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AUTOEXEC.BAT is the name of a system file found originally on the MS-DOS operating system. It is a plain-text batch file that is located in the root directory of the boot device. The name of the file stands for "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[1], but this can be worked around.[2]
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. [3] 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