[文本处理] BAT批处理截取字符串为何无法获得结果？

@echo off
setlocal enabledelayedexpansion
set str=12345679
set n=6
echo %str%
set strNEW=!str:~0,%n%!
echo -------------------------------------
echo %strNEW%
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SET str=12345679
set n=5
setlocal enabledelayedexpansion
echo !str!
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@echo off & SETLOCAL enabledelayedexpansion
(
SET str=12345679

SET n=6

echo !str!
for %%a in (!n!) do echo,!str:~0,%%a!

)
echo -------------------------------------
echo %str%
echo !str:~0,%n%!
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@ECHO OFF & SETLOCAL enabledelayedexpansion
FOR /F %%i in ('dir 20*.txt /b') DO (SET str=%%i
	FOR /F "tokens=*" %%j in (%%i) DO (SET str=%%j

		IF "!str:~0,10!" == "tablename=" SET str1=!str:~10!
		IF not "!str:~0,10!" == "tablename=" SET str2=!str!

	) 

	set strlen=0
	for /l %%i in (0,1,30) do ( 
		if "!str1:~%%i,1!"=="" (if !strlen!==0 set strlen=%%i)
		)

	echo !str2:strlen,100!
)
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@echo off
setlocal enabledelayedexpansion
for /f %%i in ('dir /b 20*.txt') do (
    set str=%%i
    for /f "tokens=*" %%j in (%%i) do (
        set str=%%j
        if "!str:~0,10!" == "tablename=" (
            set str1=!str:~10!
        ) else (
            set str2=!str!
        )
    )
    set strlen=0
    for /l %%i in (0,1,30) do (
        if "!str1:~%%i,1!"=="" (
            if !strlen!==0 (
                set strlen=%%i
            )
        )
    )
    call echo %%str2:~!strlen!,100%%
)
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