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%