.BAT File Format

Introduction

A batch file is a text file of commands for the DOS or Windows command interpreter, run one line after another as if they had been typed. The .bat extension goes back to MS-DOS, where COMMAND.COM ran them and AUTOEXEC.BAT ran at every start.

Windows NT brought a new interpreter, cmd.exe, and the .cmd extension, which OS/2 also used. cmd.exe runs both today. The difference is small: a few built-in commands, such as set and path, reset ERRORLEVEL on success in a .cmd file and leave it alone in a .bat file.

Structure

A batch file is plain text, normally with Windows line ends, and cmd.exe reads it in the console’s code page, not as Unicode. Many files start with @echo off, which stops each command being printed before it runs. Comments use rem, or ::, which is really a label nothing jumps to.

• %NAME% expands a variable, set with set NAME=value. With setlocal enabledelayedexpansion, !NAME! expands it when the line runs and not when it is read, which matters inside loops.
• %1 to %9 are the arguments, %* is all of them, and %~dp0 is the folder holding the script.
• A line starting with a colon is a label. goto jumps to one and call :label runs it as a subroutine.
• A for loop variable is written %%F in a batch file and %F at the prompt.
• ^ escapes the next character, and at the end of a line it continues the command on the next.

@echo off
rem Copy each file named on the command line into a backup folder.

if "%~1"=="" goto usage

for %%F in (%*) do (
    copy "%%~F" "%USERPROFILE%\Backup" >nul
    if errorlevel 1 echo Could not copy %%~F
)
exit /b 0

:usage
echo Usage: %~nx0 file [file ...]
exit /b 1

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