systemd, systemctl, journald, journalctl

###### Systemd unit example ######
[Unit]
Description=Server RZD

[Service]
User=minidlna
#Type=forking
WorkingDirectory=/home/pi/ServerRZD/v1
#ExecStartPre=
ExecStart=/home/pi/ServerRZD/v1/srzd
#ExecStartPost=
ExecReload=/usr/sbin/nginx -g 'daemon on; master_process on;' -s reload
Restart=always
RemainAfterExit=yes

TimeoutSec=10

#### LOGGING:
#Environment=PYTHONUNBUFFERED=1
StandardOutput=tty
#StandardOutput=syslog            # Output to syslog
#StandardOutput=file:/home/user/log1.log
#StandardError=append:/home/user/log2.log
#StandardError=syslog             # Output to syslog
#SyslogIdentifier=<your program identifier>

[Install]
WantedBy=multi-user.target

####################################
Minimal unit runs rc.local at restart:
[Unit]
Description=Runs /usr/local/bin/rc.local

[Service]
ExecStart=/usr/local/bin/rc.local

[Install]
WantedBy=multi-user.target
####################################
# systemd resolve
# Show DNS status:
resolvectl status
systemd-resolve --statistics

# Clean DNS caches:
systemd-resolve --flush-caches
####################################

# Disable not need services:
for i in avahi-daemon wpa_supplicant bluealsa.service bluetooth.service alsa-restore.service hciuart.service ModemManager.service triggerhappy.service graphical.target rdnssd.service incron.service aakore.service acp-update-controller.service acronis_mms.service acronis_schedule.service active-protection.service unattended-upgrades.service cloud-init.service
do
 systemctl disable ${i}
 systemctl stop ${i}
done
##### User's units: https://wiki.archlinux.org/index.php/Systemd_(%D0%A0%D1%83%D1%81%D1%81%D0%BA%D0%B8%D0%B9)/User_(%D0%A0%D1%83%D1%81%D1%81%D0%BA%D0%B8%D0%B9)

# Current runlevel:
systemctl get-default

# Get PID of unit:
systemctl show --property MainPID --value speakphone

# View loaded targets:
systemctl list-units --type target
# View all targets:
systemctl list-units --type target --all
# View running units
systemctl list-unit-files --state=disabled

# Journald /etc/systemd/journald.conf
SystemMaxUse=1G
systemctl restart systemd-journald.service