DOS String manipulation - Replace on empty variable

Question

I have the following DOS commands in a batch file:

SET "REALM="
ECHO %REALM:/=.%

SET "REALM=Westeros"
ECHO %REALM:/=.%

SET "REALM=Westeros/"
ECHO %REALM:/=.%

The last two commands give me the expected results: Westeros Westeros.

But the first, replacing / for . on an empty variable, gives: /=.

Why does this happen and how can I do this right?

What is the "expected result" when you try to replace characters from a value that does not exists? — Aacini
– Aacini, Commented Sep 4, 2014 at 15:03
with an undefined %REALM%, ECHO %REALM:/=.% gives me %REALM:/=.% (as I would expect), not /=. (on cmd.exe; I have no DOS available any more) — Stephan
– Stephan, Commented Sep 4, 2014 at 16:49

Community · Accepted Answer · 2017-05-23 12:12:46Z

3

It's a problem/feature of the parser with undefined variables.

How does the Windows Command Interpreter (CMD.EXE) parse scripts?

See also How does the Windows Command Interpreter (CMD.EXE) parse scripts?

To solve this you need to test first for undefined variables like.

set "REALM="
if defined REALM SET "REALM=%REALM:/=.%"
echo( %REALM%

I used here echo( to avoid the output of "ECHO IS OFF" when the variable is empty.

edited May 23, 2017 at 12:12

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answered Sep 4, 2014 at 12:40

jeb

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2 Comments

Martin Giessen Over a year ago

So, as I understand it, if REALM is undefined then the command-line processor will remove %REALM: from the command-line, leaving "/=." and continue the scan. Thus; To solve this puzzle I would have to define an empty variable, which I can't, to get the expected result.

jeb Over a year ago

Yes, replacing works only when the variable is defined, else it fails and you get rubbish

SachaDee · Accepted Answer · 2014-09-04 10:59:04Z

0

If you make aSET "REALM=". Then %Realm% is undefined (without any value).

answered Sep 4, 2014 at 10:59

SachaDee

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1 Comment

Martin Giessen Over a year ago

I understand the variable is undefined, but why does the replace return /=. instead of an empty string or nothing?

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