Today  I wrote my first service file for arch! It's so easy!...

Before today I connected to the internet by executing

################################
ip link set enp2s0 up
pppoe-start

################################

at start up of xfce. How inelegant!

By just following instructions on the wiki I created easily my own service file:

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[Unit]
Description=Network Connectivity
Wants=network.target
Before=network.target
BindsTo=sys-subsystem-net-devices-enp2s0.device
After=sys-subsystem-net-devices-enp2s0.device

[Service]
Type=oneshot
RemainAfterExit=yes
ExecStart=/sbin/ip link set enp2s0 up
ExecStart=/usr/sbin/pppoe-start

ExecStop=/usr/sbin/pppoe-stop
ExecStop=/sbin/ip link set enp2s0 down

[Install]
WantedBy=multi-user.target

################################

systemctl can now start and stop the service, or enable it on boot!