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Customizing the OSbuild templates

There are three ways one can customize an OSbuild template:

  • Manually edit the template
  • Overriding variables on the CLI
  • Overriding variables via a custom distro file

We will not be covering here the first method as this is the least sustainable, changes made to the template would need to be rebased every time a change is made to the repository.

First, let’s discuss about these variables.

Template variables

The manifests currently include a number of variables, which appear in different mpp-vars sections of the OSbuild template manifests.

For example, a mpp-vars section may look like the following example:

  "mpp-vars": {
    "rootfs_uuid": {"mpp-format-string": "{__import__('uuid').uuid1()}"},
    "bootfs_uuid": {"mpp-format-string": "{__import__('uuid').uuid1()}"},
    "rootfs_size": 4294967296,
    "homefs_size": {"mpp-format-string": "{rootfs_size}"}
  },

This example defines four variables: rootfs_uuid, bootfs_uuid, rootfs_size and homefs_size.

  • rootfs_uuid and bootfs_uuid dynamically generate when you run the equivalent of the following python code:
import uuid
uuid.uuid1()

  • rootfs_size is hardcoded to 4294967296 bytes.

  • homefs_size is equal to rootfs_size.

Overriding variables via a custom distro file

The simplest way to override variables using a customized file in the distro/ folder in the sample-images repository.

As an example, we will create a customized image pulling two RPMs from the EPEL repository.

Create the custom file

cp distro/cs9.ipp.yml distro/custom.ipp.yml

Edit the custom file

Open distro/custom.ipp.yml in your favorite text editor and adjust it so that the diff between cs9.ipp.yml and custom.ipp.yml is as follow:

--- distro/cs9.ipp.yml 2022-04-12 10:20:18.130917511 +0200
+++ distro/custom.ipp.yml 2022-04-22 16:18:11.203590871 +0200
@@ -10,6 +10,8 @@
     baseurl: $distro_baseurl/AppStream/$arch/os/
   - id: automotive
     baseurl: https://buildlogs.centos.org/9-stream/automotive/$arch/packages-main/
+  - id: epel-9
+    baseurl: https://dl.fedoraproject.org/pub/epel/9/Everything/$arch/
   distro_devel_repos:
   - id: crb
     baseurl: $distro_baseurl/CRB/$arch/os/
@@ -21,3 +23,7 @@
   - id: crb-debug
     baseurl: $distro_baseurl/CRB/$arch/debug/tree/
   distro_module_id: platform:el9
+  extra_rpms:
+  - gnuplot
+  - python3-zmq
+  root_password: $6$3c5hyALn6Ge/XXwL$Qr961XQbJeCC/aRLNd4CagSRUa/x7tOoG2MrnKpPCrZxoNYSZ4N3bfFQO99A6vQUuAWOOoUclqxPa2DJ0Ylg90

As you can see, we’ve added a new repo named epel-9 with its baseurl. Lower down, we have defined a extra_rpms variable corresponding to a list with two packages: gnuplot and python3-zmq. We have also changed the root password from password to password2 (the guest password remaining the same).

Note

To encrypt the password you can use: mkpasswd --method=SHA-512 --stdin

Create the image

Now that we have this distro/custom.ipp.yml created, if we ran make help we will this in its output:

...
custom-qemu-container-regular.x86_64.[img,qcow2,oci.tar,repo,rootfs,ext4,tar]
custom-qemu-container-ostree.x86_64.[img,qcow2,oci.tar,repo,rootfs,ext4,tar]
custom-qemu-container-direct.x86_64.[img,qcow2,oci.tar,repo,rootfs,ext4,tar]
custom-qemu-developer-regular.x86_64.[img,qcow2,oci.tar,repo,rootfs,ext4,tar]
...

So we can create the image of our choice:

make custom-qemu-developer-regular.x86_64.qcow2

Boot and check

Once the image has been built, we can easily run it using runvm:

./runvm custom-qemu-developer-direct.x86_64.qcow2

We can then log into the image with the root user, using password2 as password and check that the two new packages were installed:

[root@localhost ~]# rpm -q python3-zmq gnuplot
python3-zmq-22.3.0-2.el9.x86_64
gnuplot-5.4.3-2.el9.x86_64

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