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Telegram (Messenger)?

Yes. Communicating with telegram from bash is simple. Check out https://www.curry-software.com/en/blog/telegram_unit_fail/ for example.

I'm wondering if it is possible also with Signal (signal.org)

Harder, since they don't have an open API and don't want people using non-standard clients. The clients are open-source though, so probably you can do it.

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