How to check command line parameter in ".bat" file?

The short answer - use square brackets:

if [%1]==[] goto :blank

or (when you need to handle quoted args, see the Edit below):

if [%~1]==[] goto :blank

Why? you might ask. Well, just as Jeremiah Willcock mentioned: http://ss64.com/nt/if.html - they use that! OK, but what's wrong with the quotes?

Again, short answer: they are "magical" - sometimes double (double) quotes get converted to a single (double) quote. And they need to match, for a start.

Consider this little script:

@rem argq.bat
@echo off

:loop 
if "%1"=="" goto :done
echo %1
shift
goto :loop

:done
echo Done.

Let's test it:

C:\> argq bla bla
bla
bla
Done.

Seems to work. But now, lets switch to second gear:

C:\> argq "bla bla"
bla""=="" was unexpected at this time.

Boom This didn't evaluate to true, neither did it evaluate to false. The script DIED. If you were supposed to turn off the reactor somewhere down the line, well - tough luck. You now will die like Harry Daghlian.

You may think - OK, the arguments can't contain quotes. If they do, this happens. Wrong Here's some consolation:

C:\> argq ""bla bla""
""bla
bla""
Done.

Oh yeah. Don't worry - sometimes this will work.

Let's try another script:

@rem args.bat
@echo off

:loop 
if [%1]==[] goto :done
echo %1
shift
goto :loop

:done
echo Done.

You can test yourself, that it works OK for the above cases. This is logical - quotes have nothing to do with brackets, so there's no magic here. But what about spicing the args up with brackets?

D:\>args ]bla bla[
]bla
bla[
Done.

D:\>args [bla bla]
[bla
bla]
Done.

No luck there. The brackets just can't choke cmd.exe's parser.

Let's go back to the evil quotes for a moment. The problem was there, when the argument ended with a quote:

D:\>argq "bla1 bla2"
bla2""=="" was unexpected at this time.

What if I pass just:

D:\>argq bla2"
The syntax of the command is incorrect.

The script won't run at all. Same for args.bat:

D:\>args bla2"
The syntax of the command is incorrect.

But what do I get, when the number of "-characters "matches" (i.e. - is even), in such a case:

D:\>args bla2" "bla3
bla2" "bla3
Done.

NICE - I hope you learned something about how .bat files split their command line arguments (HINT: *It's not exactly like in bash). The above argument contains a space. But the quotes are not stripped automatically.

And argq? How does it react to that? Predictably:

D:\>argq bla2" "bla3
"bla3"=="" was unexpected at this time.

So - think before you say: "Know what? Just use quotes. [Because, to me, this looks nicer]".

Edit

Recently, there were comments about this answer - well, sqare brackets "can't handle" passing quoted arguments and treating them just as if they weren't quoted.

The syntax:

if "%~1"=="" (...)

Is not some newly found virtue of the double quotes, but a display of a neat feature of stripping quotes from the argument variable, if the first and last character is a double quote.

This "technology" works just as well with square brackets:

if [%~1]==[] (...)

It was a useful thing to point this out, so I also upvote the new answer.

Finally, double quote fans, does an argument of the form "" exist in your book, or is it blank? Just askin' ;)

You need to check for the parameter being blank: if "%~1"=="" goto blank

Once you've done that, then do an if/else switch on -b: if "%~1"=="-b" (goto specific) else goto unknown

Surrounding the parameters with quotes makes checking for things like blank/empty/missing parameters easier. "~" ensures double quotes are stripped if they were on the command line argument.

Look at http://ss64.com/nt/if.html for an answer; the command is IF [%1]==[] GOTO NO_ARGUMENT or similar.