chmod
Original author(s)AT&T Bell Laboratories
Developer(s)Various open-source and commercial developers
Initial release3 November 1971 (1971-11-03)
Written inPlan 9: C
Operating systemUnix, Unix-like, Plan 9, Inferno, IBM i
PlatformCross-platform
TypeCommand
Licensecoreutils: GPLv3
Plan 9: MIT License

In Unix and Unix-like operating systems, chmod is the command and system call used to change the access permissions and the special mode flags (the setuid, setgid, and sticky flags) of file system objects (files and directories). Collectively these were originally called its modes,[1] and the name chmod was chosen as an abbreviation of change mode.[2]

History

A chmod command first appeared in AT&T Unix version 1, along with the chmod system call.

As systems grew in number and types of users, access-control lists[3] 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.[4] The command is available as a separate package for Microsoft Windows as part of the UnxUtils collection of native Win32 ports of common GNU Unix-like utilities.[5] The chmod command has also been ported to the IBM i operating system.[6]

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 ...][7]

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). The script findPhoneNumbers.sh 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.

The main parts of the chmod permissions:

For example: rwxr-x---

Each group of three characters 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, r-x, define permissions for the Group class (i.e. the group owning the file)
  • the rightmost 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 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.

#Sum rwx Permission
7 4(r) + 2(w) + 1(x) rwx read, write and execute
6 4(r) + 2(w) rw- read and write
5 4(r)        + 1(x) r-x read and execute
4 4(r) r-- read only
3        2(w) + 1(x) -wx write and execute
2        2(w) -w- write only
1               1(x) --x execute only
0 0 --- none

For example, 754 would allow:

  • "read" (4), "write" (2), and "execute" (1) for the User class; i.e., 7 (4 + 2 + 1).
  • "read" (4) and "execute" (1) for the Group class; i.e., 5 (4 + 1).
  • Only "read" (4) for the Others class.

A numerical code permits execution if and only if it is odd (i.e. 1, 3, 5, or 7). A numerical code permits "read" if and only if it is greater than or equal to 4 (i.e. 4, 5, 6, or 7). A numerical code permits "write" if and only if it is 2, 3, 6, or 7.

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,[8] 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:

ReferenceClassDescription
uuserfile owner
ggroupmembers of the file's group
oothersusers who are neither the file's owner nor members of the file's group
aallall three of the above, same as ugo
(empty)defaultsame as "all", except that bits in the umask will be unchanged

The chmod program uses an operator to specify how the modes of a file should be adjusted. The following operators are accepted:

OperatorDescription
+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:

ModeNameDescription
rreadread a file or list a directory's contents
wwritewrite to a file or directory
xexecuteexecute a file or recurse a directory tree
Xspecial executewhich 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
ssetuid/gid
tsticky

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.[9]

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 jsmitt  northregion 96 Apr 8 12:53 shared_dir
$ chmod  g+w shared_dir
$ ls -ld shared_dir  # show access modes after chmod
drwxrwxr-x   2 jsmitt  northregion 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 tmiller  northregion 96 Apr 8 12:53 ourBestReferenceFile
$ chmod a-w ourBestReferenceFile
$ ls -l ourBestReferenceFile
-r--r--r--   2 tmiller  northregion 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 from adding files.
    $ ls -ld referenceLib
drwxr-----   2 ebowman  northregion 96 Apr 8 12:53 referenceLib
$ chmod ug=rx referenceLib
$ ls -ld referenceLib
dr-xr-x---   2 ebowman  northregion 96 Apr 8 12:53 referenceLib
  • 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 rsanchez  budget       96 Dec  8 12:53 sample
  • Remove all permissions, allowing no one to read, write, or execute the file named sample to no useful end.
    $ chmod a-rwx sample
$ ls -l sample
----------   2 rswven  planning       96 Dec  8 12:53 sample
  • Change 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 oschultz  warehousing       96 Dec  8 12:53 NY_DBs
$ chmod ug=rx sample
$ ls -ld sample
dr-xr-x---   2 oschultz  warehousing       96 Dec  8 12:53 NJ_DBs

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 numerically, particularly in octal, but some do not. On these systems, only the symbolic modes can be used.

Command line examples

CommandExplanation
chmod a+r publicComments.txtAdds read permission for all classes (i.e. user, Group and Others)
chmod a-x publicComments.txtRemoves execute permission for all classes
chmod a+rx viewer.shAdds read and execute permissions for all classes
chmod u=rw,g=r,o= internalPlan.txtSets read and write permission for user, sets read for Group, and denies access for Others
chmod -R u+w,go-w docsAdds 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.txtSets read and write permissions for user and Group
chmod 664 global.txtSets read and write permissions for user and Group, and provides read to Others.
chmod 744 Show_myCV.shsets read, write, and execute permissions for user, and sets read permission for Group and Others
chmod 1755 findReslts.shSets sticky bit (this suggests that the script be retained in memory), sets read, write, and execute permissions for owner, and sets read and execute permissions for group and others
chmod 4755 setCtrls.shSets UID, sets read, write, and execute permissions for user, and sets read and execute permissions for Group and Others
chmod 2755 setCtrls.shsets GID, Sets read, write, and execute permissions for user, and sets read and execute permissions for Group and Others
chmod -R u+rw,g-,o-rx privateStuffRecursively (i.e. on all files and directories in privateStuff) adds read, write permissions for user, removes read, write, and execution permissions for Group, and removes read and execution permissions for Others
chmod -R a-x+X publicDocsRecursively (i.e. on all files and directories in publicDocs) removes execute permission for all classes and adds special execution permission for all classes

See also

References

  1. ↑ The modes/permissions are shown when listing files in long format.
  2. ↑ "Tutorial for chmod". catcode.com.
  3. ↑ "AIX 5.3 System management". IBM knowledge Center. IBM. Retrieved 30 August 2015.
  4. ↑ "chmod(1): change file mode bits - Linux man page". linux.die.net.
  5. ↑ "Native Win32 ports of some GNU utilities". unxutils.sourceforge.net.
  6. ↑ IBM. "IBM System i Version 7.2 Programming Qshell" (PDF). IBM. Retrieved 5 September 2020.
  7. ↑ "chmod Man Page with examples and calculator - Linux - SS64.com". ss64.com.
  8. ↑ "AIX 5.5 Commands Reference". IBM Knowledge Center. IBM. Retrieved 30 August 2015.
  9. ↑ "Permissions masking with umask, chmod, 777 octal permissions". teaching.idallen.com.
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