chmod

chmod
	The chmod command
Developer(s)	AT&T Bell Laboratories
Initial release	3 November 1971; 53 years ago
Operating system	Unix and Unix-like
Type	Command

In Unix and Unix-like operating systems, chmod is the command and system call which is used to change the access permissions of file system objects (files and directories). It is also used to change special mode flags. The request is filtered by the umask. The name is an abbreviation of change mode.^[1]

History

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

As systems grew in number and types of users, access control lists^[2] were added to many file systems in addition to these most basic modes to increase flexibility.

The version of chmod bundled in GNU coreutils was written by David MacKenzie and Jim Meyering.^[3]

Command syntax

Throughout this section, user refers to the owner of the file, as a reminder that the symbolic form of the command uses "u".

chmod [options] mode[,mode] file1 [file2 ...]^[4]

Usually implemented options include:

-R Recursive, i.e. include objects in subdirectories.
-v verbose, show objects changed (unchanged objects are not shown).

If a symbolic link is specified, the target object is affected. File modes directly associated with symbolic links themselves are typically not 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. 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 dgerman; read and executed by members of the staff group; and only read by any other users.

Octal modes

The main parts of the chmod permissions:

For example: drwxrwx---

The characters to the right of the "d" define permissions for each class:

the three leftmost characters, rwx, define permissions for the user class (i.e. the file owner).
the middle three characters, rwx, define permissions for the Group class.
the last three characters, ---, define permissions for the Others class. In this example, users who are not the owner of the file and who are not members of the Group (and, thus, are in the Others class) have no permission to access the file.

Numerical permissions

The chmod numerical format accepts up to four octal digits. The three rightmost digits define permissions for the file user, the group, and others. The optional leading digit, when 4 digits are given, specifies the special setuid, setgid, and sticky flags. Each digit of the three rightmost digits represents a binary value, which controls the "read", "write" and "execute" permissions respectively. A value of 1 means a class is allowed that action, while a 0 means it is disallowed.

#	Permission	rwx	Binary
7	read, write and execute	rwx	111
6	read and write	rw-	110
5	read and execute	r-x	101
4	read only	r--	100
3	write and execute	-wx	011
2	write only	-w-	010
1	execute only	--x	001
0	none	---	000

For example, 754 would allow:

"read" (4), "write" (2), and "execute" (1) for the user class, as the binary value of 7 (4+2+1) is 111.
"read" (4) and "execute" (1) for the Group class, as the binary value of 5 (4+1) is 101.
Only "read" (4) for the Others class, as the binary value of 4 (4) is 100.

Numeric example

Change permissions to permit 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

The chmod command also accepts a finer-grained symbolic notation,^[5] 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 ...

Classes of users are used to distinguish to whom the permissions apply. If no classes are specified "all" is implied. The classes are represented by one or more of the following letters:

Reference	Class	Description
u	user	file owner
g	group	members of the file's group
o	others	users who are neither the file's owner 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 Others). It is only really useful when used with `+` and usually in combination with the `-R` flag for giving Group or Others 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). If a user is not specified, chmod will check the umask and the effect will be as if "a" was specified except bits that are set in the umask are not affected.^[6]

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 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

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`	adds read permission for all classes (i.e. user, Group and Others)
`chmod a-x publicComments.txt`	removes execute permission for all classes
`chmod a+rx viewer.sh`	adds read and execute permissions for all classes
`chmod u=rw,g=r,o= internalPlan.txt`	sets read and write permission for user, sets read for Group, and denies access for Others
`chmod -R u+w,go-w docs`	adds write permission to the directory docs and all its contents (i.e. Recursively) for owner, and removes write permission for group and others
`chmod ug=rw groupAgreements.txt`	sets read and write permissions for user and Group
`chmod 664 global.txt`	sets read and write permissions for user and Group, and provides read to Others.
`chmod 744 myCV.txt`	sets read, write, and execute permissions for user, and sets read permission for Group and Others
`chmod 1755 findReslts.sh`	sets sticky bit, sets read, write, and execute permissions for owner, and sets read and execute permissions for group and others (this suggests that the script be retained in memory)
`chmod 4755 setCtrls.sh`	sets UID, sets read, write, and execute permissions for user, and sets read and execute permissions for Group and Others
`chmod 2755 setCtrls.sh`	sets GID, sets read, write, and execute permissions for user, and sets read and execute permissions for Group and Others
`chmod -R u+rwX,g-rwx,o-rx personalStuff`	Recursively (i.e. on all files and directories in personalStuff) adds read, write, and special execution permissions for user, removes read, write, and execution permissions for Group, and removes read and execution permissions for Others
`chmod -R a-x+X publicDocs`	Recursively (i.e. on all files and directories in publicDocs) removes execute permission for all classes and adds special execution permission for all classes

System call

The POSIX standard defines the following function prototype:^[7]

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 user
S_IWUSR, S_IWRITE	00200	Write by user
S_IXUSR, S_IEXEC	00100	Execute/search by user
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

References

^ Tutorial for chmod
^ "AIX 5.3 System management". IBM knowledge Center. IBM. Retrieved 30 August 2015.
^ https://linux.die.net/man/1/chmod
^ chmod
^ "AIX 5.5 Commands Reference". IBM Knowledge Center. IBM. Retrieved 30 August 2015.
^ http://teaching.idallen.com/cst8207/19w/notes/510_umask.html
^ "chmod function". The Open Group Base Specifications Issue 7, 2013 Edition. The Open Group. Retrieved 30 August 2015.

External links

chmod(1): change file modes – FreeBSD General Commands Manual
chmod(1) – Plan 9 Programmer's Manual, Volume 1
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

[1] Tutorial for chmod

[2] "AIX 5.3 System management". IBM knowledge Center. IBM. Retrieved 30 August 2015.

[3] ttps://linux.die.net/man/1/chmod

[4] ^ chmod

[5] "AIX 5.5 Commands Reference". IBM Knowledge Center. IBM. Retrieved 30 August 2015.

[6] ttp://teaching.idallen.com/cst8207/19w/notes/510_umask.html

[7] "chmod function". The Open Group Base Specifications Issue 7, 2013 Edition. The Open Group. Retrieved 30 August 2015.

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v t e Unix command-line interface programs and shell builtins
File system	cat chattr chmod chown chgrp cksum cmp cp dd du df file fuser ln ls mkdir mv pax pwd rm rmdir split tee touch type umask
Processes	at bg crontab fg kill nice ps time
User environment	env exit logname mesg talk tput uname who write
Text processing	awk basename comm csplit cut diff dirname ed ex fold head iconv join m4 more nl paste patch printf read sed sort strings tail tr troff uniq vi wc xargs
Shell builtins	alias cd echo test unset wait
Searching	find grep
Documentation	man
Software development	ar ctags lex make nm strip yacc
Miscellaneous	bc cal expr lp od sleep true and false
Categories Standard Unix programs Unix SUS2008 utilities List