Installer is an application included in Mac OS X

Mac OS X

Mac OS X is a series of Unix-based operating systems and graphical user interfaces developed, marketed, and sold by Apple Inc. Since 2002, has been included with all new Macintosh computer systems...

 (and in its progenitors OPENSTEP

OpenStep

OpenStep was an object-oriented application programming interface specification for an object-oriented operating system that used a non-NeXTSTEP operating system as its core, principally developed by NeXT with Sun Microsystems. OPENSTEP was a specific implementation of the OpenStep API developed...

 and NeXTSTEP

NEXTSTEP

NeXTSTEP was the object-oriented, multitasking operating system developed by NeXT Computer to run on its range of proprietary workstation computers, such as the NeXTcube...

) which extracts and installs files out of .pkg

.pkg

For other uses see PKG..pkg is a file format used by Apple Inc. on its Macintosh line of computers, and the iPhone. It's also used by Sony's PlayStation 3 on downloadable content over PlayStation Network....

 packages

Software package (installation)

In package management systems, which are commonly used with Linux-based operating systems, a package is a specific piece of software which the system can install and uninstall....

. It was created by NeXT

NeXT

Next, Inc. was an American computer company headquartered in Redwood City, California, that developed and manufactured a series of computer workstations intended for the higher education and business markets...

, and is now maintained by Apple Inc. Its purpose is to help software developers create uniform software installers.

Installer resides at /Applications/Utilities/Installer.app on Macintosh computers running Mac OS X

Mac OS X

Mac OS X is a series of Unix-based operating systems and graphical user interfaces developed, marketed, and sold by Apple Inc. Since 2002, has been included with all new Macintosh computer systems...

 versions prior to 10.5, and launches when a package or metapackage file is opened. In Mac OS X v10.5

Mac OS X v10.5

Mac OS X Leopard is the sixth major release of Mac OS X, Apple's desktop and server operating system for Macintosh computers. Leopard was released on 26 October 2007 as the successor of Tiger , and is available in two variants: a desktop version suitable for personal computers, and a...

 the Installer application is located at /System/Library/CoreServices/Installer.app. The installation process itself can vary substantially, as Installer allows developers to customize the information the user is presented with. For example, it can be made to display a custom welcome message, software license and readme

README

A readme file contains information about other files in a directory or archive and is commonly distributed with computer software. Such a file is usually a text file called README.TXT, README.1ST, READ.ME, or simply README, although some Microsoft Windows software may occasionally include a...

. Installer also handles authentication, checks that packages are valid before installing them, and allows developers to run custom scripts

Scripting language

A scripting language, script language, or extension language is a programming language that allows control of one or more applications. "Scripts" are distinct from the core code of the application, as they are usually written in a different language and are often created or at least modified by the...

 at several points during the installation process.

There is also a command line version located at /usr/sbin/installer.