From Luke Jackson

The chmod command (abbreviated from change mode) is a UNIX shell command in Unix and Unix-like environments.

When executed, the command can change file system modes of Computer file|files and directory (file systems)|directories. The modes include permissions and special modes.

Contents 1 History 2 Usage 3 Options 4 String modes 4.1 String mode examples 5 Octal numbers 6 Special modes 7 Examples 8 Sources

History

A chmod command first appeared in AT&T Unix version 1, and is still used today on Unix-like machines.

Usage

The chmod command options are specified like this:

$ chmod [options] mode file1 ...

To view what the permissions currently are, type:

$ ls -l

Options

The chmod command has a number of command line options that affect its behavior. The most common options are:

• -R: Changes the modes of directories and files recursion|recursively
• -v: Verbose mode; lists all files as they are being processed

String modes

See also: File system permissions#Symbolic notation|Symbolic notation of file system permissions

To the chmod utility, all permissions and special modes are represented by its mode parameter. One way to adjust the mode of files or directories is to specify a symbolic mode. The symbolic mode is composed of three components, which are combined to form a single string of text:

$ chmod [references][operator][modes] file1 ...

The references (or classes) are used to distinguish the users to whom the permissions apply. If no references are specified it defaults to "all". They 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 taken away 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	Special modes section
t	sticky	Special modes section

The combination of these three components produces a string that is understood by the chmod command. Multiple changes can be specified by separating multiple symbolic modes with commas.

String mode examples

For example, the following command would be used to add the read and write permissions to the user and group classes of a file or directory named sample:

$ chmod ug+rw sample
$ ls -ld sample
drw-rw----   2 unixguy  unixguy       96 Dec  8 12:53 sample

This command removes all permissions, and allows no one to read, write, or execute the file named sample.

$ chmod a-rwx sample
$ ls -l sample
----------   2 unixguy  unixguy       96 Dec  8 12:53 sample

The following command changes the permissions for the user and the group to read and execute only (no write permission) on sample .

Sample file permissions before command
$ ls -ld sample
drw-rw----   2 unixguy  unixguy       96 Dec  8 12:53 sample
$ chmod ug=rx sample
$ ls -ld sample
dr-xr-x---   2 unixguy  unixguy       96 Dec  8 12:53 sample

Octal numbers

The chmod command also accepts three- and four-digit octal numbers representing modes. See the article mentioned above for more. Using a four-digit octal number to set the modes of a file or directory named sample would look something like this:

$ chmod 0664 sample

Assuming that the setuid, setgid and sticky bits are not set, this is equivalent to:

$ chmod 664 sample

or

$ chmod +r,-x,ug+w sample

Special modes

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 bit|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.

Examples

• chmod +r file – read is added for all
• chmod -x file – execute permission is removed for all
• chmod u=rw,go= file – read and write is set for the owner, all permissions are cleared for the group and others
• chmod +rw file – change the permissions of the file file to read and write for all.
• 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 666 file – ADDS! read and write access for the owner, the group, and 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 Setuid|set user ID.
• find path/ -type d -exec chmod a-x {} \; – sets a-x for all directories in tree starting from path/ (use '-type f' to match files only).
• find path/ -type d -exec chmod 777 {} \; allows directory browsing ls for example if you've reset permissions for Samba write access.
• 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 all.

Sources

• http://en.wikipedia.org/wiki/Chmod