[kovidgoyal]

You should write a python script for that, not bash, it will be easier. Something like:

import subprocess

users = tuple(filter(None, open('users.txt').read().splitlines()))
if users:
    subprocess.check_call('systemctl stop calibre-server'.split())
    for user in users:
         username, password = user.split()
         subprocess.check_call(['calibre-server', '--manage-users', '--', 'add', username, password])
    subprocess.check_call('systemctl start calibre-server')