An extension to <a href="https://stackoverflow.com/users/781339/mechaflash">Mechaflash</a>'s answer, and solves the problem of overwriting the file by generating a unique filename for the "testing" file.
@ECHO OFF
SETLOCAL ENABLEDELAYEDEXPANSION
SET "a=%~1"
SET "b="
SET "g=0"
:a
SET "c= `1234567890-=qwertyuiop[]asdfghjkl;'zxcvbnm,.~!@#$%%^&()_+QWERTYUIOP{}ASDFGHJKLZXCVBNM"
SET /A "d=0, e=1"
:b
IF "!c!" NEQ "" (
IF "!c:~%d%,1!" NEQ "" (
IF EXIST "!a!\!b!!c:~%d%,1!" (
SET "c=!c:~0,%d%!!c:~%e%!"
) ELSE (
SET /A "d=!d!+1, e=!e!+1"
)
GOTO :b
)
)
IF "!c!" EQU "" (
SET "c= `1234567890-=qwertyuiop[]asdfghjkl;'zxcvbnm,.~!@#$%%^&()_+QWERTYUIOP{}ASDFGHJKLZXCVBNM"
:c
IF "!c!" NEQ "" (
IF "!c:~%d%,1!" NEQ "" (
SET /A "d=!d!+1"
GOTO :c
)
)
SET /A "d=!d!-1"
SET /A "f=%RANDOM%*!d!/32768"
SET "b=!b!!c:~%f%,1!"
GOTO :a
) ELSE (
SET /A "d=!d!-1"
SET /A "f=%RANDOM%*!d!/32768"
SET "b=!b!!c:~%f%,1!"
)
((ECHO EXIT>"!a!\!b!" && SET "g=1") & IF EXIST "!a!\!b!" DEL /F "!a!\!b!") >NUL 2>&1
ENDLOCAL & (SET "a=%g%")
IF "%a%" EQU "1" ECHO TRUE
(`%~1` is the input directory)<br/>
EDIT: If you want a more safe option<br/>
@ECHO OFF
SETLOCAL ENABLEDELAYEDEXPANSION
SET "a=%~1"
SET "b="
SET "g=0"
:a
SET "c= `1234567890-=qwertyuiop[]asdfghjkl;'zxcvbnm,.~!@#$%%^&()_+QWERTYUIOP{}ASDFGHJKLZXCVBNM"
SET /A "d=0, e=1"
:b
IF "!c!" NEQ "" (
IF "!c:~%d%,1!" NEQ "" (
IF EXIST "!a!\!b!!c:~%d%,1!" (
SET "c=!c:~0,%d%!!c:~%e%!"
) ELSE (
SET /A "d=!d!+1, e=!e!+1"
)
GOTO :b
)
)
IF "!c!" EQU "" (
SET "c= `1234567890-=qwertyuiop[]asdfghjkl;'zxcvbnm,.~!@#$%%^&()_+QWERTYUIOP{}ASDFGHJKLZXCVBNM"
:c
IF "!c!" NEQ "" (
IF "!c:~%d%,1!" NEQ "" (
SET /A "d=!d!+1"
GOTO :c
)
)
SET /A "d=!d!-1"
SET /A "f=%RANDOM%*!d!/32768"
SET "b=!b!!c:~%f%,1!"
GOTO :a
) ELSE (
SET /A "d=!d!-1"
SET /A "f=%RANDOM%*!d!/32768"
SET "b=!b!!c:~%f%,1!"
)
IF EXIST "!a!\!b!" (
SET "b=!b:~0,-1!"
GOTO :a
) ELSE (
((ECHO EXIT>"!a!\!b!" && SET "g=1") & IF EXIST "!a!\!b!" DEL /F "!a!\!b!") >NUL 2>&1
)
ENDLOCAL & (SET "a=%g%")
IF "%a%" EQU "1" ECHO TRUE