ingo's installations

me as admin

Add my own user as an admin (i.e. being in the same groups as the current user).

$ sudo adduser inkarkat
$ sudo usermod --append --groups "$(sed -e "/.$USER/"'!d' -e 's/^\([^:]\+\):.*$/\1/' /etc/group | sed -n -e H -e '${ g; s/^\n//; s/\n/,/g; p }')" inkarkat
$ su - inkarkat

user to me

a)

Rename an existing user to my own username. For example, an AWK EC2 Ubuntu instance has the ubuntu default username.

When you can directly login as superuser (or another user and then switch user), this is as simple as this:

$ usermod -l inkarkat -d /home/inkarkat -m OLD_USER_NAME && groupmod -n inkarkat OLD_GROUP

On an AWS EC2 Ubuntu instance, the passwordless sudo is achieved via a special configuration that has the username hard-coded; this needs to be adapted as well if the user doesn't have a password:

$ sed -i.orig -e 's/OLD_USER_NAME/inkarkat/g' /etc/sudoers.d/90-cloud-init-users

b)

When login is only possible through the original user, this will fail, as even with exec sudo -s, the SSH login process is still owned by the original user.

usermod: user OLD_USER_NAME is currently used by process 745
# ps -eaf | grep OLD_USER_NAME
OLD_USER_NAME     745       1  0 12:39 ?        00:00:00 /lib/systemd/systemd --user
OLD_USER_NAME     746     745  0 12:39 ?        00:00:00 (sd-pam)
root            28832     612  0 13:14 ?        00:00:00 sshd: OLD_USER_NAME [priv]
OLD_USER_NAME   28915   28832  0 13:14 ?        00:00:00 sshd: OLD_USER_NAME@pts/0
OLD_USER_NAME   28916   28915  0 13:14 pts/0    00:00:00 -bash

We need to create a temporary local user, give that sudo rights and enable SSH login with my key, and login as them to do the renaming:

$ sudo adduser temp && sudo adduser temp sudo
Adding user `temp' ...
Adding new group `temp' (1001) ...
Adding new user `temp' (1001) with group `temp' ...
Creating home directory `/home/temp' ...
Copying files from `/etc/skel' ...
New password:
Retype new password:
passwd: password updated successfully
Changing the user information for temp
Enter the new value, or press ENTER for the default
        Full Name []:
        Room Number []:
        Work Phone []:
        Home Phone []:
        Other []:
Is the information correct? [Y/n]
Adding user `temp' to group `sudo' ...
Adding user temp to group sudo
Done.
$ sudo install -d -o temp -m 700 ~temp/.ssh && sudo install -D -o temp -m 600 -t ~temp/.ssh/ ~/.ssh/authorized_keys
$ exit

$ export OLD_USER_NAME=ubuntu; export OLD_GROUP=$OLD_USER_NAME
$ sudo usermod -l inkarkat -d /home/inkarkat -m $OLD_USER_NAME && sudo groupmod -n inkarkat $OLD_GROUP
$ sudo sed -i.orig -e "s/$OLD_USER_NAME/inkarkat/g" /etc/sudoers.d/90-cloud-init-users
$ exit 

$ sudo rm -r ~temp && sudo deluser temp
Removing user `temp' ...
Warning: group `temp' has no more members.
Done.

Source: Ask Ubuntu - How do I change my username?

Nextcloud

$ sudo add-apt-repository ppa:nextcloud-devs/client
$ sudo apt update && sudo apt install nautilus-nextcloud

- Applications > Accessories > Nextcloud desktop sync client
  Server address: https://cloud.karkat.de/
  (Login in browser)
  (O) Sync everything from server
  [v] Ask for confirmation for larger than 500 MB
  [v] Ask before syncing external storage
  Local Folder: /home/inkarkat/cloud
  General > General Settings
    [v] Launch on System Startup
  Network > Proxy Settings
    (O) Use system proxy

Ingo Karkat, 20-Dec-2020, updated 14-Apr-2021