Check for null variable in Windows batch

I'm working on a Windows batch file that will bcp three text files into SQL Server. If something goes wrong in production, I want to be able to override the file names. So I'm thinking of doing something like this.

bcp.exe MyDB..MyTable1 in %1 -SMyServer -T -c -m0
bcp.exe MyDB..MyTable2 in %2 -SMyServer -T -c -m0
bcp.exe MyDB..MyTable3 in %3 -SMyServer -T -c -m0

I would like to be able to enter default names for all three files, to be used if the positional parameters are not supplied. The idea would be either to execute

myjob.bat

with no parameters, and have it use the defaults, or execute

myjob.bat "c:\myfile1" "c:\myfile2" "c:\myfile3"

and have it use those files. I haven't been able to figure out how to tell if %1, %2 and %3 exist and/or are null. I also don't know how to set those values conditionally. Is this possible? Any suggestions would be appreciated.

Solution 1:

To test for the existence of a command line paramater, use empty brackets:

IF [%1]==[] echo Value Missing

or

IF [%1] EQU [] echo Value Missing

The SS64 page on IF will help you here. Under "Does %1 exist?".

You can't set a positional parameter, so what you should do is do something like

SET MYVAR=%1

You can then re-set MYVAR based on its contents.

Solution 2:

The right thing would be to use a "if defined" statement, which is used to test for the existence of a variable. For example:

IF DEFINED somevariable echo Value exists

In this particular case, the negative form should be used:

IF NOT DEFINED somevariable echo Value missing

PS: the variable name should be used without "%" caracters.