https://github.com/saracen/bitcoin-all-key-generator
in python, because thats what i have installed 2.7 and 3.xx.
Where i can set the starting point for example start with
0000000000000000000000000000000000000000000000000000251462abc584
and output to a txt file.
You want a random 256-bit number
like this:
import random
random.getrandbits(256)
this will output a 256-bit number, just like any bitcoin private key. I ran this here and it returned:
92190443112138794064351826137983695759100025828546020995296739378362186540927
To create a file you can use ETFBitcoin solution, like this:
import random
path = './file.txt'
with open(path, 'w+') as f:
f.write(str(random.getrandbits(256)))
https://en.bitcoin.it/wiki/Private_key#:~:text=In%20Bitcoin%2C%20a%20private%20key,range%200%2D9%20or%20A%2DF.
E9873D79C6D87DC0FB6A5778633389_SAMPLE_PRIVATE_KEY_DO_NOT_IMPORT_F4453213303DA61 F20BD67FC233AA33262
Edit: For security purposes, using secrets lib like NotATether suggested is better.