This is an example of systemd service configuration file.

foo.service

[Unit]
Description=In this sample , 'foo' is the name of the software.
Documentation=
After=network-online.target
Wants=network-online.target systemd-networkd-wait-online.service

[Service]
Restart=on-failure

; User and group the process will run as.
User=www-data
Group=www-data

; Some sort of files will be written to this directory.
Environment=HOME=/etc/foo/bar

; Always set "-root" to something safe in case it gets forgotten in the bar.conf .
ExecStart=/usr/local/bin/foo -log stdout -agree=true -conf=/etc/foo/bar.conf -root=/var/tmp
ExecReload=/bin/kill -USR1 $MAINPID

; Limit the number of file descriptors; see `man systemd.exec` for more limit settings.
LimitNOFILE=1048576
; Unmodified foo is not expected to use more than that.
LimitNPROC=64

; Use private /tmp and /var/tmp, which are discarded after foo stops.
PrivateTmp=true
; Use a minimal /dev
PrivateDevices=true
; Hide /home, /root, and /run/user. Nobody will steal your SSH-keys.
ProtectHome=true
; Make /usr, /boot, /etc and possibly some more folders read-only.
ProtectSystem=full
; … except /etc/foo/bar, because we want the specified files there.
;   This merely retains r/w access rights, it does not add any new. Must still be writable on the host!
ReadWriteDirectories=/etc/foo/bar

; Drop all other capabilities. Important if you run foo as privileged user (which you should not).
CapabilityBoundingSet=CAP_NET_BIND_SERVICE
; … but permit foo to open ports reserved for system services.
;   This could be redundant here, but is needed in case foo runs as nobody:nogroup.
AmbientCapabilities=CAP_NET_BIND_SERVICE
; … and prevent gaining any new privileges.
NoNewPrivileges=true

; Caveat: Some plugins need additional capabilities. Add them to both above lines.
; - plugin "upload" needs: CAP_LEASE

[Install]
WantedBy=multi-user.target

The provided file is written for systemd version 229 or later!

Got an error? Revisit permissions!

Prefer systemctl edit over modifying the unit file directly:
systemctl edit foo.service to make user-local modifications
systemctl edit --full foo.service for system-wide ones

To view the resulting configuration use
systemctl cat foo