Windows Batch files: what is variable expansion, and what does EnableDelayedExpansion mean?

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  • Variable expansion means replace a variable enclosed in % or ! by its value.
  • The %normal% expansion happen just once, before a line is executed. This means that a %variable% expansion have the same value no matters if the line is executed several times (like in a for command).
  • The !delayed! expansion is performed each time that the line is executed.

See this example:

@echo off
setlocal EnableDelayedExpansion
set "var=Original"
set "var=New" & echo Normal: "%var%", Delayed: "!var!"

Output:

Normal: "Original", Delayed: "New"

Another one:

@echo off
setlocal EnableDelayedExpansion
set "var1=Normal"
set "var2=Delayed"
for /L %%i in (1,1,10) do (
   set "var1=%var1% %%i"
   set "var2=!var2! %%i"
)
echo Normal:  "%var1%"
echo Delayed: "%var2%"

Output:

Normal:  "Normal 10"
Delayed: "Delayed 1 2 3 4 5 6 7 8 9 10"

Normal expansion is not necessarily a disadvantage, but depends on the specific situation it is used. For example, in any other programming languages, to exchange the value of two variables you need the aid of a third one, but in Batch it can be done in just one line:

set "var1=%var2%" & set "var2=%var1%"

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