I wrote a Python script which sends any message via Telegram:

#!/opt/anaconda3/bin/python

import asyncio
import telegram
import os
import sys


async def main():
    bot = telegram.Bot(os.environ["TELEGRAM_TOKEN"])
    async with bot:

        #print(await bot.get_me())
        #print((await bot.get_updates())[0])
        await bot.send_message(text=' '.join(sys.argv[1:]), chat_id=MYCHATID)


if __name__ == '__main__':
    asyncio.run(main())

TELEGRAM_TOKEN is set in /etc/environment, MYCHATID is hardcoded.

Created two scripts to run it

# cat /etc/init.d/sendbot_boot.sh
#!/bin/bash

/opt/anaconda3/bin/python /usr/local/bin/sendbot "Booting..."

# cat /etc/init.d/sendbot_shutdown.sh
#!/bin/bash

/opt/anaconda3/bin/python /usr/local/bin/sendbot "Shutting down..."

Added one of them to cron

# cat /var/spool/cron/crontabs/root
@reboot /etc/init.d/sendbot_boot.sh

And another one to rc6.

# ls -l /etc/rc6.d/K99sendbot_shutdown
lrwxrwxrwx 1 root root 29 фев 10 02:08 /etc/rc6.d/K99sendbot_shutdown -> ../init.d/sendbot_shutdown.sh

All scripts are executable and work well when I run them from terminal. But neither of them work on actual reboot.

What I did wrong?

# lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 18.04.6 LTS
Release:        18.04
Codename:       bionic