apt / dpkg — package management

sudo apt update                   # refresh the package INDEX (does not install upgrades)
sudo apt upgrade                  # install available upgrades
sudo apt full-upgrade             # + remove conflicting packages (riskier)
sudo apt install nginx            # install
sudo apt install nginx=1.24\*     # a specific version
sudo apt install --reinstall nginx
sudo apt remove nginx             # remove (config files stay)
sudo apt purge nginx              # remove together with config files
sudo apt autoremove --purge       # remove orphaned dependencies
apt search keyword ; apt show nginx ; apt list --installed
apt-cache policy nginx            # available versions and which repo they come from
sudo apt-mark hold nginx          # pin a version (unhold to release it)
sudo apt download nginx           # download the .deb without installing

dpkg -l | grep nginx              # is it installed
dpkg -L nginx                     # which files the package brought in
dpkg -S /usr/sbin/nginx           # which package owns a file
sudo dpkg -i package.deb          # install a local .deb (then apt -f install)
dpkg --get-selections             # a list to carry over to another machine

# add a third-party repository (modern way, with a key):
curl -fsSL https://repo.example.com/key.gpg | sudo gpg --dearmor -o /etc/apt/keyrings/example.gpg
echo "deb [signed-by=/etc/apt/keyrings/example.gpg] https://repo.example.com stable main" \
  | sudo tee /etc/apt/sources.list.d/example.list
sudo apt update