Unfortunately, free version of ngrok changes external address each start.I got a little creative and decided to transfer the address through Telegram using a bot.
keys.sh is what keys should look like.
API_KEY can be retrieved from BotFather.
CHAT_ID is a group. You need to create it, then add bot there.
Somehow bot must send a message there. IIRC I just sent a few API requests manually. Then bot will edit that message every time new information arrives.
I finally created this file. Its path should be
$HOME/.local/share/systemd/user/ngrok-ssh.service (I just symlinked it):
mkdir -p $HOME/.local/share/systemd/user
ln -s $(realpath ngrok-ssh.service) $HOME/.local/share/systemd/user
There is a problem which might be solved eventually: the unit relies on the fact that directory containing all the files is in the home directory. As earlier, I just symlinked it:
ln -s $(realpath .) $HOME
Then the unit shall be activated:
systemctl --user enable --now ngrok-ssh.service
If autostart even before login is desired, run:
sudo loginctl enable-linger $USERNAME
It is more natural to attain the same result via python-telegram-bot or cpphttplib (for example) and what I am actually going to do, but ths bot has been written in Bash just for me learning new concepts at a time.