Copying local SSH key to remote host if it doesn't exist already

Just use a variable, like this:

KEY=$(cat ~/.ssh/id_rsa.pub) ssh -p <port> <user>@<hostname> "if [ -z \"\$(grep \"$KEY\" ~/.ssh/authorized_keys )\" ]; then echo $KEY >> ~/.ssh/authorized_keys; echo key added.; fi;"

Why not just use ssh-copy-id?

NAME

 ssh-copy-id -- copy public   keys to a remote host

SYNOPSIS

 ssh-copy-id [-lv] [-i keyfile] [-o   option] [-p port] [user@]hostname

DESCRIPTION

 The ssh-copy-id utility copies public keys   to a remote host's
 ~/.ssh/authorized_keys file (creating the file and   directory, if
 required).

 The following options are available:

 -i   file
   Copy the public key contained in file.  This option can be speci-
   fied multiple times and can be combined with the -l option.  If a
   private key is specified and a public key is found then the pub-
   lic key will be used.

 -l        Copy the keys currently held by ssh-agent(1).  This is the
   default if the -i option was not specified.

 -o   ssh-option
   Pass this option directly to ssh(1).  This option can be speci-
   fied multiple times.

 -p   port
   Connect to the specified port on the remote host instead of the
   default.

 -v        Pass -v to ssh(1).

 The remaining arguments are a list   of remote hosts to connect to, each
 one optionally qualified by a user   name.

EXIT STATUS

 The ssh-copy-id utility exits 0 on   success, and >0 if an error occurs.

EXAMPLES

 To   send a specific key to multiple hosts:
 $ ssh-copy-id -i /path/to/keyfile.pub user@host1 user@host2
 user@host3

Old question maybe, but you can also execute:

ssh -q -o "BatchMode=yes" user@hostname exit

Which if ssh-copy-id already happend will succeed and return 0. If not return an error.