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, Sep 04 '14 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, Sep 04 '14 at 16:49

score 3 · Accepted Answer · edited May 23 '17 at 12:12

3

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

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 '17 at 12:12

Community

answered Sep 04 '14 at 12:40

jeb

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. – Martin Giessen Sep 05 '14 at 07:27
Yes, replacing works only when the variable is defined, else it fails and you get rubbish – jeb Sep 05 '14 at 13:42

score 0 · Answer 2 · answered Sep 04 '14 at 10:59

0

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

answered Sep 04 '14 at 10:59

SachaDee

I understand the variable is undefined, but why does the replace return /=. instead of an empty string or nothing? – Martin Giessen Sep 04 '14 at 11:47

2 Answers2