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cat ~/.ssh/id_rsa.pub | ssh user@123.45.56.78 "mkdir -p ~/.ssh && cat >> ~/.ssh/authorized_keys"
can someone explain how the quotations works.
From what I understand. I am reading the public key on my disk then piping it to the server, making a directory if it doesn't exist and then... am I using this second concatenation to read from my disk? what is being concatenated?
It get confusing because im used to seeing a file after the cat command.
The command cat is able to concatenate files and to print them on the standard output.
When it is invoked without specifying any file or with a -, it will read the standard input.
In your command it is used in both the ways.
Step by step:
cat ~/.ssh/id_rsa.pub) does exactly what you understood: it prints the local file ~/.ssh/id_rsa.pub to the standard output.| you redirect the local standard output as standard input to the following command (ssh).ssh command practically will make execute the command inside the string "" on the remote machine passing the standard input from the pipe.~/.ssh and thanks to the option -p it will not generate error if the directory just exists.&& you will execute the following cat only if the previous mkdir exits without errors.cat >> ~/.ssh/authorized_keys is an invocation of cat without file. Its output will be appended (if possible) to the file ~/.ssh/authorized_keys thanks to the redirection operator (>>).Notes:
On many systems it exists the command ssh-copy-id to copy the local keys on the remote machine. For more info type man ssh-copy-id
ssh-copy-id — use locally available keys to authorise logins on a remote machine
As stated by roalma in the comment the original command line can work or not depending of (restrictive) settings of ssh configuration.
