A few notes on the setup process for a new user under Linux/UNIX systems.

Create a new user $USER by running:

sudo useradd $USER

After setting up a user password for $USER:

sudo passwd $USER

Make sure to create a home directory at the selected location $USER_HOME by running:

sudo mkdir $USER_HOME
sudo cp -rT /etc/skel $USER_HOME
sudo usermod -d $USER_HOME $USER
sudo chown -R $USER:$USER $USER_HOME

To set a default shell for $USER, run:

sudo usermod --shell /bin/bash $USER

To create a new group $GROUP, run:

sudo groupadd $GROUP

To add $USER to $GROUP, run:

sudo usermod -a -G $GROUP $USER

To change the group and user ownership of a folder/file $FILE to that of $USER and $GROUP, run:

sudo chown -R $USER:$GROUP $FILE

To change the group ownership of a folder/file $FILE to that of $GROUP, run:

sudo chgrp -R $GROUP $FILE

After logging in with the desired user, to generate an (RSA 4096) SSH keypair {$KEY, $KEY.pub} run:

ssh-keygen -t rsa -b 4096 -f $USER_HOME/.ssh/$KEY

To turn on and off the password/SSH publickey access, edit the associated lines in the SSH daemon config file and then restart the daemon:

sudo nano /etc/ssh/sshd_config
sudo service ssh restart

After the keypair is generated, turn off the publickey only authentication. Using the terminal on the machine you want to install the key on, connect to the remote server. After copying the private key locally (either SSH'ing and copying the content of the private key file, or using scp - e.g., scp $USER@$IP_ADDR_REMOTE:$USER_HOME/.ssh/$KEY $LOCAL_HOME/.ssh), run the following command:

ssh-copy-id -i $LOCAL_HOME/.ssh/$KEY $USER@$IP_ADDR_REMOTE

After this operation, turn off the with-password access on the remote host. After rebooting the SSH daemon on the remote machine, the following command should not work anymore (and give the error):

ssh $USER@$IP_ADDR_REMOTE

>> $USER@$IP_ADDR_REMOTE: Permission denied (publickey).

On the contrary, the following should:

ssh $USER@$IP_ADDR_REMOTE -i $LOCAL_HOME/.ssh/$KEY

The Complete Guide to “useradd” Command in Linux

Managing Users on Linux Systems

SSH Keygen

Force SSH to only allow users with a key to log in

Copy SSH Keys to a Remote Host