Why does this batch variable never change even when set?

Question

@echo off
SET first=0
FOR %%N IN (hello bye) DO (
SET first=1
echo %first%
echo %%N
)

It seems that the variable "first" is always 0. Why?

Answer 1

With batch files, variables are expanded when their command is read - so that would be as soon as the for executes. At that point, it no longer says echo %first%, it literally says echo 0, because that was the value at the point of expansion.

To get around that, you need to use delayed expansion by surrounding your variable name with ! instead of % - so that would be echo !first!. This may require you to start cmd.exe with the /V parameter, or use setlocal enabledelayedexpansion in the beginning of your batch file (just after echo off).

If you type set /?, you'll see a much more detailed explanation of this at the end of the output.