Everything about Configuration File totally explained
In
computing,
configuration files, or
config files, are used to configure the initial settings for some
computer programs. They are used for user
applications,
server processes and
operating system settings. The files are often written in
ASCII (rarely
UTF-8) and line-oriented, with lines terminated by a
newline or
carriage return/
line feed pair, depending on the operating system. They may be considered a simple
database. Some files are created and modified using an ASCII editor. Others are created and modified as a side-effect of changing settings in a
graphical user interface (GUI) program. The formats of configuration files are often poorly
documented.
Some applications provide tools to create, modify, and verify the
syntax of their configuration files. For server processes and operating system settings, the only documentation may be the source code. Some configuration files are partially described by
man or help pages.
Some computer programs only read the configuration files at
startup. Others periodically check the configuration files for changes. Some can be told 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 standards or strong conventions.
Recently,
XML and
YAML have become popular as configuration file formats. They have the advantages of having well-defined syntaxes, and tools to validate and verify the syntax of the files that are created in those formats.
UNIX
Among the
Unix variants, there are hundreds of configuration file formats. Each application or service may have a unique format. Historically,
Unix operating system settings were often modified only by editing configuration files. Some formats allow entries to be disabled by
prepending a special comment character.
Unix user applications often create a file or directory in the home directory of the user upon startup. To hide the file or directory from casual listing of the contents of the home directory, the name of the file or directory is prepended with a
period, giving rise to the nickname "dotfile" or "dot file". Server processes often use configuration files stored in
/etc, but they may also use their installation directory, the root directory, or a location defined by the system administrator.
Configuration files also did more than just modify settings, they often (in the form of an "rc file") ran a set of commands upon startup (ex. a shell rc file might instruct the shell to change directories, run certain programs, delete or create files - many things which didn't involve modifying variables in the shell itself and so were not in the shell's dotfiles); according to the
Jargon File, this convention is borrowed from "runcom files" on the
CTSS operating system.
(External Link
) This functionality can and has been extended for programs written in
interpreted languages such that the configuration file is actually another program rewriting or extending or customizing the original program;
Emacs is the most prominent such example.
On
UNIX variants dot files are 'hidden' from listing by default, On Mac OS X these files are sometimes called "
hidden files" although other mechanisms exist on Mac OS X to hide a file from view in various tools. The
Explorer interface of
Microsoft Windows XP doesn't allow you to rename a file with an initial '.' though it does allow access to such files, and Windows' Notepad program does allow files to be saved with such names.
IBM's
AIX uses an Object Data Manager (ODM) database to store some system settings,
some of which need to be available at boot time.
Microsoft DOS
Primarily relied upon two files called
CONFIG.SYS and
AUTOEXEC.BAT. These were retained up to Windows 98SE, but were not strictly required to run Windows applications.
Microsoft Windows
Within the
Microsoft Windows family of operating systems and their attendant applications, the situation is similar.
Windows 3.0 had an API for
INI files (from "initialization"), but that format is
deprecated and many modern Windows programs forgo configuration files to use only the
Windows Registry to store information.
IBM OS/2
IBM's
OS/2 uses a binary format also named INI but it's different from the Windows versions.
It contains a
list of lists of untyped key-value pairs.
There are two files for system-wide settings, OS2.INI and OS2SYS.INI.
Application developers can choose whether to use them or create a specific file for their applications.
Platform independent
The trend in the increase of XML and YAML (among other formats) for use as configuration file formats is at least partially attributable to the increase in popularity of
Open source and
Platform neutral software applications and libraries. Moreover, the
specifications describing these formats are routinely made available to the public, thus increasing the availability of
parsers and
emitters across programming languages.
Further Information
Get more info on 'Configuration File'.
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