How to survive “delayed variable expansion” in a DOS batch script

Question

This is my script:

@echo off
setlocal

for /f %%i in ('echo aaa/') do set REPO=%%i
if "%REPO%"=="" (
  echo No input
) else (
  echo %REPO:~-1%
  echo %REPO:~0,-1%
  if %REPO:~-1%==/ set REPO=%REPO:~0,-1%
  echo %REPO%
)

endlocal

Please, observe:

c:\dev\shunra\GlobalLibrary\Server>c:\Utils\hgbackup.cmd
/
aaa
aaa/

c:\dev\shunra\GlobalLibrary\Server>

What is going on?

EDIT

Note, that I am assigning to REPO something that evaluates to "aaa", hence I expect it to print "aaa", not "aaa/". It drives me crazy.

EDIT2

Apparently, here is the culprit (from help on the set command):

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%

But I tried using the ! sign, still it does not work for me. I use get ! printed on the screen or the wrong result again.

Answer 1

As has been discussed in the comments, and in your edited question, you need delayed expansion.

Delayed expansion must be enabled before you can use it. Within a batch script you can use setlocal enableDelayedExpansion

@echo off
setlocal enableDelayedExpansion

for /f %%i in ('echo aaa/') do set REPO=%%i
if "%REPO%"=="" (
  echo No input
) else (
  echo %REPO:~-1%
  echo %REPO:~0,-1%
  if %REPO:~-1%==/ set REPO=%REPO:~0,-1%
  echo !REPO!
)

endlocal

EDIT

The above fails if the IN() clause is changed such that REPO is undefined. For example: in (echo.)

It fails because the entire IF/ELSE construct must have valid syntax, even it the ELSE clause will not be executed.

If REPO is undefined, then

if %REPO:~-1%==/ set REPO=%REPO:~0,-1%

expands to

if ~-1REPO:~0,-1

which is invalid syntax.

The problem again is solved by using delayed expansion.

@echo off
setlocal enableDelayedExpansion

for /f %%i in ('echo.') do set REPO=%%i
if "%REPO%"=="" (
  echo No input
) else (
  echo %REPO:~-1%
  echo %REPO:~0,-1%
  if !REPO:~-1!==/ set REPO=%REPO:~0,-1%
  echo !REPO!
)

endlocal

Answer 2

Note, that I am assigning to REPO something that evaluates to "aaa"

Actually, you're conditionally assigning something. Have you testing whether the then-part is actually executing (for example, echo If Entered ).

Answer 3

This works for me (just an extract from my whole script)

choice /C 1234567H /M "Select an option or ctrl+C to cancel"
set _dpi=%ERRORLEVEL%

if "%_dpi%" == "8" call :helpme && goto menu 

for /F "tokens=%_dpi%,*" %%1 in ("032 060 064 096 0C8 0FA 12C") do set _dpi=%%1
echo _dpi:%_dpi%: