I have installed geth on an offline-computer and created a account with ./geth account new. Than I have copied (USB-Stick) the resulting file from the offline-computers keystore/ to an online-computer, where I can check the balance of that account. Of course I will never type the passphrase for that account on the online-computer.
How can I transfer Ethers from the account using only geth? (I do not want to install any third-party software on the offline-computer.)
I have read other posts saying "using the unofficial RPC method eth.signTransaction". Unfortunately it's not clear to me, if this is related to geth or an other tool. I would very much appreciate to sign offline just with geth.
the file created by geth account new is the file you don't want to end up on a computer connected to a network. It's the file that you want to have hidden somewhere, where nobody else knows where it is
you can check the balance for any account either locally in a synced geth client or on https://etherscan.io, without ever uploading that file anywhere. With geth it's done by running in the console web3.fromWei(web3.eth.getBalance('0x....'), 'ether') where you replace 0x.... with your actual account
In order to send money out of that new account using geth, without storing the wallet on an internet connected computer is documented here on stackexchange.