systemd is a Linux system manager.
Example service to start tmux on boot:
[Unit]
Description=tmux default session (detached)
Documentation=man:tmux(1)
[Service]
Type=forking
Environment=DISPLAY=:0
ExecStart=/usr/bin/tmux new-session -d
ExecStop=/home/ngrogan/.tmux/plugins/tmux-resurrect/scripts/save.sh
ExecStop=/usr/bin/tmux kill-server
KillMode=none
RestartSec=2
[Install]
WantedBy=default.target