chmod

In Unix-like operating systems, chmod is the system 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.^[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.

Command syntax

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

^[3]

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 owner, read and executed by other members of the staff group and can also be read by others.

Octal modes

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

The chmod command also accepts a finer-grained symbolic notation,^[4] 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`	owner	file's owner
`g`	group	users who are 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 owner, 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). 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.^[5]

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 owner 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

Command line examples

`Command`	Explanation
`chmod a+r publicComments.txt`	adds read permission for all classes (i.e. owner, group and others)
`chmod +r publicComments.txt`	adds read permission for all classes depending on umask
`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 owner, 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 owner and group
`chmod 664 global.txt`	sets read and write permissions for owner and group, and provides read to others.
`chmod 0744 myCV.txt`	sets read, write, and execute permissions for owner, and sets read permission for group and others (the `0` specifies no special modes)
`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 owner, and sets read and execute permissions for group and others
`chmod 2755 setCtrls.sh`	sets GID, sets read, write, and execute permissions for owner, 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 owner, 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:^[6]

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

References

↑ Tutorial for chmod
↑ "AIX 5.3 System management". IBM knowledge Center. IBM. Retrieved 30 August 2015.
↑ chmod
↑ "AIX 5.5 Commands Reference". IBM Knowledge Center. IBM. Retrieved 30 August 2015.
↑ http://teaching.idallen.com/cst8207/12f/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 — 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

Unix command-line interface programs and shell builtins

File system	cat chmod chown chgrp cksum cmp cp dd du df file fsck fuser ln ls mkdir mount mv pax pwd rm rmdir split tee touch type umask

Processes	at bg chroot cron fg kill killall nice pgrep pkill ps pstree time top

User environment	clear env exit finger history logname mesg passwd su sudo uptime talk tput uname w wall who whoami write

Text processing	awk banner basename comm csplit cut diff dirname ed ex fmt fold head iconv join less more nl paste printf sed sort spell strings tail tr uniq vi wc xargs yes

Shell builtins	alias cd echo test unset wait

Networking	dig host ifconfig inetd netcat netstat nslookup ping rdate rlogin route ssh traceroute

Searching	find grep locate whatis whereis

Documentation	apropos help man

Miscellaneous	bc dc cal expr lp od sleep true and false

List of Unix commands

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