_IEXEC	00100	Execute/search by owner
S_IRGRP	00040	Read by group
S_IWGRP	00020	Write by group
S_IXGRP	00010	Execute/search by group
S_IROTH	00004	Read by others
S_IWOTH	00002	Write by others
S_IXOTH	00001	Execute/search by others

Where alternate flag names are given, one of the pair of names might not be supported on some OSs. The octal values of the flags are summed or combined in a bitwise OR operation to give the desired permission mode.

The function returns an error code.

See also

• File system permissions
• Modes (Unix)
• chown, the command used to change the owner of a file or directory on Unix-like systems
• chgrp, the command used to change the group of a file or directory on Unix-like systems
• cacls, a command used on Windows NT and its derivatives to modify the access control lists associated with a file or directory
• attrib
• User identifier
• Group identifier
• List of Unix programs

External links

• chmod(1): change file modes – FreeBSD General Commands Manual
• chmod — manual page from GNU coreutils.
• GNU "Setting Permissions" manual
• Solaris 9 chmod man page
• Mac OS X chmod man page, which also supports access control lists.
• CHMOD-Win 3.0 — Freeware Windows' ACL ←→ CHMOD converter.
• Beginners tutorial with on-line "live" example
• chmod examples Searchable examples