Weird scope issue in .bat file - scope

Question

I'm writing a simple .bat file and I've run into some weird behavior. There are a couple places where I have to do a simple if/else, but the code inside the blocks don't seem to be working correctly.
Here's a simple case that demonstrates the error:
#echo off
set MODE=FOOBAR
if "%~1"=="" (
set MODE=all
echo mode: %MODE%
) else (
set MODE=%~1
echo mode: %MODE%
)
echo mode: %MODE%
The output I'm getting is:
C:\>test.bat test
mode: FOOBAR
mode: test
Why is the echo inside the code block not getting the new value of the variable? In the actual code I'm writing I need to build a few variables and reference them within the scope of the if/else. I could switch this to use labels and gotos instead of an if/else, but that doesn't seem nearly as clean.
What causes this behavior? Is there some kind of limit on variables within code blocks?

Answer 1

You are running into the problem of cmd's static variable expansion. The MODE variable is only evaluated once. You can see this if you omit the #echo off line.
From the set /? documentation:
Finally, support for delayed environment variable expansion has
been added. This support is always
disabled by default, but may be
enabled/disabled via the /V command
line switch to CMD.EXE. See CMD /?
Delayed environment variable expansion is useful for getting around
the limitations of the current
expansion which happens when a line of
text is read, not when it is executed.
The following example demonstrates the
problem with immediate variable
expansion:
set VAR=before
if "%VAR%" == "before" (
set VAR=after
if "%VAR%" == "after" #echo If you see this, it worked
)
would never display the message, since
the %VAR% in BOTH IF statements is
substituted when the first IF
statement is read, since it logically
includes the body of the IF, which is
a compound statement. So the IF
inside the compound statement is
really comparing "before" with "after"
which will never be equal. Similarly,
the following example will not work as
expected:
set LIST=
for %i in (*) do set LIST=%LIST% %i
echo %LIST%
in that it will NOT build up a list of
files in the current directory, but
instead will just set the LIST
variable to the last file found.
Again, this is because the %LIST% is
expanded just once when the FOR
statement is read, and at that time
the LIST variable is empty. So the
actual FOR loop we are executing is:
for %i in (*) do set LIST= %i
which just keeps setting LIST to the
last file found.
Delayed environment variable expansion
allows you to use a different
character (the exclamation mark) to
expand environment variables at
execution time. If delayed variable
expansion is enabled, the above
examples could be written as follows
to work as intended:
set VAR=before
if "%VAR%" == "before" (
set VAR=after
if "!VAR!" == "after" #echo If you see this, it worked
)
set LIST=
for %i in (*) do set LIST=!LIST! %i
echo %LIST%

Answer 2

setlocal EnableDelayedExpansion
will enable the /v flag

Answer 3

Additionally to already anwered here.
Long answer article:
https://rsdn.org/article/winshell/NTCommandProcessor.xml
The google translated variant is pretty broken, but you can at least read the raw text:
https://translate.google.com/?sl=ru&tl=en&text=https%3A%2F%2Frsdn.org%2F%3Farticle%2Fwinshell%2FNTCommandProcessor.xml&op=translate
Start read from Conditional block section.
Short answer:
The block operator (...) blocks a %-variable expansion until the top most scope. You have to exit the scope out to be able to use %-variable as is:
#echo off
set "MODE="
(
(
set MODE=all
echo MODE=%MODE%
)
echo MODE=%MODE%
)
echo MODE=%MODE%
Or use call prefix to reevaluate it in place:
#echo off
set "MODE="
(
(
set MODE=all
call echo MODE=%%MODE%%
)
)

Answer 4

Looks like the read and write use different scoping rules.
If you eliminate this line
set MODE=FOOBAR
it will work as expected. So you'll probably need to have a complex series if if/elses to get the variables populated as you'd like.