How can I make my batch script add code to itself - batch-file

Question

I have made a batch script that is just a basic login screen, however, I want to make it so it adds a code to itself AFTER you register (i.e it lets you skip registering) I want to add this code:
:alreadyregistered
goto login
However for it to work properly, I also need it to remember your username/password. This script stores this text in a text document, but i don't know how to make it compare the username/password you signed in to the ones in the document. (something like this: if %username% == "what is in the notepad" the goto logged in if else goto error)
It would also be cool if you could fix some errors I have. I want to make it so if there was an error making your account a "user account" (get system error 5) it takes you to the accounterror section. I tried using errorlevel but it does not work. (if ERRORLEVEL NEQ 0 goto accounterror) Thanks for all your help, as I'm a beginner.
Here is my code:
#echo off
color fc
rem register
:register
title Register
cls
set /p username="Please type a username: "
echo username = %username% > logindetails.txt
powershell -Command $pword = read-host "Please Enter a Password" -AsSecureString ; $BSTR=[System.Runtime.InteropServices.Marshal]::SecureStringToBSTR($pword) ; [System.Runtime.InteropServices.Marshal]::PtrToStringAuto($BSTR) > .tmp.txt & set /p password=<.tmp.txt & del .tmp.txt
echo password = %password% >> logindetails.txt
powershell -Command $pword = read-host "Please Retype your Password" -AsSecureString ; $BSTR=[System.Runtime.InteropServices.Marshal]::SecureStringToBSTR($pword) ; [System.Runtime.InteropServices.Marshal]::PtrToStringAuto($BSTR) > .tmp.txt & set /p confirmpassword=<.tmp.txt & del .tmp.txt
if %username% == %password% goto matcherror
if %confirmpassword% NEQ %password% goto passwordnotmatch
goto login1
rem if passwords don't match
:passwordnotmatch
title ERROR
cls
(
mshta javascript:alert^("Passwords do not match, try again"^);close^(^);
)
goto register
rem this is for credential matching
:matcherror
title ERROR
cls
(
mshta javascript:alert^("Credientials cannot match, try again"^);close^(^);
)
goto register
rem username for login
:login1
title Login
cls
set /p usernamelogin="Please type your username: "
if %usernamelogin% == %username% goto login2
if %usernamelogin% NEQ %username% goto loginerror
rem this is incorrect username/password, will go back to username entering
:loginerror
title ERROR
cls
(
mshta javascript:alert^("Incorrect username or password"^);close^(^);
)
goto login1
rem password for login
:login2
title Login
powershell -Command $pword = read-host "Please Enter your Password" -AsSecureString ; $BSTR=[System.Runtime.InteropServices.Marshal]::SecureStringToBSTR($pword) ; [System.Runtime.InteropServices.Marshal]::PtrToStringAuto($BSTR) > .tmp.txt & set /p passwordlogin=<.tmp.txt & del .tmp.txt
if %passwordlogin% == %password% goto loggedin
if %passwordlogin% NEQ %password% goto loginerror
rem after login is complete
:loggedin
title Welcome %username%
cls
echo Hello %username%, what would you like to do?
echo 1) Make your account an actual user account
echo 2) Exit
set /p something=">"
if %something% == 1 goto work
if %something% == 2 exit
if not defined something (
(
mshta javascript:alert^("Please enter a valid number"^);close^(^);
)
goto loggedin
)
rem make a user account using credentials from this file
:work
net user %username% %password% /add
(
mshta javascript:alert^("Congratulations, your account %username% was created!"^);close^(^);
)
goto loggedin
rem error making account an actual user account
:accounterror
cls
(
mshta javascript:alert^("Sorry, %username%, your account was not created due to an error"^);close^(^);
)
goto loggedin

Answer 1

(something like this: if %username% == "what is in the notepad" the
goto logged in if else goto error
that would be:
set /p "EnteredName=Please enter Username: "
<file.txt set /p NameFromFile=
if %NameFromFile%==%EnteredName% (
echo they are the same
) else (
echo they are NOT the same
)
Please don't use username as variable name, because it's a systemvariable used by windows (like date and time). Changing it could lead to unexpected results.
I want to add this code:
appending code to the batchfile is easy: just append it:
(echo :alreadyregistered
echo goto login)>>%dpnx0
%~dpnx0 stands for Drive, Path, Name and eXtension of the currently running batchfile