What is the usage of "." in Windows DOS Batch Scripting?

Question

For Eg:- "if %1.==Sub. goto %2"
what does the above given example mean?

Answer 1

Let us call %1 as variable1
So here in this statement the script means
If contents of variable1 begins with Sub then goto.

In this example it is used as a regular expression to indicate
' whatever after wards '

Answer 2

Typically, extra characters are put into expressions like this to prevent an error condition when a variable is empty, null, or not provided.

For instance, if the test expression was used without the period, then it would look like this,

if %1==Sub goto %2

and you ran your batch file without providing a value for %1, then the null value of %1 will translate into a line like this,

if ==Sub goto %2

and you would get an error like, "==Sub was unexpected at this time." Because the "if" expression isn't properly defined.

So, by putting a default placeholder around the variable, this prevents the error condition. If %1 is left empty in your original line, then the expression becomes

if .==Sub. goto %2

And the "if" line can be understood by the batch file because there is something on both sides of the test.

Usually, I use quotes around my variable tests, like

If "%1"=="Sub" goto %2

or, to test for blank or null values, like this

If "%1"=="" goto :Syntax

and I've have a :SYNTAX section to explain how to run the batch file with proper inputs, but that's just a preference. Periods do the same thing.