variables expansion inside for in batch

Question

I am new to batch and I don\'t understand when to use late variable expansion or normal expansion. Below I have a test script in which I have tested the variables expansion.

Answer 1

When a line or a block of code (the code enclosed in parenthesis in your for, if, ... ) is reached, the parser removes all variable reads, replacing them with the value inside the variable before the line/block starts to execute. So, if the value of a variable is changed inside a line/block, this changed value is not visible inside that same line/block since all access to the variable content has been replaced with its value.

So, if the value of the variable is changed inside a line/block AND the changed value of the variable needs to be read/accessed inside the same line/block, delayed expansion is needed.

for and if commands are places where this is usually more evident, but constructs as

set "data=test"
....
set "data=other test" & echo %data%

shows the same problem. When the parser handles the last line, %data% is replaced with its value, and then the line is executed. So the final line executed is

set "data=other test" & echo test

Answer 2

This can work depending on the text being parsed, as poison characters can affect it.

@echo off
set var=0
echo late var=%var%
echo var=%var%

for /F "delims= " %%A in (temp.txt) do call :next "%%A"
goto :EOF
:next
        echo Analyzing %~1
        set line=%~1
        echo line=%line%
        echo late line=%line%