Strange question about windows batch file

Question

I got 1.txt and 2.txt in my working directory. I use the following batch to list all the files.

The batch is this:

@echo off
for /f "tokens=*" %%a in ('dir *.txt /b') do (
    echo ---------------
    set file_variable=%%a
    echo file_variable=%file_variable%
    echo filename=%%a
    )    

The result is below:

---------------
file_variable=2.txt   <---------------why it is not 1.txt here??
filename=1.txt
---------------
file_variable=2.txt
filename=2.txt

Thanks.

Answer 1

You need to put:

@setlocal enableextensions enabledelayedexpansion

at the top of your file and

endlocal

at the end.

Then you need to use the delayed expansion substitution characters.

@setlocal enableextensions enabledelayedexpansion
@echo off
for /f "tokens=*" %%a in ('dir *.txt /b') do (
    echo ---------------
    set file_variable=%%a
    echo file_variable=!file_variable!
    echo filename=%%a
)
endlocal

C:\Documents and Settings\Pax\My Documents> qq.cmd
---------------
file_variable=1.txt
filename=1.txt
---------------
file_variable=2.txt
filename=2.txt

---

What you're seeing without delayed expansion is that the entire for loop is being evaluated before running. That includes the substitution, so that %file_variable% will be replaced with the value it held before the loop started. Using delayed expansion defers the evaluation until the actual line is executed.

---

There are all sorts of wonderful Windows scripting tricks over at Rob van der Woude's site, containing quite a lot of different ways of doing things under Windows with various tools.

Answer 2

In this particular case you could get the correct output if you ECHOed file_variable like this:

CALL ECHO file_variable=%%file_variable%%

This approach is less flexible and probably less performant than the one described in @paxdiablo's answer. It's probably just a quick-and-dirty method of delaying the expansion of a variable without the need to enable the special syntax for that.