A Unix shell is a command-line interpreter or shell that provides a traditional user interface for the Unix operating system and for Unix-like systems...
command that lets a user tell the system how much (or little) access it should permit to a file. It changes the file system modes of files
Computer file
A computer file is a block of arbitrary information, or resource for storing information, which is available to a computer program and is usually based on some kind of durable storage. A file is durable in the sense that it remains available for programs to use after the current program has finished...
In computing, a folder, directory, catalog, or drawer, is a virtual container originally derived from an earlier Object-oriented programming concept by the same name within a digital file system, in which groups of computer files and other folders can be kept and organized.A typical file system may...
Most current file systems have methods of administering permissions or access rights to specific users and groups of users. These systems control the ability of the users to view or make changes to the contents of the filesystem....
C is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system....
Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...
A Unix-like operating system is one that behaves in a manner similar to a Unix system, while not necessarily conforming to or being certified to any version of the Single UNIX Specification....
AT&T Inc. is an American multinational telecommunications corporation headquartered in Whitacre Tower, Dallas, Texas, United States. It is the largest provider of mobile telephony and fixed telephony in the United States, and is also a provider of broadband and subscription television services...
Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...
version 1, and is still used today on Unix like machines.
Usage
The chmod command options are specified like this:
$ chmod [options] mode[,mode] file1 [file2 ...]
This is used to control the file mode.
To view the current file mode:
$ ls -l file
or use the stat command to view the octal numerical values ("*" lists all files in current directory)
$ stat -c '%A %a %n' *
Octal numbers
See also: Octal notation of file system permissions
The chmod command accepts up to four digits to represent an octal number. The octets refer to bits applied to the file owner, group and other users, respectively. Use of three digits is discouraged because it leaves the fourth as the default and this value is not fixed. The least significant digit sets/resets an additional mode for each of these three sets of bits. Experienced Unix and Linux users tend to recommend that the user of this command check the man page (man chmod) on the system of interest.
Particular care should be taken when a directory is the target because the effect is not intuitive. In addition, it will not work on all file types. For example, it has no effect on a symbolic link. myfile :
Since the setuid, setgid and sticky bits are not set, this is equivalent to:
$ chmod 0664 myfile
Symbolic modes
See also: Symbolic notation of file system permissions
chmod also accepts finer-grained symbolic notation, 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” but modifies only the permissions allowed by the umask
Umask
umask is a command and a function in POSIX environments that sets the file mode creation mask of the current process which limits the permission modes for files and directories created by the process...
. 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 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.
Numerical Permissions
#
Permission
7
full
6
read and write
5
read and execute
4
read only
3
write and execute
2
write only
1
execute only
0
none
Symbolic examples
Add the read and write permissions to the user and group classes of a directory:
$ chmod ug+rw mydir
$ ls -ld mydir
drw-rw---- 2 unixguy uguys 96 Dec 8 12:53 mydir
For a file, remove write permissions for all classes:
$ chmod a-w myfile
$ ls -l myfile
-r-xr-xr-x 2 unixguy uguys 96 Dec 8 12:53 myfile
Set the permissions for the user and the group to read and execute only (no write permission) on mydir.
$ chmod ug=rx mydir
$ ls -ld mydir
dr-xr-x--- 2 unixguy uguys 96 Dec 8 12:53 mydir
Most current file systems have methods of administering permissions or access rights to specific users and groups of users. These systems control the ability of the users to view or make changes to the contents of the filesystem....
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
Setuid
setuid and setgid are Unix access rights flags that allow users to run an executable with the permissions of the executable's owner or group...
In computing, the sticky bit is an access-right flag that can be assigned to files and directories on Unix systems.-History:The sticky bit was introduced in the Fifth Edition of Unix in 1974 for use with pure executable files. When set, it instructed the operating system to retain the text segment...
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.
setuid and setgid are Unix access rights flags that allow users to run an executable with the permissions of the executable's owner or group...
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.
Function details
For C programming language, the function prototype is defined:
int chmod(const char *path, mode_t mode);
The function takes a parameter of type mode_t, which 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.
Various error codes can be returned, and these are detailed in the associated man page for the function.
Most current file systems have methods of administering permissions or access rights to specific users and groups of users. These systems control the ability of the users to view or make changes to the contents of the filesystem....
The chown command is used on Unix-like systems to change the owner of a file. In most implementations, it can only be executed by the superuser. Unprivileged users who wish to change the group of a file that they own may use chgrp.-Usage examples:These examples illustrate typical syntax and use...
, the command used to change the owner of a file or directory on Unix-like systems
cacls and icacls are Microsoft Windows native command line utilities capable of displaying and modifying the access control lists on folders and files...
Windows NT is a family of operating systems produced by Microsoft, the first version of which was released in July 1993. It was a powerful high-level-language-based, processor-independent, multiprocessing, multiuser operating system with features comparable to Unix. It was intended to complement...
and its derivatives to modify the access control lists associated with a file or directory
Unix-like operating systems identify users within the kernel by an unsigned integer value called a user identifier, often abbreviated to UID or User ID...
In Unix-like systems, multiple users can be categorized into groups. POSIX and conventional Unix file system permissions are organized into three classes, user, group, and others. The use of groups allows additional abilities to be delegated in an organized fashion, such as access to disks,...
An access control list , with respect to a computer file system, is a list of permissions attached to an object. An ACL specifies which users or system processes are granted access to objects, as well as what operations are allowed on given objects. Each entry in a typical ACL specifies a subject...
