# Write startup script on Ubuntu 22.04

###### tags: `linux` `network`

An example of systemd startup script.

The script configures a network interface (eno2) to be attached on a linux bridge (virbr1) created by libvirt.
This could be done by using "netplan", but it has issue on libvirt to start virtual network of the bridge.

root@nuc112:/etc/netplan# cat 00-installer-config.yaml 
# This is the network config written by 'subiquity'
network:
  ethernets:
    eno1:
      dhcp4: true
    eno2:
      dhcp4: false
      link-local: []
      match:
        macaddress: c8:4d:44:22:01:ee
      set-name: eno2
    #enxc84d442201ee:
    #  dhcp4: true
  bridges:
    virbr1:
      interfaces: [eno2]
      macaddress: 52:54:00:fe:cc:b7
      dhcp4: false
      dhcp6: false

  version: 2

root@nuc112:/etc/netplan# brctl show virbr1
bridge name	bridge id		STP enabled	interfaces
virbr1		8000.525400feccb7	no		eno2


root@nuc112:/etc/netplan# virsh net-list --all
 Name       State      Autostart   Persistent
-----------------------------------------------
 default    active     yes         yes
 external   inactive   yes         yes

root@nuc112:/etc/netplan# virsh net-info external
Name:           external
UUID:           0a6d6648-8fac-477a-b129-66fbe65bd9b4
Active:         no
Persistent:     yes
Autostart:      yes
Bridge:         virbr1

root@nuc112:/etc/netplan# virsh net-start external
error: Failed to start network external
error: error creating bridge interface virbr1: File exists

A workaround is to have a systemd service to run a script which attach the interface to the bridge instead of using netplan.

#### Steps

##### 1. write systemd service file

root@nuc112:~# cd /etc/systemd/system/
root@nuc112:/etc/systemd/system# vim attach-interface-to-linux-bridge.service

[Unit]
After=libvirtd.service

[Service]
ExecStart=/usr/local/bin/attach-interface-to-linux-bridge.sh virbr1 eno2

[Install]
WantedBy=default.target

##### 2. write script to be executed on system startup

root@nuc112:/etc/systemd/system# vim /usr/local/bin/attach-interface-to-linux-bridge.sh

#!/bin/bash

BR=$1
IF=$2

STOP_FILE='/tmp/stop-attach-interface-to-linux-bridge'

if [ "$#" != 2 ]; then
  echo "Invalid arguments, exit."
  exit 2
fi

# prevent attaching interface has IP assigned
ret=$(ip -4 addr show ${IF})
if [ "${ret}" != "" ]; then
  echo "Interface ${IF} has IP address assigned, exit."
  exit 2
fi

while true; do

  ret=$(brctl show ${BR} | grep ${IF} &> /dev/null; echo $?)
  if [ "${ret}" == "0" ]; then
    echo "Already attached, continue. (sleep 60 seconds)"
    sleep 60
    continue
  fi

  ret=$(brctl show ${BR} &> /dev/null; echo $?)
  if [ "${ret}" == "0" ]; then

    ret=$(ip link show ${IF} &> /dev/null; echo $?)
    if [ "${ret}" == "0" ]; then
	echo "Attach ${IF} to ${BR}."
        brctl addif ${BR} ${IF}
    else
      echo "Interface ${IF} not exit."
    fi
    
  else
    echo "Bridge ${BR} not exit."
  fi

  if [ -f "$STOP_FILE" ]; then
    echo "Stop script."
    break
  fi

  sleep 10
done

##### 3. set permission

root@nuc112:/etc/systemd/system# chmod 744 /usr/local/bin/attach-interface-to-linux-bridge.sh 
root@nuc112:/etc/systemd/system# chmod 644 /etc/systemd/system/attach-interface-to-linux-bridge.service 

##### 4. enable and start the systemd service

root@nuc112:/etc/systemd/system# systemctl daemon-reload
root@nuc112:/etc/systemd/system# systemctl enable attach-interface-to-linux-bridge.service
Created symlink /etc/systemd/system/default.target.wants/attach-interface-to-linux-bridge.service → /etc/systemd/system/attach-interface-to-linux-bridge.service.

root@nuc112:/etc/systemd/system# systemctl start attach-interface-to-linux-bridge.service
root@nuc112:/etc/systemd/system# systemctl status attach-interface-to-linux-bridge.service
● attach-interface-to-linux-bridge.service
     Loaded: loaded (/etc/systemd/system/attach-interface-to-linux-bridge.service; enabled; vendor preset: enabled)
     Active: active (running) since Sun 2023-01-01 16:42:07 AEDT; 2s ago
   Main PID: 506436 (attach-interfac)
      Tasks: 2 (limit: 38086)
     Memory: 536.0K
        CPU: 22ms
     CGroup: /system.slice/attach-interface-to-linux-bridge.service
             ├─506436 /bin/bash /usr/local/bin/attach-interface-to-linux-bridge.sh virbr1 eno2
             └─506448 sleep 10

Jan 01 16:42:07 nuc112 systemd[1]: Started attach-interface-to-linux-bridge.service.
Jan 01 16:42:07 nuc112 attach-interface-to-linux-bridge.sh[506436]: Attach eno2 to virbr1.

root@nuc112:/etc/systemd/system# brctl show virbr1
bridge name	bridge id		STP enabled	interfaces
virbr1		8000.525400feccb7	yes		eno2

##### 5. reboot & check

ubuntu@nuc112:~$ brctl show
bridge name	bridge id		STP enabled	interfaces
docker0		8000.0242b6edc974	no		
virbr0		8000.52540046ab1a	yes		
virbr1		8000.525400feccb7	yes		eno2

ubuntu@nuc112:~$ virsh net-list
 Name       State    Autostart   Persistent
---------------------------------------------
 default    active   yes         yes
 external   active   yes         yes

ubuntu@nuc112:~$ systemctl status attach-interface-to-linux-bridge
● attach-interface-to-linux-bridge.service
     Loaded: loaded (/etc/systemd/system/attach-interface-to-linux-bridge.service; enabled; vendor preset: enabled)
     Active: active (running) since Sun 2023-01-01 16:45:28 AEDT; 1min 56s ago
   Main PID: 932 (attach-interfac)
      Tasks: 2 (limit: 38086)
     Memory: 680.0K
        CPU: 51ms
     CGroup: /system.slice/attach-interface-to-linux-bridge.service
             ├─ 932 /bin/bash /usr/local/bin/attach-interface-to-linux-bridge.sh virbr1 eno2
             └─1578 sleep 60

Jan 01 16:45:28 nuc112 systemd[1]: Started attach-interface-to-linux-bridge.service.
Jan 01 16:45:28 nuc112 attach-interface-to-linux-bridge.sh[936]: bridge virbr1 does not exist!
Jan 01 16:45:28 nuc112 attach-interface-to-linux-bridge.sh[932]: Bridge virbr1 not exit.
Jan 01 16:45:38 nuc112 attach-interface-to-linux-bridge.sh[932]: Attach eno2 to virbr1.
Jan 01 16:45:48 nuc112 attach-interface-to-linux-bridge.sh[932]: Already attached, continue. (sleep 60 seconds)
Jan 01 16:46:48 nuc112 attach-interface-to-linux-bridge.sh[932]: Already attached, continue. (sleep 60 seconds)

# reference
https://linuxconfig.org/how-to-run-script-on-startup-on-ubuntu-22-04-jammy-jellyfish-server-desktop
https://wiki.archlinux.org/title/Systemd