chmod

In Unix-like operating systems, chmod is the command and system call which may change the access permissions to file system objects (files and directories). It may also alter special mode flags. The request is filtered by the umask. The name is an abbreviation of change mode.

History

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

As systems grew in number and types of users access control lists were added to many file systems in addition to these most basic modes to increase flexibility.

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 findPhoneNumbers.sh
-rwxr-xr--  1 dgerman  staff  823 Dec 16 15:03 findPhoneNumbers.sh
$ stat -c %a findPhoneNumbers.sh
754

The r, w, and x specify the read, write, and execute access, respectively. The first character of the ls display denotes the object type; a hyphen represents a plain file. This script 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 numerical format accepts up to four octal digits. The three rightmost digits refer to permissions for the file owner, the group, and other users. The optional leading digit (when 4 digits are given) specifies the special setuid, setgid, and sticky flags.

Numerical permissions

#	Permission	rwx
7	read, write and execute	rwx
6	read and write	rw-
5	read and execute	r-x
4	read only	r--
3	write and execute	-wx
2	write only	-w-
1	execute only	--x
0	none	---

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 neither the owner of the file nor members of the file's group
a	all	all three of the above, 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 one 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 (without spaces).

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

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 referenceLib
dr-xr-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 publicComments.txt	read is added for all classes (i.e. User, Group and Others).
chmod +r publicComments.txt	omitting the class defaults to all classes, but the resultant permissions are dependent on umask
chmod a-x publicComments.txt	execute permission is removed for all classes.
chmod a+rx viewer.sh	add read and execute for all classes.
chmod u=rw,g=r,o= internalPlan.txt	user(i.e. owner) can read and write, group can read, Others cannot access.
chmod -R u+w,go-w docs	add write permissions to the directory docs and all its contents (i.e. Recursively) for user and deny write access for everybody else.
chmod ug=rw groupAgreements.txt	User and Group members can read and write (update the file).
chmod 664 global.txt	sets read and write and no execution access for the user and group, and read, no write, no execute for all others.
chmod 0744 myCV.txt	equivalent to u=rwx (400+200+100),go=r (40+ 4). The 0 specifies no special modes.
chmod 1755 findReslts.sh	the 1000 specifies set sticky bit and the rest is equivalent to u=rwx (400+200+100),go=rx (40+10 + 4+1) This suggests that the script be retained in memory.
chmod 4755 SetCtrls.sh	the 4 specifies set user ID and the rest is equivalent to u=rwx (400+200+100),go=rx (40+10 + 4+1).
chmod 2755 SetCtrls.sh	the 2 specifies set group ID and the rest is equivalent to u=rwx (400+200+100),go=rx (40+10 + 4+1).
chmod -R u+rwX,g-rwx,o-rx PersonalStuff	Recursively set a directory tree to rwx for owner directories, rw for owner files, --- (i.e. no access) for group and others.
chmod -R a-x+X publicDocs	remove the execute permission on all files in a directory tree (i.e. Recursively), 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

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
• umask, restricts mode (permissions) at file or directory creation on Unix-like systems
• 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
• CHMOD-Win 3.0 — Freeware Windows' ACL ←→ CHMOD converter.
• Beginners tutorial with on-line "live" example
• Online Chmod Calculator