concatenate variables

set ROOT=c:\programs 
set SRC_ROOT=%ROOT%\System\Source

Note that if strings has spaces then quotation marks are needed at definition and must be chopped while concatenating:

rem The retail files set
set FILES_SET="(*.exe *.dll"

rem The debug extras files set
set DEBUG_EXTRA=" *.pdb"

rem Build the DEBUG set without any
set FILES_SET=%FILES_SET:~1,-1%%DEBUG_EXTRA:~1,-1%

rem Append the closing bracket
set FILES_SET=%FILES_SET%)

echo %FILES_SET%

Cheers...

If you need to concatenate paths with quotes, you can use = to replace quotes in a variable. This does not require you to know if the path already contains quotes or not. If there are no quotes, nothing is changed.

@echo off
rem Paths to combine
set DIRECTORY="C:\Directory with spaces"
set FILENAME="sub directory\filename.txt"

rem Combine two paths
set COMBINED="%DIRECTORY:"=%\%FILENAME:"=%"
echo %COMBINED%

rem This is just to illustrate how the = operator works
set DIR_WITHOUT_SPACES=%DIRECTORY:"=%
echo %DIR_WITHOUT_SPACES%