windows batch files: setting variable in for loop

Question

I have a number of files with the same naming scheme. As a sample, four files are called \"num_001_001.txt\", \"num_002_001.txt\", \"num_002_002.txt\", \"num_002_003.txt\"

Answer 1

Your problem is that the variable get replaced when the batch processor reads the for command, before it is executed.

Try this:

SET temp=Hello, world!
CALL yourbatchfile.bat

And you'll see Hello printed 5 times.

The solution is delayed expansion; you need to first enable it, and then use !temp! instead of %temp%:

@echo off
SETLOCAL ENABLEDELAYEDEXPANSION
FOR /R %%X IN (*.txt) DO (
  set temp=%%~nX
  echo directory !temp:~0,7!
)

See here for more details.

Answer 2

I'm not sure whether this is officially documented, but you can simulate delayed expansion using the call statement:

@echo off
FOR /R %%X IN (*.txt) DO (
  set temp=%%~nX
  call echo directory %%temp:~0,7%%
)

Doubling the percent signs defers the variable substitution to the second evaluation. However, delayed expansion is much more straightforward.

Answer 3

Another solution is to move the body of the for loop to a subroutine and call it.

@echo off
FOR /R %%X IN (*.txt) DO call :body %%X
goto :eof

:body
set temp=%~n1
echo directory %temp:~0,7%
goto :eof

Why do this? One reason is that the Windows command processor is greedy about parentheses, and the results may be surprising. I usually run into this when dereferencing variables that contain C:\Program Files (x86).

If the Windows command processor was less greedy, the following code would either print One (1) Two (2) or nothing at all:

@echo off
if "%1" == "yes" (
   echo 1 (One)
   echo 2 (Two)
)

However, that's not what it does. Either it prints 1 (One 2 (Two), which missing a ), or it prints 2 (Two). The command processor interprets the ) after One as the end of the if statement's body, treats the second echo as if it's outside the if statement, and ignores the final ).