Delayed Variable Expansion

Delayed variable expansion is a feature in Windows batch scripting that allows you to access the current value of a variable at execution time rather than at parse time. This is particularly useful when dealing with variables inside blocks of code (like if statements or for loops).

Key Points:

1. Syntax: Uses exclamation marks !var! instead of percent signs %var%
2. Activation: Requires SETLOCAL EnableDelayedExpansion at the beginning of your script
3. When Needed: Essential when modifying and reading variables within the same block

Example Without Delayed Expansion:

@echo off
set var=original
if 1==1 (
    set var=new
    echo %var%  :: Will still show "original"
)

Example With Delayed Expansion:

@echo off
SETLOCAL EnableDelayedExpansion
set var=original
if 1==1 (
    set var=new
    echo !var!  :: Correctly shows "new"
)

Mathematical Operations:

When performing calculations in loops, delayed expansion is often necessary:

SETLOCAL EnableDelayedExpansion
set /a count=0
for /l %%i in (1,1,5) do (
    set /a count+=1
    echo Iteration !count!
)

The main difference can be summarized with the equation: \(\text{Regular Expansion} \rightarrow \%var\% = \text{Value at parse time}\) \(\text{Delayed Expansion} \rightarrow !var! = \text{Value at execution time}\)

This feature is crucial for writing robust batch scripts that need to handle dynamic variable values within code blocks.