Batch file fails to set environment variable within conditional statement

Question

Why does the following Windows Batch File output Foo followedby Bar, rather than Baz?

@echo off
setlocal

set _=Foo
echo %         


        

Answer 1

The answer to this is the same as the answer to:Weird scope issue in batch file. See there for more details. Basically variable expansion is done at line read time, not at execution time.

Answer 2

try this

@echo off
setlocal

set _=Foo
echo %_%
set _=Bar
if "1" NEQ "2" goto end
set _=Baz
echo %_%
:end

Answer 3

What's happening is that variable substitution is done when a line is read. What you're failing to take into account is the fact that:

if 1==1 (
    set _=Baz
    echo %_%
)

is one "line", despite what you may think. The expansion of "%_%" is done before the set statement.

What you need is delayed expansion. Just about every single one of my command scripts starts with "setlocal enableextensions enabledelayedexpansion" so as to use the full power of cmd.exe.

So my version of the script would be:

@echo off
setlocal enableextensions enabledelayedexpansion

set _=Foo
echo !_!
set _=Bar
if 1==1 (
    set _=Baz
    echo !_!
)

endlocal

This generates the correct "Foo", "Baz" rather than "Foo", "Bar".