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Here is what I have tried, and I got an error:

$ cat /home/tim/.ssh/id_rsa.pub | ssh tim@just.some.other.server 'cat >> .ssh/authorized_keys'
Password: 
cat: >>: No such file or directory
cat: .ssh/authorized_keys: No such file or directory
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Why not do it in two steps? Copy it across and then append it? – Faheem Mitha Jan 18 '12 at 20:01
    
@FaheemMitha: That works, thanks! I actually might realize the cause of trouble. Please see my new post? – Tim Jan 18 '12 at 21:23
up vote 38 down vote accepted

OpenSSH comes with a command to do this, ssh-copy-id. You just give it the remote address and it adds your public key to the authorized_keys file on the remote machine:

$ ssh-copy-id tim@just.some.other.server
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1  
Thanks! Why my command doesn't work? – Tim Jan 18 '12 at 20:18
    
@Tim This answer explained it; >> is handled by your shell, and you're running the command through SSH instead of through a shell. His fix of having SSH run a shell, which then runs your command, should work – Michael Mrozek Jan 18 '12 at 20:22
    
Thanks! (1) ssh-copy-id doesn't work either. I manually copy the file to the remote and append its content, then it works. I wonder why is this? I found that my default shell on the server is some script, which I update to my original post, and might be the reason. Please have a look. (2) I wonder if ssh-copy-id is just copy the public key to the remote, it doesn't create the private and public key, does it? – Tim Jan 18 '12 at 20:37
    
Okay, I crated a new post for my default shell problem – Tim Jan 18 '12 at 21:19
1  
@Abdull Not unless you already have some other key on that machine to connect with. It's just connecting over SSH – Michael Mrozek Jun 7 at 15:31

You could always do something like this:

scp ~/.ssh/id_rsa.pub user@remote.example.com:/tmp/id_rsa.pub
ssh user@remote.example.com 
cat /tmp/id_rsa.pub >> ~/.ssh/authorized_keys"

I'm not sure if you can cat from a local machine into an ssh session. Just move it to /tmp as suggested.

Edit: This is exactly what ssh-copy-id does. Just like Michael said.

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Thanks! I wonder if ssh-copy-id is just copy the public key to the remote. It doesn't create the private and public key, does it? – Tim Jan 18 '12 at 20:51
    
No it doesn't create it. Just adds it. – Mr. Monkey Jan 18 '12 at 21:22
    
@Mr.Monkey Yes, you can pipe data into an ssh session (from cat or otherwise). What you're describing is the old-fashioned way; ssh-copy-id is recommended because there's less risk of typos or giving files wrong permissions. – Gilles Jan 18 '12 at 23:19

This answer describes how to make the intended way shown in the question working.

You can execute a shell on the remote computer to interpret the special meaning of the >> redirection operator:

ssh tim@just.some.other.server sh -c "'cat >> .ssh/authorized_keys'" < /home/tim/.ssh/id_rsa.pub

The redirection operator >> is normally interpreted by a shell.

When you execute ssh host 'command >> file' then it is not guaranteed that command >> file will be interpreted by a shell. In your case command >> file is executed instead of the shell without special interpretation and >> was given to the command as an argument -- the same way as running command '>>' file in a shell.

Some versions of SSH (OpenSSH_5.9) will automatically invoke shell on the remote server and pass the command(s) to it when they detect tokens to be interpreted by a shell like ; > >> etc.

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openssh does provide ssh-copy-id. The sequence would be:

  • Generate a decent 4k key

    ssh-keygen -t rsa -b 4096 -f ~/.ssh/id_rsa4k
    
  • Start your ssh-agent up and suck in information like SSH_AGENT_PID, etc.

    ssh-agent -s > ~/mysshagent
    source ~/mysshagent
    rm ~/mysshagent
    
  • Now start loading keys into your SSH Agent

    ssh-add ~/.ssh/id_rsa4k
    
  • Check that it is loaded

    ssh-add -l
    ssh-add -L
    

    This will show you what you have in the ssh-agent

  • Now actually SSH to a remote system

    ssh username@remotehost.network
    
  • Now you can run ssh-copy-id with no arguments:

    ssh-copy-id
    

    This creates ~/.ssh/authorized_keys and fills in the basic info required from ssh-agent.

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BTW, I created a small script at github.com/centic9/generate-and-send-ssh-key which runs most of these steps in one go and additionally ensures file/directory permissions which usually always caused me headaches... – centic Oct 7 '15 at 11:24

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