Batch file
A batch file is a script file in DOS, OS/2 and Microsoft Windows. It consists of a series of commands to be executed by the command-line interpreter, stored in a plain text file. A batch file may contain any command the interpreter accepts interactively and use constructs that enable conditional branching and looping within the batch file, such as
IF, FOR, and GOTO labels. The term "batch" is from batch processing, meaning "non-interactive execution", though a batch file might not process a batch of multiple data.Similar to Job Control Language, DCL and other systems on mainframe and minicomputer systems, batch files were added to ease the work required for certain regular tasks by allowing the user to set up a script to automate them. When a batch file is run, the shell program reads the file and executes its commands, normally line-by-line. Unix-like operating systems, such as Linux, have a similar, but more flexible, type of file called a shell script.
The filename extension .bat is used in DOS and Windows. Windows NT and OS/2 also added .cmd. Batch files for other environments may have different extensions, e.g., .btm in 4DOS, 4OS2 and 4NT related shells.
The detailed handling of batch files has changed significantly between versions. Some of the detail in this article applies to all batch files, while other details apply only to certain versions.
Variants
DOS
In MS-DOS, a batch file can be started from the command-line interface by typing its name, followed by any required parameters and pressing the key. When DOS loads, the file AUTOEXEC.BAT, when present, is automatically executed, so any commands that need to be run to set up the DOS environment may be placed in this file. Computer users would have the AUTOEXEC.BAT file set up the system date and time, initialize the DOS environment, load any resident programs or device drivers, or initialize network connections and assignments.A.bat file name extension identifies a file containing commands that are executed by the command interpreter COMMAND.COM line by line, as if it were a list of commands entered manually, with some extra batch-file-specific commands for basic programming functionality, including a
GOTO command for changing flow of line execution.Early Windows
was introduced in 1985 as a graphical user interface-based overlay on text-based operating systems and was designed to run on DOS. In order to start it, theWIN command was used, which could be added to the end of the AUTOEXEC.BAT file to allow automatic loading of Windows. In the earlier versions, one could run a.bat type file from Windows in the MS-DOS Prompt. Windows 3.1x and earlier, as well as Windows 9x invoked COMMAND.COM to run batch files.OS/2
The IBM OS/2 operating system supported DOS-style batch files. It also included a version of REXX, a more advanced batch-file scripting language. IBM and Microsoft started developing this system, but during the construction of it broke up after a dispute; as a result of this, IBM referred to their DOS-like console shell without mention of Microsoft, naming it just DOS, although this seemingly made no difference with regard to the way batch files worked from COMMAND.COM.OS/2's batch file interpreter also supports an EXTPROC command. This passes the batch file to the program named on the EXTPROC file as a data file. The named program can be a script file; this is similar to the #! mechanism used by Unix-like operating systems.
Windows NT
Unlike Windows 98 and earlier, the Windows NT family of operating systems does not depend on MS-DOS. Windows NT introduced an enhanced 32-bit command interpreter that could execute scripts with either the.CMD or.BAT extension. Cmd.exe added additional commands, and implemented existing ones in a slightly different way, so that the same batch file might work differently with cmd.exe and COMMAND.COM. In most cases, operation is identical if the few unsupported commands are not used. Cmd.exe's extensions to COMMAND.COM can be disabled for compatibility.Microsoft released a version of cmd.exe for Windows 9x and ME called WIN95CMD to allow users of older versions of Windows to use certain cmd.exe-style batch files.
, cmd.exe is the normal command interpreter for batch files; the older COMMAND.COM can be run as well in 32-bit versions of Windows able to run 16-bit programs.
Filename extensions
;.bat: The first filename extension used by Microsoft for batch files. This extension runs with DOS and all versions of Windows, under COMMAND.COM or cmd.exe, despite the different ways the two command interpreters execute batch files.;.cmd: Used for batch files in Windows NT family and sent to cmd.exe for interpretation. COMMAND.COM does not recognize this file name extension, so cmd.exe scripts are not executed in the wrong Windows environment by mistake. In addition,
append, dpath, ftype, set, path, assoc and prompt commands, when executed from a.bat file, alter the value of the errorlevel variable only upon an error, whereas from within a.cmd file, they would affect errorlevel even when returning without an error. It is also used by IBM's OS/2 for batch files.;.btm: The extension used by 4DOS, 4OS2, 4NT and Take Command. These scripts are faster, especially with longer ones, as the script is loaded entirely ready for execution, rather than line-by-line.
COMMAND.COM and cmd.exe can run a batch file even if its filename is typed without an extension. For instance, if
DoThis is entered, the interpreter tries the following extensions in the order given: COM, .EXE, .BAT, .CMD, and seven other extension unrelated to this topic. The PATHEXT environment variable can change the aforesaid default.Batch file parameters
COMMAND.COM and cmd.exe support special variables in order to refer to the path and name of the batch job and the first nine calling parameters from within the batch job, see also SHIFT. Non-existent parameters are replaced by a zero-length string. They can be used similar to environment variables, but are not stored in the environment. Microsoft and IBM refer to these variables as replacement parameters or replaceable parameters, whereas Digital Research, Novell and Caldera established the term replacement variables for them. JP Software calls them batch file parameters.Examples
This example batch file displaysHello World!, prompts and waits for the user to press a key, and then terminates. @ECHO OFF
ECHO Hello World!
PAUSE
To execute the file, it must be saved with the filename extension suffix.bat in plain text format, typically created by using a text editor such as Microsoft Notepad or a word processor working in plain text mode.
When executed, the following is displayed:
Hello World!
Press any key to continue...
Explanation
The interpreter executes each line in turn, starting with the first. The@ symbol at the start of any line prevents the prompt from displaying that command as it is executed. The command ECHO OFF turns off the prompt permanently, or until it is turned on again. The combined @ECHO OFF is often as here the first line of a batch file, preventing any commands from displaying, itself included. Then the next line is executed and the ECHO Hello World! command outputs Hello World!. The next line is executed and the PAUSE command displays Press any key to continue... and pauses the script's execution. After a key is pressed, the script terminates, as there are no more commands. In Windows, if the script is executed from an already running command prompt window, the window remains open at the prompt as in MS-DOS; otherwise, the window closes on termination.Limitations and exceptions
Null values in variables
Variable expansions are substituted textually into the command, and thus variables which contain nothing simply disappear from the syntax, and variables which contain spaces turn into multiple tokens. This can lead to syntax errors or bugs.For example, if %foo% is empty, this statement:
parses as the erroneous construct:
Similarly, if
%foo% contains abc def, then a different syntax error results:The usual way to prevent this problem is to surround variable expansions in quotes so that an empty variable expands into the valid expression
IF """bar" instead of the invalid IF bar. The text that is being compared to the variable must also be enclosed in quotes, because the quotes are not special delimiting syntax; these characters represent themselves.The delayed !VARIABLE! expansion available in Windows 2000 and later may be used to avoid these syntactical errors. In this case, null or multi-word variables do not fail syntactically because the value is expanded after the IF command is parsed:
Another difference in Windows 2000 or higher is that an empty variable is not substituted. As described in previous examples, previous batch interpreter behaviour would have resulted in an empty string. Example:
C:\>set MyVar=
C:\>echo %MyVar%
%MyVar%
C:\>if "%MyVar%""" else
MyVar is %MyVar%
Batch interpreters prior to Windows 2000 would have displayed result
MyVar is not defined.