AUTOEXEC.BAT

  is a system file that was originally on DOS-type operating systems. It is a plain-text batch file in the root directory of the boot device. The name of the file is an abbreviation of "automatic execution", which describes its function in automatically executing commands on system startup; the filename was coined in response to the 8.3 filename limitations of the FAT file system family.

Usage
is read upon startup by all versions of DOS, including MS-DOS version 7.x as used in Windows 95 and Windows 98. Windows ME only parses environment variables as part of its attempts to reduce legacy dependencies, but this can be worked around.

The filename was also used by (DCP), an MS-DOS derivative by the former East-German VEB Robotron.

In Korean versions of MS-DOS/PC DOS 4.01 and higher (except for PC DOS 7 and 2000), if the current country code is set to 82 (for Korea) and no  is given and no default   is found,   will look for a file named   instead in order to ensure that the DBCS frontend drivers will be loaded even without properly set up   and   files.

Under DOS, the file is executed by the primary copy of the command-line processor (typically ) once the operating system has booted and the   file processing has finished. While DOS by itself provides no means to pass batch file parameters to  for   processing, the alternative command-line processor 4DOS supports a     directive and   startup option to define such parameters. Under Concurrent DOS, Multiuser DOS and REAL/32, three initial parameters will be passed to either the corresponding  (if it exists) or the generic   startup file,   holds the virtual console number,   the 2-digit terminal number (xx) (with 00 being the main console) and   the 1-digit session number (y).

Windows NT and its descendants Windows XP and Windows Vista parse  when a user logs on. As with Windows ME, anything other than setting environment variables is ignored. Unlike, the commands in   can be entered at the interactive command line interpreter. They are just standard commands that the computer operator wants to be executed automatically whenever the computer is started, and can include other batch files.

is most often used to set environment variables such as keyboard, soundcard, printer, and temporary file locations. It is also used to initiate low level system utilities, such as the following:


 * Virus scanners
 * Disk caching software
 * Mouse drivers
 * Keyboard drivers
 * CD drivers
 * Miscellaneous other drivers

Example
In early versions of DOS,  was by default very simple. The  and   commands were necessary as early PC and XT class machines did not have a battery backed-up real-time clock as default.

In non-US environments, the keyboard driver (like  for the French keyboard) was also included. Later versions were often much expanded with numerous third-party device drivers. The following is a basic DOS 5 type  configuration, consisting only of essential commands:

This configuration sets common environment variables, loads a disk cache, places common directories into the default PATH, and initializes the DOS mouse / keyboard drivers. The  command sets the prompt to "C:\>" (when the working directory is the root of the C drive) instead of simply "C>" (the default prompt, indicating only the working drive and not the directory therein).

In general, device drivers were loaded in, and programs were loaded in the   file. Some devices, such as mice, could be loaded either as a device driver in, or as a TSR in  , depending upon the manufacturer.

In MS-DOS 6.0 and higher, a DOS boot menu is configurable. This can be of great help to users who wish to have optimized boot configurations for various programs, such as DOS games and Windows.

The  line informs DOS to look up menu entries that were defined within. Then, these profiles are named here and configured with the desired specific drivers and utilities. At the desired end of each specific configuration, a  command redirects DOS to the   section. Lines after  will be used by all profiles.

Dual-booting DOS and Windows 9x
When installing Windows 95 over a preexisting DOS/Windows install,  and   are renamed to   and. This is intended to ease dual booting between Windows 9x and DOS. When booting into DOS, they are temporarily renamed  and. Backups of the Windows 9x versions are made as  files.

Windows 9x also installs, a configuration file, which will not boot Windows 95/98 if parameter  is loaded, and instead a DOS prompt will appear on the screen (Windows can still be loaded by calling the   command (file WIN.COM). This file contains some switches that designate how the system will boot, one of which controls whether or not the system automatically goes into Windows. This "BootGUI" option must be set to "0" in order to boot to a DOS prompt. By doing this, the system's operation essentially becomes that of a DOS/Windows pairing like with earlier Windows versions. Windows can be started as desired by typing   at the DOS prompt.

When installing Caldera DR-DOS 7.02 and higher, the Windows version retains the name, while the file used by the DR-DOS   is named  , referred to by the startup parameter   in the SHELL directive. It also differentiates the  file by using the name.

OS/2
The equivalent to  under OS/2 is the OS/2   file. Genuine DOS sessions booted under OS/2 continue to use.

Windows NT
On Windows NT and its derivatives, Windows 2000, Windows Server 2003 and Windows XP, the equivalent file is called  and is located in the   directory. The file is not used during the operating system boot process; it is executed when the MS-DOS environment is started, which occurs when a DOS application is loaded.

The  file may often be found on Windows NT in the root directory of the boot drive. Windows only considers the  and   statements which it contains, in order to define environment variables global to all users. Setting environment variables through this file may be interesting if for example MS-DOS is also booted from this drive (this requires that the drive be FAT-formatted) or to keep the variables across a reinstall. This is an exotic usage today so the file usually remains empty. The Tweak UI applet from the Microsoft PowerToys collection allows to control this feature (Parse AUTOEXEC.BAT at logon).