Configuration file

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. 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. Since the formats were poorly documented and there were no syntax checking tools for the configuration files, it was easy to misconfigure the operating system or service. Some configuration files have sample entries that can be used as templates. However, the examples may be out-of-date or incorrect. 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.

On UNIX variants dot files are 'hidden' from listing by default, On OS X these files are called "hidden files." The Explorer interface of Microsoft Windows XP does not allow you to rename a file with an initial '.' though it does allow access to such files.

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 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 is different from the Windows versions. It contains a list of lists of untyped key-value pairsThe OS/2 INI Files by James J. Weinkam.. 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.

Implementation Shortcomings

Some of the common problems with configuration file implementations are:

No defined behavior if the configuration file is missing, truncated, unreadable, unwritable, or has inappropriate ownership
No defined minimum or maximum file size
No defined minimum or maximum entry size
No revision control
No defined precedence—i.e., if there are two identical entries, does the program use the value of the first, the second, or both?
No formal syntax definition
- No defined character set
- No defined line-continuation syntax
- Poorly defined comment syntax
- undefined or idiosyncratic treatment of whitespace

Reference

External links

dotfiles.com – a site for users to share configuration files for Unix applications and utilities
libprf1 - alpha cross-platform, multi-language support for accessing the Preferences Registry Format (PRF) 1.0 configuration files
www.libelektra.org - Elektra provides a universal and secure framework to store configuration parameters in a hierarchical key-value pair mechanism, instead of each program using its own text configuration files
gconf - GConf is a system for storing application preferences. It is intended for user preferences; not configuration of something like Apache, or arbitrary data storage
rc.conf - BSD system startup configuration file
libconfig - Libconfig (LGPL) is a simple library for parsing structured configuration files, with a file format that is more compact and more readable than XML. And unlike XML, it is type-aware, so it is not necessary to do string parsing in application code. The library includes bindings for both the C and C++ languages.
GetPot GetPot (LGPL) is a sophisticated command line and input file parsing library, contained in a header file.

Computer data