ssh-copy-id and duplicates in authorized_keys

Question

I want to type password only once when connecting to SSH so I use ssh-copy-id and install my pubkey into authorized keys.

But I don't keep track which servers already have my key and which does not, so I issue ssh-copy-id again sometimes which adds duplicate key to authorized_keys?

1. How to prevent ssh-copy-id from installing the key when it is already installed?
2. /* How to make key installation automatic and transparent when connecting to SSH (without explicit ssh-copy-id? */

Answer 1

How to prevent ssh-copy-id from installing the key when it is already installed?

Write your own script. All ssh-copy-id does is append a line to a file. The following would check for key's existence:

#!/bin/bash
cat ~/.ssh/id_* | ssh "$@" 'mkdir -pm 0700 ~/.ssh &&
    while read -r ktype key comment; do
        if ! (grep -Fw "$ktype $key" ~/.ssh/authorized_keys | grep -qsvF "^#"); then
            echo "$ktype $key $comment" >> ~/.ssh/authorized_keys
        fi
    done'

---

How to make key installation automatic and transparent when connecting to SSH (without explicit ssh-copy-id?

You cannot, because if the server doesn't have your public key already, it will not know where to get it from, either.

Answer 2

Will http://thinkinginsoftware.blogspot.com/2012/07/avoid-duplicates-in-authorizedkeys.html work for you? Basically the trick is you first add the key and after that you delete all occurrences of the key with the exception of the last one (Dollar+Exclamation Mark):

#!/bin/bash -ex
# ssh-copy-id-uniq.sh

user=$1
host=$2
publicKey=$3
privateKey=$4

LOCAL_HOST_NAME=`hostname`

USAGE="Usage: `basename $0`    "

if [ $# -ne "4" ] 
then
 echo $USAGE
  exit 1 
fi

su $user -c "ssh-copy-id -i $publicKey $user@$host"
ssh -i $privateKey $user@$host "sed -i \"\\\$!{/$user@$LOCAL_HOST_NAME/d;}\" ~/.ssh/authorized_keys"