DosTips.com

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@echo off
:buildString
setlocal enableDelayedExpansion
if %~1 geq 8192 exit /b
set /a "n=%~1, leftBit=4096"
set "str="
echo target len=!n!
for /l %%n in (1 1 13) do (
  echo(
  call :strLen str len
  echo before double len=!len!
  set "str=!str!!str!"
  call :strLen str len
  echo after double len=!len!
  set /a "bit=n&leftBit, n<<=1"
  if !bit! neq 0 (
    set "str=!str!."
    call :strLen str len
    echo after append len=!len!
  )
)
exit /b
echo(
call :strLen str len
echo final len=!len!
exit /b


:strLen string len -- returns the length of a string
::                 -- string [in]  - variable name containing the string being measured for length
::                 -- len    [out] - variable to be used to return the string length
:: Many thanks to 'sowgtsoi', but also 'jeb' and 'amel27' dostips forum users helped making this short and efficient
:$created 20081122 :$changed 20101116 :$categories StringOperation
:$source http://www.dostips.com
(   SETLOCAL ENABLEDELAYEDEXPANSION
    set "str=A!%~1!"&rem keep the A up front to ensure we get the length and not the upper bound
                     rem it also avoids trouble in case of empty string
    set "len=0"
    for /L %%A in (12,-1,0) do (
        set /a "len|=1<<%%A"
        for %%B in (!len!) do if "!str:~%%B,1!"=="" set /a "len&=~1<<%%A"
    )
)
( ENDLOCAL & REM RETURN VALUES
    IF "%~2" NEQ "" SET /a %~2=%len%
)
EXIT /b

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>buildstring 8185
target len=8185

before double len=0
after double len=0
after append len=1

before double len=1
after double len=2
after append len=3

before double len=3
after double len=6
after append len=7

before double len=7
after double len=14
after append len=15

before double len=15
after double len=30
after append len=31

before double len=31
after double len=62
after append len=63

before double len=63
after double len=126
after append len=127

before double len=127
after double len=254
after append len=255

before double len=255
after double len=510
after append len=511

before double len=511
after double len=1022
after append len=1023

before double len=1023
after double len=2046

before double len=2046
after double len=4092

before double len=4092
after double len=8184
after append len=8184

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@echo off
setlocal EnableDelayedExpansion
set "str=."
for /l %%n in (1 1 12) do (
  set "str=!str!!str!"
)
set str=!str!!str:~-4091!
call :strLen str len
echo !len!
pause

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for %%B in (!len!) do if "!str:~%%B,1!"=="" set /a "len&=~1<<%%A"

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SET    "VAR=!STR!xxxx"

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SET/P=!STR!xxxx

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REM %STR%

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REM %STR%x

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REM !STR!1234567890
for /f %A in ("!STR!") do REM %%A1234567890

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setlocal enableDelayedExpansion
set "X=."
for /L %%n in (1,1, 13) do set "X=!X:~0,4000!!X:~0,4000!"
for /L %%n in (1,1, 180) do set "X=!X!."
REM Now X has 8180 dots

set Y=%X%^
12345678#901
REM Y has now 8191 characters, ending with 12345678901, the # was removed by the 8192 "BUG"

echo pos 8189=!Y:~8188,1! 
echo pos 8190=!Y:~8189,1! 
echo pos 8191=!Y:~8190,1! 
echo pos 8192=!Y:~8191,1!

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echo %Y:~0,8185%^
abcdefghijklmnop
echo -------

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(
echo(!Y!#
) > tmpFile