Add a service file /lib/systemd/system/picam.service similar to this:

[Service]
WorkingDirectory=/home/pi/mydir
ExecStart=/usr/bin/python xxx.py
Restart=always
StandardOutput=syslog
StandardError=syslog
SyslogIdentifier=picam
User=pi
Group=pi
[Install]
WantedBy=multi-user.target

enable then start/stop using:

sudo systemctl enable picam.service 
sudo systemctl start picam.service 
sudo systemctl stop picam.service

to view the output:

 sudo journalctl -u picam.service