The
chmod command (abbreviated from
change
mode) is a
shell command in
Unix and
Unix-like environments.
When executed, the command can change file system modes of
files and
directories. The modes include
permissions and special modes.
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 recursively
-v: Verbose mode; lists all files as they are being processed
String modes
:''See also:
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 || details in Special modes section
|-
|t || sticky || details in 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
:''See also:
Octal notation of file system permissions''
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
:''See also:
Additional 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.
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 – removes 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 ''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 – set a directory tree to rwx for owner directories, rw for owner files, --- for group and all.