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julio
julio

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Automating Ubuntu VM Creation with libvirt (KVM/QEMU) and Cloud-Init

#virtualization #ubuntu #libvirt #script

Ever needed to spin up multiple Ubuntu VMs quickly for testing, development, or learning? In this post, I'll walk you through a bash script that automates the entire process using libvirt/KVM and cloud-init.

What This Script Does

  • Downloads the official Ubuntu Noble cloud image automatically
  • Creates multiple VMs with customizable resources (memory, CPUs, disk)
  • Configures static IPs in the 192.168.122.2-254 range
  • Sets up SSH access via your GitHub public keys
  • Pre-installs useful packages including Docker, Python 3.13.7, and a minimal desktop environment (Openbox)

Prerequisites

Before running this script, make sure you have:

  • A Linux host with KVM/QEMU installed
  • libvirt and virt-install packages
  • cloud-localds (from cloud-image-utils package)
  • A bridge network interface (br0) configured
  • wget for downloading the cloud image

Below is the script

#!/bin/bash

usage() {
  cat <<EOF
Usage: $(basename "$0") [OPTIONS]

Launch Ubuntu VMs using libvirt/KVM with cloud-init configuration.

OPTIONS:
  -n, --num-vms NUM       Number of VMs to create (default: 1, max: 5)
  -m, --memory GB         Memory per VM in GB (default: 1, max: 8)
  -c, --cpus NUM          Number of CPUs per VM (default: 1, max: 2)
  -d, --disk-size GB      Disk size in GB (default: 10, min: 10)
  -g, --github USER       GitHub username for SSH key import (required)
  -h, --help              Show this help message and exit

EXAMPLES:
  $(basename "$0") -g myuser
  $(basename "$0") -n 3 -m 2 -c 2 -d 20 -g myuser
  $(basename "$0") --num-vms 5 --memory 4 --github myuser

NOTES:
  - VMs will be assigned IPs in the range 192.168.1.100-150
  - The script will automatically find available network octets
  - Ubuntu Noble cloud image will be downloaded if not present
EOF
  exit 0
}

NUM_VMS=1
MEMORY=1
CPUS=1
DISK_SIZE=10
GITHUB_USER=""

while [[ $# -gt 0 ]]; do
  case $1 in
    -n|--num-vms)
      NUM_VMS="$2"
      shift 2
      ;;
    -m|--memory)
      MEMORY="$2"
      shift 2
      ;;
    -c|--cpus)
      CPUS="$2"
      shift 2
      ;;
    -d|--disk-size)
      DISK_SIZE="$2"
      shift 2
      ;;
    -g|--github)
      GITHUB_USER="$2"
      shift 2
      ;;
    -h|--help)
      usage
      ;;
    *)
      echo "Error: Unknown option $1"
      echo "Use -h or --help for usage information."
      exit 1
      ;;
  esac
done

if [ -z "$GITHUB_USER" ]; then
  echo "Error: GitHub username is required for SSH key import."
  echo "Use -g or --github to specify your GitHub username."
  echo "Use -h or --help for usage information."
  exit 1
fi

BASE_IMG="noble-server-cloudimg-amd64.img"
IMG_URL="https://cloud-images.ubuntu.com/noble/current/noble-server-cloudimg-amd64.img"
IMG_DIR="/var/lib/libvirt/images"
SUBNET="192.168.122"
GATEWAY="$SUBNET.1"
DHCP_START=2
DHCP_END=254

if [ $NUM_VMS -gt $((DHCP_END - DHCP_START)) ]; then
  echo "Error: Max 5 VMs to stay within $DHCP_START-$DHCP_END range."
  exit 1
fi

if [ $MEMORY -gt 8 ]; then
  echo "Error: Max memory is 8GB"
  exit 1
fi

if [ $CPUS -gt 2 ]; then
  echo "Error: Max CPUs is 2"
  exit 1
fi

if [ $DISK_SIZE -lt 10 ]; then
  echo "Error: Disk size must be at least 10GB to accommodate base image."
  exit 1
fi

if [ ! -f "$IMG_DIR/$BASE_IMG" ]; then
  echo "Downloading Ubuntu Noble cloud image..."
  wget -O "$IMG_DIR/$BASE_IMG" "$IMG_URL"
  if [ $? -ne 0 ]; then
    echo "Error: Failed to download the base image from $IMG_URL"
    exit 1
  fi
  echo "Base image downloaded successfully."
fi

is_octet_available() {
  local octet=$1
  local ip="$SUBNET.$octet"

  for vm in $(virsh list --all --name 2>/dev/null); do
    if [ -n "$vm" ]; then
      if [ "$vm" = "server-$octet" ]; then
        return 1
      fi
    fi
  done

  if ping -c 1 -W 1 "$ip" &>/dev/null; then
    return 1
  fi

  return 0
}

AVAILABLE_OCTETS=()
for octet in $(seq $DHCP_START $DHCP_END); do
  if is_octet_available $octet; then
    AVAILABLE_OCTETS+=($octet)
  fi
  if [ ${#AVAILABLE_OCTETS[@]} -ge $NUM_VMS ]; then
    break
  fi
done

if [ ${#AVAILABLE_OCTETS[@]} -lt $NUM_VMS ]; then
  echo "Error: Not enough available network addresses. Found ${#AVAILABLE_OCTETS[@]}, need $NUM_VMS."
  echo "Available range is $SUBNET.$DHCP_START-$DHCP_END. Some addresses may already be in use."
  exit 1
fi

echo "Found ${#AVAILABLE_OCTETS[@]} available network addresses."

for i in $(seq 0 $(($NUM_VMS - 1))); do
  OCTET=${AVAILABLE_OCTETS[$i]}
  HOSTNAME="server-$OCTET"
  IMG="$IMG_DIR/vm-$OCTET.qcow2"
  SEED="$IMG_DIR/seed-$OCTET.iso"
  USER_DATA="user-data-$OCTET.yaml"
  NETWORK_DATA="network-data-$OCTET.yaml"

  qemu-img create -f qcow2 -F qcow2 -b "$IMG_DIR/$BASE_IMG" "$IMG" ${DISK_SIZE}G

  cat <<EOF > "$USER_DATA"
#cloud-config
hostname: $HOSTNAME

package_update: true
package_upgrade: true

users:
  - name: ubuntu
    shell: /bin/bash
    groups: [users, sudo]
    sudo: "ALL=(ALL) NOPASSWD:ALL"
    lock_passwd: false
    passwd: 123456789
    ssh_import_id:
      - gh:$GITHUB_USER

packages:
  - apt-transport-https
  - xorg
  - xterm
  - openbox
  - chromium-browser
  - spice-vdagent

runcmd:
  - mkdir -p /home/ubuntu/.config/openbox
  - wget https://raw.githubusercontent.com/Mikachu/openbox/refs/heads/master/data/rc.xml -P /home/ubuntu/.config/openbox
  - sed -i '/<\/keyboard>/i \   <keybind key="C-A-t">\n     <action name="Execute">\n       <command>xterm</command>\n     </action>\n   </keybind>' /home/ubuntu/.config/openbox/rc.xml
  - sed -i '/<\/keyboard>/i \   <keybind key="C-A-b">\n     <action name="Execute">\n       <command>chromium</command>\n     </action>\n   </keybind>' /home/ubuntu/.config/openbox/rc.xml

  - sudo apt-get install -y make build-essential libssl-dev zlib1g-dev libbz2-dev libreadline-dev libsqlite3-dev wget curl llvm libncurses5-dev libncursesw5-dev xz-utils tk-dev liblzma-dev
  - cd /tmp/
  - wget https://www.python.org/ftp/python/3.13.7/Python-3.13.7.tgz
  - tar xzf Python-3.13.7.tgz
  - cd Python-3.13.7
  - sudo ./configure --enable-optimizations
  - sudo make -j\$(nproc)
  - sudo make altinstall
  - sudo ln -s /usr/local/bin/python3.13 /usr/local/bin/python
  - sudo ln -s /usr/local/bin/pip3.13 /usr/local/bin/pip

  - echo 'if [[ -z \$DISPLAY ]] && [[ \$(tty) = /dev/tty1 ]]; then exec startx; fi' | sudo tee -a /home/ubuntu/.profile
  - echo "alias displays='ps e | grep -Po " DISPLAY=[\\.0-9A-Za-z:]* " | sort -u'" >> /home/ubuntu/.bashrc
  - sudo groupadd -g 3001 docker
  - sudo usermod -aG docker ubuntu
  - curl -fsSL https://get.docker.com -o get-docker.sh
  - sudo sh get-docker.sh
  - sudo reboot
EOF

  cat <<EOF > "$NETWORK_DATA"
#cloud-config
network:
  version: 2
  ethernets:
    interface0:
      match:
        name: "en*s0"
      dhcp4: false
      addresses: [$SUBNET.$OCTET/24]
      routes:
        - to: default
          via: $GATEWAY
      nameservers:
        addresses: [8.8.8.8,8.8.4.4]
EOF

  echo "$(cat $USER_DATA)"
  echo "$(cat $NETWORK_DATA)"

  cloud-localds --network-config "$NETWORK_DATA" "$SEED" "$USER_DATA"

  rm "$USER_DATA"
  rm "$NETWORK_DATA"

  virt-install \
    --name "$HOSTNAME" \
    --memory $(( $MEMORY * 1024 )) \
    --vcpus $CPUS \
    --disk path="$IMG",bus=virtio,format=qcow2 \
    --disk path="$SEED",bus=virtio,format=raw \
    --network default \
    --os-variant ubuntu24.04 \
    --graphics spice,listen=0.0.0.0 \
    --channel spicevmc \
    --video virtio \
    --import \
    --noautoconsole

  echo "Launched VM $((i + 1)): $HOSTNAME with IP $SUBNET.$OCTET (disk size: ${DISK_SIZE}GB)"
done

echo "All $NUM_VMS VMs launched. Use 'virsh list --all' to see VMs, 'virsh start/stop/destroy <name>', 'virsh console <name>' for access. Clean up images/ISOs when done."
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If you want to use another version of Python, you can find here and update the area into the runcmd that install the python

When the machine start you need to wait the cloud-init to configure everything, you can see the logs with sudo tail -f /var/log/cloud-init-output.log

The network that I used was this

<network>
  <name>default</name>
  <uuid>2dba4e25-611d-44db-9b54-ce38bd7c5a62</uuid>
  <forward mode="nat">
    <nat>
      <port start="1024" end="65535"/>
    </nat>
  </forward>
  <bridge name="virbr0" stp="on" delay="0"/>
  <mac address="ad:98:81:e2:9a:74"/>
  <dns>
    <forwarder addr="8.8.8.8"/>
    <forwarder addr="8.8.4.4"/>
  </dns>
  <ip address="192.168.122.1" netmask="255.255.255.0">
    <dhcp>
      <range start="192.168.122.2" end="192.168.122.254"/>
    </dhcp>
  </ip>
</network>
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Thanks for read until there :)

Top comments (1)

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tjuliu profile image
julio •

Into the virt-manager you can use ctrl + alt + t to open the terminal and ctrl + alt + b for the chromium