AUTOEXEC.BAT is a system file found originally on DOS-type operating systems. It is a plain-text batch file that is located 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 portmanteau was coined in response to the 8.3 filename limitations of the FAT file system family.
AUTOEXEC.BAT 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.
Under DOS, the file is executed once the operating system has booted and after the CONFIG.SYS file has been processed. Windows NT and its descendants Windows XP and Windows Vista parse AUTOEXEC.BAT when a user logs on. As with Windows Me, anything other than setting environment variables is ignored. Unlike CONFIG.SYS, the commands in AUTOEXEC.BAT 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.
AUTOEXEC.BAT 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 - SMARTDRV.EXE from Microsoft the most common
- Mouse drivers
- Keyboard drivers
- CD drivers
- Miscellaneous other drivers
In early versions of DOS, AUTOEXEC.BAT was by default extremely simple. The
TIME commands were necessary as early PC and XT class machines did not have a battery backed-up real-time clock as default.
ECHO OFF CLS DATE TIME VER
In non-US environments the keyboard driver (like KEYBFR 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.x type AUTOEXEC.BAT configuration, consisting only of essential commands:
@ECHO OFF PROMPT $P$G PATH C:\DOS;C:\WINDOWS SET TEMP=C:\TEMP SET BLASTER=A220 I7 D1 T2 LH SMARTDRV.EXE LH DOSKEY LH MOUSE.COM /Y WIN
This configuration sets common environment variables, loads the disk cache SmartDrive on line six, places common directories into the default path, and initializes the DOS mouse / keyboard drivers, before starting Windows. The
prompt command sets the command prompt to "C:\>" instead of simply "C>".
In general, .SYS files were called in CONFIG.SYS, and .EXE programs such as the popular disk caching software SmartDrive provided by Microsoft with MS-DOS 5x, were loaded in the AUTOEXEC.BAT file. Some devices, such as mice, could be loaded either as a .SYS file in CONFIG.SYS, or as a .COM in AUTOEXEC.BAT, depending upon the manufacturer.
Lines prefixed with the string "REM" are comments (remarks) and are not run as part of AUTOEXEC.BAT. The "REM" lines are used for comments or to temporarily disable drivers (e.g. for a CD-ROM). An alternative, though less common, method for commenting is using double colons (::).
In MS-DOS 6 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. (continued from CONFIG.SYS article)
@ECHO OFF PROMPT $P$G PATH C:\DOS;C:\WINDOWS SET TEMP=C:\TEMP SET BLASTER=A220 I7 D1 T2 GOTO %CONFIG% :WIN LH SMARTDRV.EXE LH MOUSE.COM /Y WIN GOTO END :XMS LH SMARTDRV.EXE LH DOSKEY GOTO END :END
goto %CONFIG% line informs DOS to look up menu entries that were defined within
CONFIG.SYS. Then, these profiles are named here and configured with the desired specific drivers and utilities. At the desired end of each specific configuration, a
goto command redirects DOS to the
:END section. Lines after
:END will be used by all profiles.
Dual-booting DOS and Win 9x 
When installing Windows 95 over a preexisting DOS/WINDOWS install, CONFIG.SYS and AUTOEXEC.BAT are renamed to CONFIG.DOS and AUTOEXEC.DOS. This is intended to ease dual booting between Windows 9.x and DOS. When booting into DOS, they are temporarily renamed CONFIG.SYS and AUTOEXEC.BAT. Backups of the Win95 versions are made as
Windows 9x also installs a fake MSDOS.SYS file. 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 "WIN" at the DOS prompt.
When installing Caldera DR-DOS 7.02 and higher, the Windows version retains the name AUTOEXEC.BAT, while the file preferred by the DR-DOS loader is named AUTODOS7.BAT. It also differentiates the CONFIG.SYS file by using the name DCONFIG.SYS.
OS/2 / NT 
On Windows NT and its derivatives, Windows 2000, Windows Server 2003 and Windows XP, the equivalent file is called AUTOEXEC.NT and is located in the
%SystemRoot%\system32 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 an MS-DOS application is loaded.
The AUTOEXEC.BAT file may often be found on Windows NT, in the root directory of the boot drive. Windows only considers the "SET" and "PATH" 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) or to keep the variables across a reinstall. This is an exotic usage today so this file usually remains empty. The TweakUI applet from the PowerToys collection allows to control this feature (Parse Autoexec.bat at logon).
OS/2 did not use the AUTOEXEC.BAT file, instead using startup.cmd.
See also 
- "Subst" Command Does Not Work in Autoexec.bat File in Windows Millennium Edition
- "Some features of MS-DOS 8.0". Archived from the original on 22 October 2010. Retrieved 2010-11-21.
- INFO: Configuring Parsing of the AUTOEXEC.BAT File
- Mouse Doesn't Work with MS-DOS Shell