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chmod

InUnix 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[edit]

Achmod 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[edit]

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 lsorstat 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[edit]

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.

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[edit]

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[edit]

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:

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

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:

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[edit]

• 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[edit]

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[edit]

See also[edit]

• File-system permissions
• chattr, the command used to change the attributes of a file or directory on Linux systems
• chown, the command used to change the owner of a file or directory on Unix-like systems
• chgrp, the command used to change the group of a file or directory on Unix-like systems
• cacls, a command used on Windows NT and its derivatives to modify the access control lists associated with a file or directory
• attrib
• umask, restricts mode (permissions) at file or directory creation on Unix-like systems
• User identifier
• Group identifier
• List of Unix commands

References[edit]

External links[edit]

The Wikibook Guide to Unix has a page on the topic of: Commands

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