permission error on modifying root owned authorized keys file - bash

Question

i need to exchange public key between two systems A and B.
These are the steps am following
copy the content of id_rsa.pub from /root/.ssh directory and save it in variable 'key'
ssh to B as ubuntu user . ssh -i key_file ubuntu#B
Move to root login by sudo su
Append the variable $key to /root/.ssh/authorized_keys
But the file authorized_keys is owned by root. Hence i get the permission error.
I cannot directory connect to system B as root. Only way is to connect as ubuntu and change to root.
I tried the following shell script
# Get all the Ips from the source file
sudo grep -o '[0-9]\{1,3\}\.[0-9]\{1,3\}\.[0-9]\{1,3\}\.[0-9]\{1,3\}' $1 | sort -u > /tmp/list_of_servers.txt
# Get the public key
pubkey=$(sudo cat /root/.ssh/id_rsa.pub)
# For each server
while read ip;
do
(echo "$ip"
# ssh to the server
ssh -i $2 $3#$ip
# append key to autorized_keys file
sudo -c "echo $pubkey >> /root/.ssh/authorized_keys" root
echo "done $ip" )
done < /tmp/list_of_servers.txt
but i didnt work. its giving me permission error.
Can someone help me in the last step.

Answer 1

A fully paranoid approach to the mechanics of the SSH connection might be something like this:
# generate a shell-escaped version of the public key (spaces, wildcards, etc)
printf -v pubkey_q '%q' "$pubkey"
# generate a shell command using that quoted form
cmd="echo $pubkey_q >>/root/.ssh/authorized_keys"
# generate a shell-quoted sudo command invoking the above in a shell
printf -v cmd_q '%q ' sudo bash -c "$cmd"
# ...and execute it on the other end of a ssh connection.
ssh -i "$2" "$3#$ip" "$cmd_q"
printf %q is a bash extension which escapes a string in such a way that being parsed by a shell -- whether in a string that's eval'd, passed to ssh with bash as the remote shell, or passed to bash -c -- evaluates back to the original data. (For regular whitespace its output is safe for sh -c as well, but for any content where bash prefers $'' to escape nonprintable characters, this output may not be POSIX compliant).

Answer 2

This code doesn't do what you think it does:
# ssh to the server
ssh -i $2 $3#$ip
# append key to autorized_keys file
sudo -c "echo $pubkey >> /root/.ssh/authorized_keys" root
The ssh command there would normally open an interactive remote shell, but since we are in a script, an interactive shell is not possible. So the remote shell immediately exits, without actually doing anything at all.
The sudo command that follows is incorrect syntax, it cannot work that way with the -c flag. Check the man page of sudo. And since you are not actually in the remote she'll as you may have believed, the command is running in your local system, not the remote one where you want to append your key.
To run sudo remotely, use something like this:
ssh -i $2 $3#$ip sudo echo hello
The echo is just an example for testing of course.
However, this whole attempt of appending a public key to the authorized list of root is deeply flawed in terms of security. Sudo should be configured to ask for the password of the user, and there is no good way to do that in a script. Or if the user can run sudo without entering a password, that's just unacceptable from a security perspective.