Thanks for the tip! I will try bitcoin-cli. Is it in the Debian repos?
I don't have a wallet. I just want to feed a seed to the tool.
I now managed to do it with electrum like this:
electrum --offline restore 'act act act act act act act act act act act act'
I don't use this for real transactions. Just to get to know how Bitcoin works. So the 'act act act ...' is simply the simplest one I came up with.That creates some kind of wallet in ~/.electrum/wallets/default_wallet
I don't know what type of seed 'act act act ...' is though. Does Electrum interpret it as a BIP39 seed?