Batch File Programming

Batch file programming is nothing but the Windows version of Unix Shell Programming. Let's start by understanding what happens when we give a DOS command. DOS is basically a file called command.com. It is this file (command.com) which handles all DOS commands that you give at the

DOS prompt---such as COPY, DIR, DEL etc. These commands are built in with the Command.com file. (Such commands which are built in are called internal commands.).DOS has something called external commands too such as FORMAT, UNDELETE, BACKUP etc.

So whenever we give a DOS command either internal or external, command.com either straightaway executes the command (Internal Commands) or calls an external separate program which executes the command for it and returns the result (External Commands.)

So why do I need Batch File Programs? Say you need to execute a set of commands over and over again to perform a routine task like Backing up Important Files, Deleting temporary files(*.tmp, .bak , ~.* etc) then it is very difficult to type the same set of commands over and over

again. To perform a bulk set of same commands over and over again, Batch files are used.

Batch Files are to DOS what Macros are to Microsoft Office and are used to perform an automated predefined set of tasks over and over again. So how do I create batch files? To start enjoying using Batch files, you need to learn to create Batch files. Batch files are basically plain text files containing DOS commands. So the best editor to write your commands in would be Notepad or the DOS Editor (EDIT) All you need to remember is that a batch file should have the extension .BAT(dot bat) Executing a batch file is quite simple too. For example if you create a Batch file and save it with the filename batch.bat then all you need to execute the batch file is to type:

C:\windows>batch.bat

So what happens when you give a Batch file to the command.com to execute?

Whenever command.com comes across a batch file program, it goes into batch mode. In the batch mode, it reads the commands from the batch file line by line. So basically what happens is, command.com opens the batch file and reads the first line, then it closes the batch file. It then executes the command and again reopens the batch file and reads the next line from it. Batch files are treated as Internal DOS commands.

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Hacking Truth: While creating a batch file, one thing that you need to keep in mind is that the filename of the batch file should not use the same name as a DOS command. For example, if you create a batch file by the name dir.bat and then try to execute it at the prompt, nothing will happen.This is because when command.com comes across a command, it first checks to see if it is an internal command. If it is not then command.com checks if it a .COM, .EXE or .BAT file with a matching filename. All external DOS commands use either a .COM or a .EXE extension, DOS never bothers to check if the batch program exits.

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Now let's move on to your first Batch file program. We will unlike always(Normally we begin with the obligatory Hello World program) first take up a simple batch file which executes or launches a .EXE program. Simply type the following in a blank text file and save it with a .BAT extension.

C:

cd windows

telnet

Now let's analyze the code, the first line tells command.com to go to the C: Next it tells it to change the current directory to Windows. The last line tells it to launch the telnet client. You may contradict saying that the full filename is telnet.exe. Yes you are right, but the .exe extension is automatically added by command.com. Normally we do not need to change the drive and the directory as the Windows directory is the default DOS folder. So instead the bath file could simply contain the below and would still work.

telnet

Now let's execute this batch file and see what results it shows. Launch command.com (DOS) and execute the batch file by typing:

C:\WINDOWS>batch_file_name

You would get the following result:

C:\WINDOWS>scandisk

And Scandisk is launched. So now the you know the basic functioning of Batch files, let' move on to Batch file commands.

The REM Command

The most simple basic Batch file command is the REM or the Remark command. It is used extensively by programmers to insert comments into their code to make it more readable and understandable. This command ignores anything there is on that line. Anything on the line after REM is not even displayed on the screen during execution. It is normally not used in small easy to understand batch programs but is very useful in huge snippets of code with geek stuff loaded into it. So if we add Remarks to out first batch file, it will become:

REM This batch file is my first batch program which launches the fav hacking tool; Telnet

telnet

The only thing to keep in mind while using Remarks is to not go overboard and putting in too many of them into a single program as they tend to slow down the execution time of the batch commands.

ECHO: The Batch Printing Tool

The ECHO command is used for what the Print command is in other programming languages: To Display something on the screen. It can be used to tell the user what the bath file is currently doing. It is true that Batch programs display all commands it is executing but sometimes they are not enough and it is better to also insert ECHO commands which give a better description of what is presently being done. Say for example the following batch program which is full of the ECHO command deletes all files in the c:\windows\temp directory:

ECHO This Batch File deletes all unwanted Temporary files from your system

ECHO Now we go to the Windows\temp directory.

cd windows\temp

ECHO Deleting unwanted temporary files....

del *.tmp

ECHO Your System is Now Clean

Now let's see what happens when we execute the above snippet of batch code.

C:\WINDOWS>batch_file_name

C:\WINDOWS>ECHO This Batch File deletes all unwanted Temporary files from your system

C:\WINDOWS>ECHO Now we go to the Windows\temp directory.

Now we go to the Windows\temp directory.

C:\WINDOWS>cd windows\temp

Invalid directory

C:\WINDOWS>ECHO Deleting unwanted temporary files

Deleting unwanted temporary files...

C:\WINDOWS>del *.tmp

C:\WINDOWS>ECHO Your System is Now Clean

Your System is Now Clean

The above is a big mess! The problem is that DOS is displaying the executed command and also the statement within the ECHO command. To prevent DOS from displaying the command being executed, simply precede the batch file with the following command at the beginning of the file:

ECHO OFF

Once we add the above line to our Temporary files deleting Batch program , the output becomes:

C:\WINDOWS>ECHO OFF

This Batch File deletes all unwanted Temporary files from your system

Now we go to the Windows\temp directory.

Invalid directory

Deleting unwanted temporary files...

File not found

Your System is Now Clean

Hey pretty good! But it still shows the initial ECHO OFF command. You can prevent a particular command from being shown but still be executed by preceding the command with a @ sign. So to hide even the ECHO OFF command, simple replace the first line of the batch file with @ECHO OFF

You might think that to display a blank line in the output screen you can simply type ECHO by itself, but that doesn't work. The ECHO command return whether the ECHO is ON or OFF. Say you have started your batch file with the command ECHO OFF and then in the later line give the command ECHO, then it will display ' ECHO is off ' on the screen. You can display a blank line by giving the command

ECHO.(ECHO followed by a dot)Simply leaving a blank line in the code too displays a blank line in the output.

You can turn ON the ECHO anytime by simply giving the command ECHO ON. After turning the echo on , if you give the command ECHO then it will return ' ECHO is on '

The PAUSE Command: Freezing Time

Say you create a batch file which shows the Directory Listing of a particular folder(DIR) before performing some other task. Or sometimes before deleting all files of a folder, you need to give the user time to react and change his mind. PAUSE, the name says it all, it is used to time out actions of a script.

Consider the following scenario:

REM This Batch program deletes *.doc files in the current folder.

REM But it gives the user to react and abort this process.

@ECHO OFF

ECHO WARNING: Going to delete all Microsoft Word Document

ECHO Press CTRL+C to abort or simply press a key to continue.

PAUSE

DEL *.doc

Now when you execute this batch program, we get the following output:

C:\WINDOWS>a.bat

WARNING: Going to delete all Microsoft Word Document

Press CTRL+C to abort or simply press a key to continue.

Press any key to continue . . .

The batch file program actually asks the user if he wishes to continue and gives the user the option to abort the process. Pressing CTRL+C cancels the batch file program(CTRL+C and CTRL+Break bring about the same results)

^C

Terminate batch job (Y/N)?y

After this you will get the DOS prompt back.

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HACKING TRUTH: Say you have saved a batch file in the c:\name directory. Now when you launch command.com the default directory is c:\windows and in order to execute the batch file program stored in the c:\name directory you need to change the directory and go to c:\name.This can be very irritating and time consuming. It is a good practice to store all your batch programs in the same folder. You can run a batch file stored in any folder(Say c:\name) from anywhere(even c:\windows\history) if you include the folder in which the batch file is stored (c:\name)in the AUTOEXEC.BAT file, so that DOS knows which folder to look for the batch program.

So simply open c:\autoexec.bat in Notepad and append the Path statement to the following line[c:\name is the folder in which all your batch files are stored.]:

SET PATH=C:\WINDOWS;C:\WINDOWS\COMMAND;C:\name

Autoexec.bat runs each time at startup and DOS knows each time, in which directory to look for the batch files.

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Parameters: Giving Information to Batch Programs

To make batch programs really intelligent you need to be able to provide them with parameters which are nothing but additional valuable information which is needed to ensure that the bath program can work efficiently and flexibly. To understand how parameters work, look at the following script:

@ECHO OFF

ECHO First Parameter is %1

ECHO Second Parameter is %2

ECHO Third Parameter is %3

The script seems to be echoing(printing) messages on the screen, but what do the strange symbols %1 , % 2 etc stand for? To find out what the strange symbols stand for save the above script and go to DOS and execute this script by passing the below parameters:

C:\windows>batch_file_name abc def ghi

This batch file produces the following result:

C:\windows>batch_file_name abc def ghi

First Parameter is abc

Second Parameter is def

Third Parameter is ghi

The first line in the output is produced by the code line:

ECHO First Parameter is %1

Basically what happens is that when DOS encounters the %1 symbol, it examines the original command used to execute the bath program and look for the first word (argument) after the batch filename and then assigns %1 the value of that word. So one can say that in the ECHO statement %1 is replaced with the value of the first argument. In the above example the first word after the batch file name is abc, therefore %1 is assigned the value of this word.

The %2 symbol too works in the similar way, the only difference being that instead of the first argument, DOS assigns it the value of the second argument, def. Now all these symbols, %1, %2 are called replaceable parameters. Actually what happens is that %1 is not assigned the value of the first argument, but in fact it is replaced by the value of the first argument.If the batch file command has more parameters than what the batch file is looking for, then the extras are ignored. For example, if while executing a batch file program , we pass four arguments, but the batch file program requires only 3 parameters, then the fourth parameter is ignored. To understand the practical usage of parameters, let's take up a real life example. Now the following script requires the user to enter the name of the files to be deleted and the folder in which they are located.

@ECHO OFF

CD\

CD %1

DEL %2

This script can be called from the DOS prompt in the following way:

C:\windows>batch_file_name windows\temp *.tmp

In a single script we cannot use more that nine replaceable parameters. This means that a particular batch file will have replaceable parameters from %1 to %9.Infact there is a tenth replaceable parameter, the %0 parameter. The %0 parameter contains the name of the batch file itself.

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HACKING TRUTH: Say you want to execute a batch file and once the procedure of execution is complete, want to leave DOS and return to Windows, what do you do? The EXIT command can be used in such situations. So simply end your batch file with the EXIT command.

EXIT

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SHIFT: Infinite Parameters

Sometimes your batch file program may need to use more than nine parameters at a time.(Actually you would never need to, but at least you are sure you can handle it if you need to.)To see how the SHIFT command works, look at the following snippet of code:

@ECHO OFF

ECHO The first Parameter is %1

ECHO.

SHIFT

ECHO The Second Parameter is %1

ECHO.

SHIFT

ECHO The Second Parameter is %1

Now execute this batch file from DOS and see what happens.

C:\windows>batch_file_name abc def ghi

The first Parameter is abc

The Second Parameter is def

The Second Parameter is ghi

How does it work? Well, each SHIFT command shuffles the parameters down one position. This means that after the first SHIFT %1 becomes def, %2 becomes ghi and abc is completely removed by DOS. All parameters change and move one position down.

Both normal parameters (%1 , % 2 etc) and the SHIFT command can be made more efficient by grouping them with the IF conditional statement to check the parameters passed by the User.

THE FOR LOOP

The syntax of the FOR LOOP is:

FOR %%PARAMETER IN(set) DO command

Most people change their mind about learning Batch Programming when they come across the syntax of the For Command. I do agree that it does seem a bit weird, but it is not as difficult as it appears to be. Let's analyze the various parts of the For command. Before we do that look at the following example,

@ECHO OFF

CLS

FOR %%A IN (abc, def, xyz) DO ECHO %%A

Basically a FOR LOOP declares a variable (%%A) and assigns it different values as it goes through the predefined set of values(abc, def, xyz) and each time the variable is assigned a new value, the FOR loop performs a command.(ECHO %%A)

The %%A is the variable which is assigned different values as the loop goes through the predefined set of values in the brackets. You can use any single letter character after the two % sign except 0 through 9.We use two %'s as DOS deletes each occurrence of a single % sign in a batch file program.

The IN(abc, def, xyz) is the list through which the FOR loop goes. The variable %%a is assigned the various values within the brackets, as the loop moves. The items in the set(The technical term for the set of values within the brackets) can be separated with commas, colons or simply spaces.

For each item in the set(The IN Thing) the FOR loop performs whatever command is given after the DO keyword.(In this example the loop will ECHO %%A)

So basically when we execute the above batch file, the output will be:

abc

def

xyz

The FOR loop becomes very powerful if used along with replaceable parameters. Take the following batch file, for example,

@ECHO OFF

ECHO.

ECHO I am going to delete the following files:

ECHO %1 %2

ECHO.

ECHO Press Ctrl+C to Abort process

PAUSE

FOR %%a IN (%1 %2 ) DO DEL %%a

ECHO Killed Files. Mission Accomplished.

At execution time, the process would be something like:

C:\WINDOWS>batchfilename *.tmp *.bak

I am going to delete the following files:

*.tmp *.bak

Press Ctrl+C to Abort process

Press any key to continue . . .

Killed Files. Mission Accomplished.

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IF: CONDITIONAL BRANCHING

The If statement is a very useful command which allows us to make the batch files more intelligent and useful. Using this command one can make the batch programs check the parameters and accordingly perform a task. Not only can the IF command check parameters, it can also checks if a particular file exists or not. On top of all this, it can also be used for the conventional checking of variables (strings).

Checking If a File Exists Or Not

The general syntax of the IF command which checks for the existence of a file is the following:

IF [NOT] EXIST FILENAME Command

This will become clearer when we take up the following example,

IF EXIST c:\autoexec.bat ECHO It exists

This command checks to see if the file, c:\autoexec.bat exists or not. If it does then it echoes or prints the string 'It exists'. On the other hand if the specified file does not exist, then it does not do anything.

In the above example, if the file autoexec.bat did not exist, then nothing was executed. We can also put in the else clause i.e. If the File exists, do this but if it does not exists, by using the GOTO command. Let's consider the following example to make it more clear:

@echo off

IF EXIST C:\ankit.doc GOTO ANKIT

Goto end

:ANKIT

ECHO ANKIT

:end

The IF statement in this code snippet checks to see if there exists a file, c:\ankit.doc. If it does then DOS is branched to :ANKIT and if it does not, then DOS goes on to the next line. The next line branches DOS to :end. The :end and :ANKIT in the above example are called labels. After the branching the respective echo statements take over.

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HACKING TRUTH: We can also check for more than one file at a time, in the following way:

IF EXIST c:\autoexec.bat IF EXIST c:\autoexec.bak ECHO Both Exist

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We can check to see if a file does not exist in the same way, the basic syntax now becomes:

IF NOT EXIST FILENAME Command

For Example,

IF NOT EXIST c:\ankit.doc ECHO It doesn't Exist

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HACKING TRUTH: How do you check for the existence of directories? No something like IF C:\windows EXISTS ECHO Yes does not work. In this case we need to make use of the NULL device. The NULL device is basically nothing, it actually stands for simply nothing. Each directory has the NULL device present in it. (At least DOS thinks so.) So to check if c:\windows exits, simply type:

IF EXIST c:\windows\nul ECHO c:\Windows exists.

One can also check if a drive is valid, by giving something like:

IF EXIST c:\io.sys ECHO Drive c: is valid.

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Comparing Strings to Validate Parameters

The basic syntax is:

IF [NOT] string1==string2 Command

Now let's make our scripts intelligent and make them perform a task according to what parameter was passed by the User. Take the following snippet of code for example,

@ECHO off

IF %1==cp GOTO COPY

GOTO DEL

:COPY

Copy %2 a:

GOTO :END

:DEL

Del %2

:END

This example too is pretty much self explanatory. The IF Statement compares the first parameter to cp, and if it matches then DOS is sent to read the COPY label else to the DEL label. This example makes use of two parameters and is called by passing at least two parameters.

We can edit the above example to make DOS check if a parameter was passed or not and if not then display an error message. Just add the following lines to the beginning of the above file.

@ECHO OFF

IF "%1" == "" ECHO Error Message Here

If no parameter is passed then the batch file displays an error message. Similarly we can also check for the existence of the second parameter.

This command too has the NOT clause.

The CHOICE Command

Before we learn how to make use of the CHOICE command, we need to what error levels really are. Now Error levels are generated by programs to inform about the way they finished or were forced to finish their execution. For example, when we end a program by pressing CTRL+C to end a program, the error level code evaluates to 3 and if the program closes normally, then the error level evaluates to 0. These numbers all by themselves are not useful but when used with the IF ERROR LEVEL and the CHIOCE command, they become very kewl.

The CHOICE command takes a letter or key from the keyboard and returns the error level evaluated when the key is pressed. The general syntax of the CHOICE command is:

CHOICE[string][/C:keys][/S][/N][/T:key,secs]

The string part is nothing but the string to be displayed when the CHOICE command is run.

The /C:keys defines the possible keys to be pressed. If options are mentioned then the default Y/N keys are used instead.

For example, The command,

CHOICE /C:A1T0

Defines A, 1, T and O as the possible keys. During execution if the user presses a undefined key, he will hear a beep sound and the program will continue as coded.

The /S flag makes the possible keys defined by the CHOICE /c flag case sensitive. So it means that if the /S flag is present then A and a would be different.

The /N flag, if present shows the possible keys in brackets when the program is executed. If the /N flag is missing then, the possible keys are not shown in brackets. Only the value contained by STRING is shown.

/T:key,secs defines the key which is taken as the default after a certain amount of time has passed.

For Example,

CHOICE Choose Browser /C:NI /T:I.5

The above command displays Choose Browser[N,I] and if no key is pressed for the next 5 seconds, then it chooses I.

Now to truly combine the CHOICE command with the IF ERROR LEVEL command, you need to know what the CHOICE command returns.

The CHOICE command is designed to return an error level according to the pressed key and its position in the /C flag. To understand this better, consider the following example,

CHOICE /C:AN12

Now remember that the error level code value depends on the key pressed. This means that if the key A is pressed, then the error level is 1, if the key N is pressed then the error level is 2, if 1 is pressed then error level is 3 and if 2 is pressed then error level is 4.

Now let us see how the IF ERROR LEVEL command works. The general syntax of this command is:

IF [NOT] ERRORLEVEL number command.

This statement evaluates the current error level number. If the condition is true then the command is executed. For Example,

IF ERRORLEVEL 3 ECHO Yes

The above statement prints Yes on the screen if the current error level is 3.

The important thing to note in this statement is that the evaluation of an error level is true when the error level us equal or higher than the number compared.

For Example, in the following statement,

IF ERRORLEVEL 2 ECHO YES

The condition is true if the error level is > or = 2.

Now that you know how to use the CHOICE and ERROR LEVEL IF command together, you can now easily create menu based programs. The following is an example of such a batch file which asks the User what browser to launch.

@ECHO OFF

ECHO.

ECHO.

ECHO Welcome to Browser Selection Program

ECHO.

ECHO 1. Internet Explorer 5.5

ECHO 2. Mozilla 5

ECHO x. Exit Browser Selection Program

ECHO.

CHOICE "Choose Browser" /C:12x /N

IF ERRORLEVEL 3 GOTO END

IF ERRORLEVEL 2 START C:\progra~1\Netscape

IF ERRORLEVEL 1 start c:\progra~1\intern~1\iexplore.exe

:END

NOTE: Observe the order in which we give the IF statements.

Redirection

Normally the Output is sent to the screen(The standard STDOUT)and the Input is read from the Keyboard(The standard STDIN). This can be pretty boring. You can actually redirect both the Input and the Output to something other than the standard I/O devices.

To send the Output to somewhere other than the screen we use the Output Redirection Operator, > which is most commonly used to capture results of a command in a text file. Say you want to read the help on how to use the net command, typing the usual Help command is not useful as the results do not fit in one screen and scroll by extremely quickly. So instead we use the Output Redirection operator to capture the results of the command in a text file.

c:\windows>net > xyz.txt

This command will execute the net command and will store the results in the text file, xyz.txt . Whenever DOS comes by such a command, it checks if the specified file exists or not. If it does, then everything in the file is erased or lost and the results are stored in it. If no such file exists, then DOS creates a new file and stores the results in this new file.

Say, you want to store the results of more than one command in the same text file, and want to ensure that the results of no command are lost, then you make use of the Double Output Re Direction Symbol, which is

the >> symbol.

For Example,

c:\windows> net >> xyz.txt

The above command tells DOS to execute the net command and append the output to the xyz.txt file, if it exits.

DOS not only allows redirection to Files, but also allows redirection to various devices.

DEVICE NAME USED DEVICE

AUX Auxiliary Device (COM1)

CLOCK$ Real Time Clock

COMn Serial Port(COM1, COM2, COM3, COM4)

CON Console(Keyboard, Screen)

LPTn Parallel Port(LPT1, LPT2, LPT3)

NUL NUL Device(means Nothing)

PRN Printer

Say for example, you want to print the results of directory listings, then you can simply give the following command:

c:\windows>dir *.* > prn

The NUL device(nothing) is a bit difficult to understand and requires special mention. This device which is also known as the 'bit bucket' literally means nothing. Redirection to the NUL device practically has no usage but can be used to suppress the messages which DOS displays on the completion of a task. For example, when DOS has successfully copied a particular file, then it displays the message: '1 file(s) copied.'

Now say you want to suppress this task completion message, then you can make use of the NUL device.

c:\windows>copy file.txt > NUL

This will suppress the task completion message and not display it.

Redirecting Input

Just like we can redirect Output, we can also redirect Input. It is handled by the Input Redirection Operator, which is the < symbol. It is most commonly used to send the contents of a text file to DOS. The other common usage of this feature is the MORE command which displays a file one screen at a time unlike the TYPE command which on execution displays the entire file.(This becomes impossible to read as the file scrolls by at incredible speed.)Thus, many people send the long text file to the MORE command by using the command:

c:\windows>more < xyz.txt

This command sends the contents of the xyz.txt file to the MORE command which displays the contents page by page.