windows - 如何在 Windows 批处理脚本中保留 "delayed variable expansion"

Question

这是我的脚本:

@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

请注意:

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

c:\dev\shunra\GlobalLibrary\Server>

这是怎么回事？

编辑

请注意，我正在为 REPO 分配一些计算结果为“aaa”的东西，因此我希望它打印“aaa”，而不是“aaa/”。这让我发疯。

EDIT2

显然，这是罪魁祸首(来自 set 命令的帮助):

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%

但我尝试使用 ! 符号，它仍然对我不起作用。我使用 get ! 打印在屏幕上还是错误的结果。

Answer 1

正如评论中所讨论的那样，在您编辑的问题中，您需要延迟扩展。

必须启用延迟扩展才能使用它。在批处理脚本中，您可以使用 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

编辑

如果 IN() 子句发生更改，导致 REPO 未定义，则上述操作将失败。例如:in (echo.)

失败是因为整个 IF/ELSE 结构必须有有效的语法，即使 ELSE 子句也不会被执行。

如果 REPO 未定义，则

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

扩展为

if ~-1REPO:~0,-1

这是无效的语法。

问题再次通过延迟扩展解决。

@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