Batch File Programming Part-5

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:\saurav.doc GOTO Saurav
Goto end
:Saurav
ECHO Saurav
:end

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

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

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:\saurav.doc ECHO It doesn't Exist

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.

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.

In the next post we will see some more concepts of Batch File Programming.