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concatenating 2 variables to create new variable in MS .bat-file

First time posting so please be patient.
I am trying to create a .bat file as follows

set a=file
set b=3
set file3=success
SETLOCAL ENABLEDELAYEDEXPANSION
echo %%a%!b!%


I was hoping it would parse as "echo %file3%" and would then print "success". Would appreciate guidance as to how to achieve the intended result. I would also appreciate it if someone could explain what actually happened.

echo !%a%%b%!

call echo %%%a%%b%%%

2 Answers
2

What you want to do with delayedExpansions is to expand one variable before an other. The one which should be expanded first is the one in %percentages%, the one which should be expanded delayed is in !exclamationMarks! (which can only be used while delayedExpansion is enabled).

So in your case you have to expand a and b first -> %a%%b% and then the value of both -> !file3!

%a%%b%

!file3!

combined: echo !%a%%b%!

echo !%a%%b%!

The important thing for such, as I call them, nested variables is that the inner ones need to be replaced/expanded before the outer ones. Variables a and b are the ones I consider as the inner ones, and file3 is the outer one.

a

b

file3

So using delayed expansion, we have two expansion phases available: the normal or immediate expansion (%) and the delayed expansion (!). Applying them in the said order the solution looks like this:

%

!

echo !%a%%b%!


So at first %a% is expanded to file and %b% to 3, hence the delayed expansion phase receives !file3!.

%a%

file

%b%

3

!file3!

There is also a method that avoids delayed expansion: using call, which introduces another parsing phase where immediate expansion is repeated. The code looks like this:

call

call echo %%%a%%b%%%


So during the first normal expansion phase %% becomes replaced by a literal %, %a% becomes expanded to file, %b% to 3 and the remaining %% becomes again replaced by %; so we have %file3% left, which is expanded by the said second parsing phase due to call.

%%

%

%a%

file

%b%

3

%%

%

%file3%

call

Both of the above methods imply normal expansion for the inner variables, which can cause problems when they are changed in a parenthesised block of code, like loop, for example. To avoid normal expansion we could use a for loop meta-variable, which is expanded later than normal expansion but before delayed expansion:

for

for %%I in ("!a!!b!") do echo !%%~I!


Or:

for /F "delims=" %%I in ("!a!!b!") do echo !%%I!


Since no immediate expansion is used herein, this would also work within another for loop even with changing variables a and b.

for

a

b

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