Editing Chmod (section)

{{short description|Shell command for changing access permissions of a file}}
{{lowercase title}}
{{Infobox software
| name                   = chmod
| logo                   = 
| caption                = Example usage of {{code|chmod}} command to change the specified file's permissions
| author                 = [[AT&T Bell Laboratories]]
| developer              = Various [[open-source software|open-source]] and [[commercial software|commercial]] developers
| released               = {{Start date and age|df=yes|1971|11|3}}
| latest release version = 
| latest release date    = 
| programming language   = Plan 9: [[C (programming language)|C]]
| operating system       = [[Unix]], [[Unix-like]], [[Plan 9 from Bell Labs|Plan 9]], [[Inferno (operating system)|Inferno]], [[IBM i]]
| platform               = [[Cross-platform]]
| genre                  = [[Command (computing)|Command]]
| license                = [[coreutils]]: [[GPLv3]]<br />Plan 9: [[MIT License]]
| website                = 
}}
'''{{code|chmod}}''' is a [[shell (computing)|shell]] [[command (computing)|command]] for changing [[File-system permissions|access permissions]] and special mode flags of [[Computer file|files]] (including [[Unix file type|special files]] such as [[computer directory|directories]]). The name is short for '''''ch'''ange '''mod'''e'' where ''mode'' refers to the permissions and flags collectively.<ref>The modes/permissions are shown when [[ls|listing files]] in long format.</ref><ref>{{Cite web|url=http://catcode.com/teachmod/|title=Tutorial for chmod|website=catcode.com}}</ref>

The command originated in [[Research Unix|AT&T Unix]] version 1 and was exclusive to [[Unix]] and [[Unix-like]] [[operating system]]s until it was ported to other operating systems such as [[Microsoft Windows|Windows]] (in [[UnxUtils]])<ref>{{Cite web|url=http://unxutils.sourceforge.net/|title=Native Win32 ports of some GNU utilities|website=unxutils.sourceforge.net}}</ref> and [[IBM i]].<ref>{{cite web |title=IBM System i Version 7.2 Programming Qshell |language=en |author=IBM |website=[[IBM]] |author-link=IBM |url=https://www.ibm.com/support/knowledgecenter/ssw_ibm_i_74/rzahz/rzahzpdf.pdf?view=kc |access-date=2020-09-05 }}</ref>

In [[Unix]] and [[Unix-like]] operating systems, a [[system call]] with the same name as the command, {{mono|chmod()}}, provides access to the underlying access control data. The command exposes the capabilities of the system call to a shell user.

As the need for enhanced [[file-system permissions]] grew, [[access-control list]]s<ref>{{cite web|title=AIX 5.3 System management|url=http://www-01.ibm.com/support/knowledgecenter/#!/ssw_aix_53/com.ibm.aix.baseadmn/doc/baseadmndita/acl.htm?cp=ssw_aix_53|website=IBM knowledge Center|publisher=IBM|access-date=30 August 2015}}</ref> were added to many file systems to augment the modes controlled via {{code|chmod}}.

The implementation of {{code|chmod}} bundled in [[GNU Core Utilities|GNU coreutils]] was written by David MacKenzie and Jim Meyering.<ref>{{Cite web|url=https://linux.die.net/man/1/chmod|title=chmod(1): change file mode bits - Linux man page|website=linux.die.net}}</ref>