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{{Short description|File holding settings for a computer program}}
{{Redirect|rc file|resource scripts|Resource (Windows)}}

A '''configuration file''', a.k.a. '''config file''', is a [[computer file|file]] that stores [[computer data|data]] used to configure a [[software system]] such as an [[application software|application]], a [[server (computing)|server]] or an [[operating system]].

Some applications provide a tool to create, modify, and verify the [[syntax]] of their configuration files {{endash}} sometimes via [[graphical user interface]] (GUI). For context, [[system administrator]]s may be expected to create and modify [[plain text|text]] config files via a text editor. For server processes and operating-system settings, there is often no standard tool, but operating systems may provide graphical interfaces such as [[YaST]] or debconf.

Some computer programs only read their configuration files at [[Booting|startup]]. Others periodically check the configuration files for changes. Users can instruct some programs to re-read the configuration files and apply the changes to the current process, or indeed to read arbitrary files as a configuration file. There are no definitive standards or strong conventions.

[[File:Gnu grub config file.png|400px|thumb|A configuration file for [[GNU GRUB]].]][GRAPHICS_QUALITY]

HIGH_FPS=2

#0-Off, 1=60FPS, 2=120FPS

MODEL_QUALITY=2 # Medium-High model detail (ad

EFFECT_QUALITY=2 # Medium-High skill effects

==File format==

In general, a config file can have any [[File format|format]]. The format that applies to a particular system is determined by the design of that system. Often, a standardized format is used since it allows for using tools designed for the format even if not designed for the consuming system. In particular, general-purpose [[serialization]] formats, such as [[JSON]], [[XML]], and [[YAML]], are often used in [[Open-source software|open-source]] and [[Cross-platform|platform-neutral]] software. The [[specification]] for these formats are generally publicly available to support wide-spread use.

The following table compares notable formats used for configuration data.

{| class="wikitable"
|+Format comparison<ref>{{Citation |title=TOML |date=2023-01-15 |url=https://github.com/toml-lang/toml |publisher=TOML |access-date=2023-01-15}}</ref>
! scope="col" |Format
! scope="col" |Formal<br>spec
! scope="col" |Allows<br>[[Comment (computer programming)|comments]]
! scope="col" |Syntax<br>typing
|-
|CUE<ref>{{cite web |title=About {{!}} CUE |url=https://cuelang.org/docs/about/ |access-date=October 6, 2022}}</ref>|| {{Yes}} || {{Yes}} || {{good|Yes}}
|-
|[[INI file|INI]]|| {{No}} || {{Yes}} || {{good|No}}
|-
|[[JSON]]|| {{Yes}}<ref>{{Cite web|url=https://datatracker.ietf.org/doc/html/rfc8259|title=The JavaScript Object Notation (JSON) Data Interchange Format}}</ref> || {{No}} || {{good|Yes}}
|-
|[[TOML]]|| {{Yes}}<ref>{{Cite web|url=https://toml.io/en/v1.0.0|title=TOML Specification}}</ref> || {{Yes}} || {{good|Yes}}
|-
|[https://github.com/vstakhov/libucl UCL]|| {{No}}<ref>{{cite web |title=Formal UCL grammar definition · Issue #231 · vstakhov/libucl |url=https://github.com/vstakhov/libucl/issues/231 |website=GitHub |language=en}}</ref> || {{Yes}} || {{good|Yes}}
|-
|[[YAML]]|| {{Yes}}<ref>{{Cite web|url=https://yaml.org/spec|title=YAML™ Specification Index}}</ref> || {{Yes}} || {{good|Yes}}
|-
|[[XML]]|| {{Yes}}<ref>{{Cite web|url=https://www.w3.org/TR/xml/|title=Extensible Markup Language (XML) 1.0 (Fifth Edition)}}</ref> || {{Yes}} || {{good|No}}
|}

The ''syntax typing'' column indicates whether the [[Syntax (programming languages)|syntax]] supports [[data type]]s. A format has syntax-typing if a value's type is specified by syntax – e.g. <code>true</code> is a [[Boolean data type|Boolean]] while <code>"true"</code> is a [[String (computer science)|string]]. A format does not have syntax-typing if a value's type is based on [[Semantics (computer science)|semantics]] – e.g. <code>true</code> and <code>"true"</code> are both Boolean if the parser expects a Boolean. Opinions on the value of syntax-typing [https://hitchdev.com/strictyaml/why-not/toml/#4-syntax-typing vary].

==Examples==
The following are examples of config files organized by the [[operating system]]s on which they are commonly used.

===Unix and Unix-like===
Many different file formats are used on [[Unix]] and [[Unix-like]] operating systems. Even so, there is a strong tradition of using human-editable, plain text formats including simple [[key–value pair]]. Filename extensions of <code>.cnf</code>, <code>.conf</code>, <code>.cfg</code>, <code>.cf</code> or <code>.ini</code> are often used.

Many formats allow [[Comment (computer programming)|comments]], in which case, individual settings can be disabled by [[comment out|prepending with the comment character]]. Often the default configuration files contain extensive internal documentation in the form of comments<ref name=":0">https://opensource.apple.com/source/postfix/postfix-174.2/Postfix.Config/main.cf.default. {{Webarchive|url=https://web.archive.org/web/20170803053000/https://opensource.apple.com/source/postfix/postfix-174.2/Postfix.Config/main.cf.default |date=2017-08-03 }}</ref><ref name=":1">http://opensource.apple.com/source/apache/apache-769/httpd.conf. {{Webarchive|url=https://web.archive.org/web/20200801023500/https://opensource.apple.com/source/apache/apache-769/httpd.conf |date=2020-08-01 }}</ref> and [[man file]]s are also typically used to document the format and options available.

System-wide software often uses configuration files stored in <code>[[/etc]]</code>, while user applications often use a "[[dotfile]]" – a file or directory in the home directory prefixed with a [[full stop|period]], which [[Hidden file and hidden directory#Unix and Unix-like environments|in Unix hides the file or directory]] from casual listing. Since this causes pollution, newer user applications generally make their own folder in the <code>.config</code> directory, a standardized subdirectory of the home directory.

Similar to config files, a [[RUNCOM|run command]] (rc) [[shell script]] can configure a [[shell (computing)|shell]] session. Often, such scripts are named with an <code>rc</code> suffix after the consuming program's name such as <code>.xinitrc</code>, <code>.vimrc</code>, <code>.bashrc</code>, <code>xsane.rc</code>.<ref name="autogenerated1">{{cite web|url=http://www.catb.org/jargon/html/R/rc-file.html |title=rc file |publisher=Catb.org |access-date=2012-02-29}}</ref>

By contrast, [[IBM]]'s [[AIX]] uses an [[Object Data Manager]] (ODM) database to store much of its system settings.

===MS-DOS===
[[MS-DOS]] primarily relied on the <code>[[CONFIG.SYS]]</code> config file; a plain text file with simple key–value pairs (e.g. <code>DEVICEHIGH=C:\DOS\ANSI.SYS</code>). MS-DOS 6 introduced the INI-file format. Similar to an rc file, the [[batch file]] named <code>[[AUTOEXEC.BAT]]</code> ran commands on [[Booting|startup]]. Both these files were retained up to [[Windows 98SE]], which still ran on top of MS-DOS.

An example CONFIG.SYS:
<syntaxhighlight lang="ini">
DOS=HIGH,UMB
DEVICE=C:\DOS\HIMEM.SYS
DEVICE=C:\DOS\EMM386.EXE RAM
DEVICEHIGH=C:\DOS\ANSI.SYS
FILES=30
SHELL=C:\DOS\COMMAND.COM C:\DOS /E:512 /P
</syntaxhighlight>

DOS applications used a wide variety of individual configuration files, most of them binary, proprietary and undocumented - and there were no common conventions or formats.{{citation needed|date=October 2016}}

===Windows===
[[File:Regedit (Wine).png|thumb|right|The REGEDIT application being used to edit Windows Registry data]]
Early [[Windows]] operating systems heavily utilized plain-text [[INI file]]s which served as the primary mechanism to configure the operating system and applications.<ref>[https://www.microsoft.com/resources/documentation/windowsnt/4/workstation/reskit/en-us/26_ini.mspx Microsoft: Windows NT Workstation Resource Kit].</ref> The [[API]]s to read and write from these still exist in Windows, but after 1993, Microsoft began to steer developers away from using INI files and toward storing settings in the [[Windows Registry|registry]], a hierarchical database to store configuration settings, which was introduced with [[Windows NT]]. Later systems use XML and other formats instead of the registry.

===macOS===
The [[Property List]] is the standard configuration file format in [[macOS]] (as well as in [[iOS]], [[NeXTSTEP]], [[GNUstep]] and [[Cocoa (API)|Cocoa]] applications). It uses the [[filename extension]] <code>.plist</code>.

===IBM OS/2===
IBM's [[OS/2]] uses a binary format, also with a .INI suffix, but this differs from the Windows versions. It contains a [[list (data structure)|list]] of lists of untyped key–value pairs.<ref name="voice">''[http://www.os2voice.org/VNL/past_issues/VNL0904H/vnewsf4.htm The OS/2 INI Files]'' by James J. Weinkam.</ref>
Two files control system-wide settings: OS2.INI and OS2SYS.INI. Application developers can choose whether to use them or to create a specific file for their applications.

===HarmonyOS and OpenHarmony operating systems ===
[[HarmonyOS]] and [[OpenHarmony]]-based operating systems use JSON config files, named {{code|config.json}}. The platform [[integrated development environment|IDE]], [[DevEco Studio]], provides methods for editing <code>config.json</code>.<ref>{{cite web |last1=Evans |first1=Melissa |title=HarmonyOS (Hongmeng) — config.json |url=https://www.mo4tech.com/harmonyos-hongmeng-config-json.html |website=Mo4Tech |publisher=Mo4Tech |access-date=24 March 2024}}</ref>

==See also==
* [[.properties]], file extension for config file mainly used in [[Java (programming language)|Java]]
* [[HOCON]], a superset of .properties and JSON
* [[Run commands]], historical origin of the "rc" suffix for startup scripts

==References==
<references />

{{DEFAULTSORT:Configuration File}}
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