windows batch files: setting variable in for loop - batch-file

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"
The first set of numbers represents which "package" it's from, and the second set of numbers is simply used to distinguish them from one another. So in this example we have one file in package 001, and three files in package 002.
I am writing a windows vista batch command to take all of the files and move them into their own directories, where each directory represents a different package. So I want to move all the files for package 001 into directory "001" and all for 002 into directory "002"
I have successfully written a script that will iterate over all of the txt files and echo them. I have also written a scrip that will move one file into another location, as well as creating the directory if it doesn't exist.
Now I figure that I will need to use substrings, so I used the %var:~start,end% syntax to get them. As a test, I wrote this to verify that I can actually extract the substring and create a directory conditionally
#echo off
set temp=num_001_001.txt
NOT IF exist %temp:~0,7%\
mkdir %temp:~0,7%
And it works. Great.
So then I added the for loop to it.
#echo off
FOR /R %%X IN (*.txt) DO (
set temp=%%~nX
echo directory %temp:~0,7%
)
But this is my output:
directory num_002
directory num_002
directory num_002
directory num_002
What's wrong? Does vista not support re-assigning variables in each iteration?
The four files are in my directory, and one of them should create num_001. I put in different files with 003 004 005 and all of it was the last package's name. I'm guessing something's wrong with how I'm setting things.
I have different workarounds to get the job done but I'm baffled why such a simple concept wouldn't work.

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

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 ).

Answer 3

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.