 | |
| Filename extensions | .bat,.cmd,.btm |
|---|---|
| Internet media type |
|
| Developed by | Microsoft |
| Type of format | Scripting |
| Container for | Scripts |
Abatch file is ascript file inDOS,OS/2 andMicrosoft Windows. It consists of a series ofcommands to be executed by thecommand-line interpreter, stored in aplain 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 asIF,FOR, andGOTOlabels. The term "batch" is frombatch processing, meaning "non-interactive execution", though a batch file might not process abatch of multiple data.
Similar toJob Control Language (JCL),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, theshell program (usuallyCOMMAND.COM orcmd.exe) reads the file and executes its commands, normally line-by-line.[1]Unix-likeoperating systems, such asLinux, have a similar, but more flexible, type of file called ashell script.[2]
Thefilename 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 in4DOS,4OS2 and4NT 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.
InMS-DOS, a batch file can be started from thecommand-line interface by typing its name, followed by any required parameters and pressing the↵ Enter key. When DOS loads, the fileAUTOEXEC.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 interpreterCOMMAND.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 aGOTO command for changing flow of line execution.
Microsoft Windows was introduced in 1985 as agraphical user interface-based (GUI) overlay on text-basedoperating 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 theAUTOEXEC.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 asWindows 9x invoked COMMAND.COM to run batch files.
TheIBMOS/2 operating system supported DOS-style batch files. It also included a version ofREXX, a more advanced batch-filescripting 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 byUnix-like operating systems.
UnlikeWindows 98 and earlier, theWindows NT family of operating systems does not depend on MS-DOS. Windows NT introduced an enhanced 32-bit command interpreter (cmd.exe) 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 (with different extension) 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.
As of Windows 8[update], 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.[nb 1]
append,dpath,ftype,set,path,assoc andprompt commands, when executed from a .bat file, alter the value of theerrorlevel variable only upon an error, whereas from within a .cmd file, they would affect errorlevel even when returning without an error.[3] It is also used by IBM's OS/2 for batch files.COMMAND.COM andcmd.exe can run a batch file even if its filename is typed without an extension. For instance, ifDoThis is entered, the interpreter tries the following extensions in the order given:COM,.EXE,.BAT,.CMD, and seven other extension unrelated to this topic. ThePATHEXTenvironment variable can change the aforesaid default.
COMMAND.COM and cmd.exe support special variables (%0,%1 through%9) in order to refer to the path and name of thebatch job and the first nine calling parameters from within the batch job, see alsoSHIFT. Non-existent parameters are replaced by a zero-length string. They can be used similar toenvironment variables, but are not stored in the environment. Microsoft and IBM refer to these variables asreplacement parameters orreplaceable parameters, whereas Digital Research, Novell and Caldera established the termreplacement variables[5] for them. JP Software calls thembatch file parameters.[6]
This example batch file displaysHello World!, prompts and waits for the user to press a key, and then terminates. (Note: It does not matter if commands are lowercase or uppercase unless working with variables)
@ECHO OFFECHO Hello World!PAUSE
To execute the file, it must be saved with thefilename extension suffix .bat (or .cmd for Windows NT-type operating systems) in plain text format, typically created by using a text editor such asMicrosoft Notepad or aword processor working in plain text mode.
When executed, the following is displayed:
Hello World!Press any key to continue . . .
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 commandECHO 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 theECHO Hello World! command outputsHello World!. The next line is executed and thePAUSE command displaysPress 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 runningcommand prompt window, the window remains open at the prompt as in MS-DOS; otherwise, the window closes on termination.
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:
IF%foo%==barECHO Equal
parses as the erroneous construct:
IF ==bar ECHO Equal
Similarly, if%foo% containsabc def, then a different syntax error results:
IF abc def==barECHO Equal
The usual way to prevent this problem is to surround variable expansions in quotes so that an empty variable expands into the valid expressionIF ""=="bar" instead of the invalidIF ==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.
IF"%foo%"=="bar"ECHO Equal
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:
IF!foo!==barECHO Equal
Another difference in Windows 2000 or higher is that an empty variable (undefined) is not substituted. As described in previous examples, previous batch interpreter behaviour would have resulted in an empty string. Example:
C:\>setMyVar=C:\>echo%MyVar%%MyVar%C:\>if"%MyVar%"==""(echo MyVar is not defined)else(echo MyVar is%MyVar%)MyVar is %MyVar%
Batch interpreters prior to Windows 2000 would have displayed resultMyVar is not defined.
Unlike Unix/POSIX processes, which receive their command-line arguments already split up by the shell into an array of strings, a Windows process receives the entire command-line as a single string, via the GetCommandLine API function. As a result, each Windows application can implement its ownparser to split the entire command line into arguments. Many applications and command-line tools have evolved their own syntax for doing that, and so there is no single convention for quoting or escapingmetacharacters on Windows command lines.
cmd.exe andwscript.exe, use their own rules.[8]Where a string contains quotation marks, and is to be inserted into another line of text that must also be enclosed in quotation marks, particular attention to the quoting mechanism is required:
C:\>setfoo="this string is enclosed in quotation marks"C:\>echo"test 1%foo%""test 1 "this string is enclosed in quotation marks""C:\>eventcreate /T Warning /ID 1 /L System /SO"Source" /D"Example:%foo%"ERROR: Invalid Argument/Option - 'string'.Type "EVENTCREATE /?" for usage.
On Windows 2000 and later, the solution is to replace each occurrence of a quote character within a value by a series of three quote characters:
C:\>setfoo="this string is enclosed in quotes"C:\>setfoo=%foo:"="""%C:\>echo"test 1%foo%""test 1 """this string is enclosed in quotes""""C:\>eventcreate /T Warning /ID 1 /L System /SO"Source" /D"Example:%foo%"SUCCESS: A 'Warning' type event is created in the 'Source' log/source.
Some characters, such as pipe (|) characters, have special meaning to the command line. They cannot be printed as text using theECHO command unless escaped using the caret ^ symbol:
C:\>echo foo| bar'bar' is not recognized as an internal or external command,operable program or batch file.C:\>echo foo^| barfoo | bar
However, escaping does not work as expected when inserting the escaped character into an environment variable. The variable ends up containing a live pipe command when merely echoed. It is necessary to escape both the caret itself and the escaped character for the character display as text in the variable:
C:\>setfoo=bar| baz'baz' is not recognized as an internal or external command,operable program or batch file.C:\>setfoo=bar^| bazC:\>echo%foo%'baz' is not recognized as an internal or external command,operable program or batch file.C:\>setfoo=bar^^^| bazC:\>echo%foo%bar | baz
The delayed expansion available with or with in Windows 2000 and later may be used to show special characters stored in environment variables because the variable value is expanded after the command was parsed:
C:\>cmd /V:ONMicrosoft Windows [Version 6.1.7601]Copyright (c) 2009 Microsoft Corporation. All rights reserved.C:\>setfoo=bar^| bazC:\>echo!foo!bar | baz
Until the TIMEOUT command was introduced with Windows Vista, there was no easy way to implement a timed pause, as the PAUSE command halts script activity indefinitely until any key is pressed.
Many workarounds were possible,[10] but generally only worked in some environments: TheCHOICE command was not available in older DOS versions,PING was only available if TCP/IP was installed, and so on. No solution was available from Microsoft, but a number of small utility programs, could be installed from other sources. A commercial example would be the 1988 Norton UtilitiesBatch Enhancer (BE) command, whereBE DELAY 18 would wait for 1 second, or the free 94-byte WAIT.COM[11] whereWAIT 5 would wait for 5 seconds, then return control to the script. Most such programs are 16-bit .COM files, so are incompatible with 64-bit Windows.
Normally, all printed text automatically has the control characters forcarriage return (CR) andline feed (LF) appended to the end of each line.
@echo foo@echo bar
C:\>batchtest.batfoobar
It does not matter if the two echo commands share the same command line; the CR/LF codes are inserted to break the output onto separate lines:
C:\>@echo Message 1&@echo Message 2Message 1Message 2
A trick discovered with Windows 2000 and later is to use the special prompt for input to output text without CR/LF trailing the text. In this example, the CR/LF does not follow Message 1, but does follow Line 2 and Line 3:
@echo offset/p="Message 1"<nulecho Message 2echo Message 3
C:\>batchtest2.batMessage 1Message 2Message 3
This can be used to output data to a text file without CR/LF appended to the end:
C:\>set/p="Message 1"<nul>data.txtC:\>set/p="Message 2"<nul>>data.txtC:\>set/p="Message 3"<nul>>data.txtC:\>type data.txtMessage 1Message 2Message 3
However, there is no way to inject this stripped CR/LF prompt output directly into an environment variable.
It is not possible to have a command prompt that uses aUNC path as the current working directory; e.g.\\server\share\directory\
The command prompt requires the use of drive letters to assign a working directory, which makes running complex batch files stored on a server UNC share more difficult. While a batch file can be run from a UNC file path, the working directory default isC:\Windows\System32\.
In Windows 2000 and later, a workaround is to use thePUSHD andPOPD command with command extensions.[nb 2]
If not enabled by default, command extensions can be temporarily enabled using the/E:ON switch for the command interpreter.
So to run a batch file on a UNC share, assign a temporary drive letter to the UNC share, and use the UNC share as the working directory of the batch file, a Windows shortcut can be constructed that looks like this:
The working directory attribute of this shortcut is ignored.
This also solves a problem related toUser Account Control (UAC) on Windows Vista and newer. When an administrator is logged on and UAC is enabled, and they try to run a batch file as administrator from a network drive letter, using the right-click file context menu, the operation will unexpectedly fail. This is because the elevated UAC privileged account context does not have network drive letter assignments, and it is not possible to assign drive letters for the elevated context via the Explorer shell or logon scripts. However, by creating a shortcut to the batch file using the abovePUSHD /POPD construct, and using the shortcut to run the batch file as administrator, the temporary drive letter will be created and removed in the elevated account context, and the batch file will function correctly.
The following syntax does correctly expand to the path of the current batch script.
%~dp0
UNC default paths are turned off by default as they used to crash older programs.[12]
The Dword registry valueDisableUNCCheck atHKEY_CURRENT_USER\Software\Microsoft\Command Processor[12] allows the default directory to be UNC.CD command will refuse to change but placing a UNC path in Default Directory in a shortcut to Cmd or by using the Start command. (C$ share is for administrators).
Batch files use an OEM character set, as defined by the computer, e.g.Code page 437. The non-ASCII parts of these are incompatible with theUnicode orWindows character sets otherwise used in Windows so care needs to be taken.[13] Non-English file names work only if entered through a DOS character set compatible editor. File names with characters outside this set do not work in batch files.
To get a command prompt with Unicode instead of Code page 437 or similar, one can use thecmd /U command. In such a command prompt, a batch file with Unicode filenames will work. Also one can usecmd /U to directly execute commands with Unicode as character set. For example,cmd /U /C dir > files.txt creates a file containing a directory listing with correct Windows characters, in theUTF-16LE encoding.
As with any other programming language, batch files can be used maliciously. Simpletrojans andfork bombs are easily created, and batch files can do a form ofDNS poisoning by modifying thehosts file. Batch viruses are possible, and can also spread themselves viaUSB flash drives by using Windows'Autorun capability.[14]
The following command in a batch file will delete all the data in the current directory (folder) - without first asking for confirmation:
del /Q *.*These three commands are a simplefork bomb that will continually replicate itself to deplete available system resources, slowing down or crashing the system:
:TOPstart""%0gotoTOP
Thecmd.exe command processor that interprets both.bat and.cmd files is supported in all versions of theWindows NT family,Windows CE, andReactOS. The olderCOMMAND.COM, which only interprets.bat files, is available inWindows 9x and 32-bit editions of Windows NT; hence, it is not available inWindows 11, which is strictly 64-bit.
Microsoft Windows, however, comes with more advanced scripting environments:
cscript.exe andwscript.exe, runs scripts written inVBScript orJScript (bearing.vbs,.js and.wsf extensions). It can run them in windowed mode (with thewscript.exe host) or in console-based mode (with thecscript.exe host). It has been deprecated inWindows 11.mshta.exe is means of creating graphically rich scripts whose source code is made ofHTML,CSS andJScript.There are other scripting languages available for Windows. However, these require the scripting language interpreter to be installed before they can be used:
COMMAND.COM at the 32-bit Windows 7 command prompt.Two consecutive double quotes inside an inQuotes region should result in a literal double quote (the parser is left in the inQuotes region). This behavior is not part of the spec of code:ParseArgumentsIntoList, but is compatible with CRT and .NET Framework.
