Starting a script on first boot via a systemd service

Make sure that you have completed the steps described in the initial setup page before starting this tutorial.

In this tutorial, we will run a script on the first boot via a systemd service. We will add the following to the Butane config from the previous scenario:

Add a script at /usr/local/bin/public-ipv4.sh.
Configure a systemd service to run the script on first boot.

Writing the script

Let’s write a simple script that uses icanhazip.com to create an issue file to display as a prelogin message on the console:

#!/bin/bash
echo "Detected Public IPv4: is $(curl https://ipv4.icanhazip.com)" > \
    /etc/issue.d/50_public-ipv4.issue

This could be useful in cloud environments where you might have different public and private addresses.

We will store this script into /usr/local/bin/public-ipv4.sh when we provision the machine.

Writing the systemd service

We need to call the script from the previous section by using a systemd unit. Here is one that works for what we want, which is to execute on first boot and not again:

[Unit]
Before=systemd-user-sessions.service
After=network-online.target
ConditionPathExists=!/var/lib/issuegen-public-ipv4

[Service]
Type=oneshot
ExecStart=/usr/local/bin/public-ipv4.sh
ExecStartPost=/usr/bin/touch /var/lib/issuegen-public-ipv4
RemainAfterExit=yes

[Install]
WantedBy=multi-user.target

We will call this unit issuegen-public-ipv4.service and we will embed it into the Butane config in the next section.

Writing the Butane config and converting to Ignition

We can now create a Butane config by including the script and the systemd unit directly as inline content into the systemd/units and storage/files sections. The final Butane config, stored in services.bu, will be:

variant: fcos
version: 1.4.0
systemd:
  units:
    - name: serial-getty@ttyS0.service
      dropins:
      - name: autologin-core.conf
        contents: |
          [Service]
          # Override Execstart in main unit
          ExecStart=
          # Add new Execstart with `-` prefix to ignore failure`
          ExecStart=-/usr/sbin/agetty --autologin core --noclear %I $TERM
    - name: issuegen-public-ipv4.service
      enabled: true
      contents: |
        [Unit]
        Before=systemd-user-sessions.service
        After=network-online.target
        ConditionPathExists=!/var/lib/issuegen-public-ipv4

        [Service]
        Type=oneshot
        ExecStart=/usr/local/bin/public-ipv4.sh
        ExecStartPost=/usr/bin/touch /var/lib/issuegen-public-ipv4
        RemainAfterExit=yes

        [Install]
        WantedBy=multi-user.target
storage:
  files:
    - path: /etc/hostname
      mode: 0644
      contents:
        inline: |
          tutorial
    - path: /etc/profile.d/systemd-pager.sh
      mode: 0644
      contents:
        inline: |
          # Tell systemd to not use a pager when printing information
          export SYSTEMD_PAGER=cat
    - path: /etc/sysctl.d/20-silence-audit.conf
      mode: 0644
      contents:
        inline: |
          # Raise console message logging level from DEBUG (7) to WARNING (4)
          # to hide audit messages from the interactive console
          kernel.printk=4
    - path: /usr/local/bin/public-ipv4.sh
      mode: 0755
      contents:
        inline: |
          #!/bin/bash
          echo "Detected Public IPv4: is $(curl https://ipv4.icanhazip.com)" > \
              /etc/issue.d/50_public-ipv4.issue

And then convert to Ignition:

butane --pretty --strict services.bu --output services.ign

Testing

Just as before we will use the following to boot the instance:

# Setup the correct SELinux label to allow access to the config
chcon --verbose --type svirt_home_t services.ign

# Start a Fedora CoreOS virtual machine
virt-install --name=fcos --vcpus=2 --ram=2048 --os-variant=fedora-coreos-stable \
    --import --network=bridge=virbr0 --graphics=none \
    --qemu-commandline="-fw_cfg name=opt/com.coreos/config,file=${PWD}/services.ign" \
    --disk=size=20,backing_store=${PWD}/fedora-coreos.qcow2

And view on the serial console that the Detected Public IPv4 is shown in the serial console output right before you are dropped to a login prompt:

[  OK  ] Finished Update UTMP about System Runlevel Changes.

Fedora CoreOS 32.20200715.3.0
Kernel 5.7.8-200.fc32.x86_64 on an x86_64 (ttyS0)

SSH host key: SHA256:1MdFJOw3JlNppvfcztG7k6EAWcLq1ktLnK9ftVybrOo (ECDSA)
SSH host key: SHA256:bXD4Dfn2AdrHOdMJ3cfWLkcT+XTWUdaj8qb4KH+bu20 (ED25519)
SSH host key: SHA256:crY6RMBzPLz/heXmYPX8fGRte+pbSihG9m9WyDyHgAo (RSA)
ens2: 192.168.122.136 fe80::5054:ff:fe74:611d
Ignition: user provided config was applied
No ssh authorized keys provided by Ignition or Afterburn
Detected Public IPv4: is 82.255.80.95
tutorial login: core (automatic login)

[core@tutorial ~]$

And the service shows it was launched successfully:

[core@tutorial ~]$ systemctl status --full issuegen-public-ipv4.service
● issuegen-public-ipv.service
     Loaded: loaded (/etc/systemd/system/issuegen-public-ipv.service; enabled; vendor preset: enabled)
     Active: active (exited) since Fri 2020-08-07 09:35:59 UTC; 57s ago
    Process: 1871 ExecStart=/usr/local/bin/public-ipv4.sh (code=exited, status=0/SUCCESS)
    Process: 1920 ExecStartPost=/usr/bin/touch /var/lib/issuegen-public-ipv4 (code=exited, status=0/SUCCESS)
   Main PID: 1871 (code=exited, status=0/SUCCESS)

Aug 07 09:35:58 tutorial systemd[1]: Starting issuegen-public-ipv.service...
Aug 07 09:35:58 tutorial public-ipv4.sh[1874]:   % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
Aug 07 09:35:58 tutorial public-ipv4.sh[1874]:                                  Dload  Upload   Total   Spent    Left  Speed
Aug 07 09:35:59 tutorial public-ipv4.sh[1874]: [158B blob data]
Aug 07 09:35:59 tutorial systemd[1]: Finished issuegen-public-ipv.service.

Cleanup

Now let’s take down the instance for the next test. First, disconnect from the serial console by pressing CTRL + ] and then destroy the machine:

virsh destroy fcos
virsh undefine --remove-all-storage fcos

You may now proceed with the next tutorial.