poky/documentation/dev-manual/customizing-images.rst

.. SPDX-License-Identifier: CC-BY-SA-2.0-UK

Customizing Images
******************

You can customize images to satisfy particular requirements. This
section describes several methods and provides guidelines for each.

Customizing Images Using ``local.conf``
=======================================

Probably the easiest way to customize an image is to add a package by
way of the ``local.conf`` configuration file. Because it is limited to
local use, this method generally only allows you to add packages and is
not as flexible as creating your own customized image. When you add
packages using local variables this way, you need to realize that these
variable changes are in effect for every build and consequently affect
all images, which might not be what you require.

To add a package to your image using the local configuration file, use
the :term:`IMAGE_INSTALL` variable with the ``:append`` operator::

   IMAGE_INSTALL:append = " strace"

Use of the syntax is important; specifically, the leading space
after the opening quote and before the package name, which is
``strace`` in this example. This space is required since the ``:append``
operator does not add the space.

Furthermore, you must use ``:append`` instead of the ``+=`` operator if
you want to avoid ordering issues. The reason for this is because doing
so unconditionally appends to the variable and avoids ordering problems
due to the variable being set in image recipes and ``.bbclass`` files
with operators like ``?=``. Using ``:append`` ensures the operation
takes effect.

As shown in its simplest use, ``IMAGE_INSTALL:append`` affects all
images. It is possible to extend the syntax so that the variable applies
to a specific image only. Here is an example::

   IMAGE_INSTALL:append:pn-core-image-minimal = " strace"

This example adds ``strace`` to the ``core-image-minimal`` image only.

You can add packages using a similar approach through the
:term:`CORE_IMAGE_EXTRA_INSTALL` variable. If you use this variable, only
``core-image-*`` images are affected.

Customizing Images Using Custom ``IMAGE_FEATURES`` and ``EXTRA_IMAGE_FEATURES``
===============================================================================

Another method for customizing your image is to enable or disable
high-level image features by using the
:term:`IMAGE_FEATURES` and
:term:`EXTRA_IMAGE_FEATURES`
variables. Although the functions for both variables are nearly
equivalent, best practices dictate using :term:`IMAGE_FEATURES` from within
a recipe and using :term:`EXTRA_IMAGE_FEATURES` from within your
``local.conf`` file, which is found in the :term:`Build Directory`.

To understand how these features work, the best reference is
:ref:`meta/classes-recipe/image.bbclass <ref-classes-image>`.
This class lists out the available
:term:`IMAGE_FEATURES` of which most map to package groups while some, such
as ``read-only-rootfs``, resolve as general configuration settings.

In summary, the file looks at the contents of the :term:`IMAGE_FEATURES`
variable and then maps or configures the feature accordingly. Based on
this information, the build system automatically adds the appropriate
packages or configurations to the
:term:`IMAGE_INSTALL` variable.
Effectively, you are enabling extra features by extending the class or
creating a custom class for use with specialized image ``.bb`` files.

Use the :term:`EXTRA_IMAGE_FEATURES` variable from within your local
configuration file. Using a separate area from which to enable features
with this variable helps you avoid overwriting the features in the image
recipe that are enabled with :term:`IMAGE_FEATURES`. The value of
:term:`EXTRA_IMAGE_FEATURES` is added to :term:`IMAGE_FEATURES` within
``meta/conf/bitbake.conf``.

To illustrate how you can use these variables to modify your image, consider an
example that selects the SSH server. The Yocto Project ships with two SSH
servers you can use with your images: Dropbear and OpenSSH. Dropbear is a
minimal SSH server appropriate for resource-constrained environments, while
OpenSSH is a well-known standard SSH server implementation. By default, the
``core-image-sato`` image is configured to use Dropbear. The
``core-image-full-cmdline`` image includes OpenSSH. The ``core-image-minimal``
image does not contain an SSH server.

You can customize your image and change these defaults. Edit the
:term:`IMAGE_FEATURES` variable in your recipe or use the
:term:`EXTRA_IMAGE_FEATURES` in your ``local.conf`` file so that it
configures the image you are working with to include
``ssh-server-dropbear`` or ``ssh-server-openssh``.

.. note::

   See the ":ref:`ref-manual/features:image features`" section in the Yocto
   Project Reference Manual for a complete list of image features that ship
   with the Yocto Project.

Customizing Images Using Custom .bb Files
=========================================

You can also customize an image by creating a custom recipe that defines
additional software as part of the image. The following example shows
the form for the two lines you need::

   IMAGE_INSTALL = "packagegroup-core-x11-base package1 package2"
   inherit core-image

Defining the software using a custom recipe gives you total control over
the contents of the image. It is important to use the correct names of
packages in the :term:`IMAGE_INSTALL` variable. You must use the
OpenEmbedded notation and not the Debian notation for the names (e.g.
``glibc-dev`` instead of ``libc6-dev``).

The other method for creating a custom image is to base it on an
existing image. For example, if you want to create an image based on
``core-image-sato`` but add the additional package ``strace`` to the
image, copy the ``meta/recipes-sato/images/core-image-sato.bb`` to a new
``.bb`` and add the following line to the end of the copy::

   IMAGE_INSTALL += "strace"

Customizing Images Using Custom Package Groups
==============================================

For complex custom images, the best approach for customizing an image is
to create a custom package group recipe that is used to build the image
or images. A good example of a package group recipe is
``meta/recipes-core/packagegroups/packagegroup-base.bb``.

If you examine that recipe, you see that the :term:`PACKAGES` variable lists
the package group packages to produce. The ``inherit packagegroup``
statement sets appropriate default values and automatically adds
``-dev``, ``-dbg``, and ``-ptest`` complementary packages for each
package specified in the :term:`PACKAGES` statement.

.. note::

   The ``inherit packagegroup`` line should be located near the top of the
   recipe, certainly before the :term:`PACKAGES` statement.

For each package you specify in :term:`PACKAGES`, you can use :term:`RDEPENDS`
and :term:`RRECOMMENDS` entries to provide a list of packages the parent
task package should contain. You can see examples of these further down
in the ``packagegroup-base.bb`` recipe.

Here is a short, fabricated example showing the same basic pieces for a
hypothetical packagegroup defined in ``packagegroup-custom.bb``, where
the variable :term:`PN` is the standard way to abbreviate the reference to
the full packagegroup name ``packagegroup-custom``::

   DESCRIPTION = "My Custom Package Groups"

   inherit packagegroup

   PACKAGES = "\
       ${PN}-apps \
       ${PN}-tools \
       "

   RDEPENDS:${PN}-apps = "\
       dropbear \
       portmap \
       psplash"

   RDEPENDS:${PN}-tools = "\
       oprofile \
       oprofileui-server \
       lttng-tools"

   RRECOMMENDS:${PN}-tools = "\
       kernel-module-oprofile"

In the previous example, two package group packages are created with
their dependencies and their recommended package dependencies listed:
``packagegroup-custom-apps``, and ``packagegroup-custom-tools``. To
build an image using these package group packages, you need to add
``packagegroup-custom-apps`` and/or ``packagegroup-custom-tools`` to
:term:`IMAGE_INSTALL`. For other forms of image dependencies see the other
areas of this section.

Customizing an Image Hostname
=============================

By default, the configured hostname (i.e. ``/etc/hostname``) in an image
is the same as the machine name. For example, if
:term:`MACHINE` equals "qemux86", the
configured hostname written to ``/etc/hostname`` is "qemux86".

You can customize this name by altering the value of the "hostname"
variable in the ``base-files`` recipe using either an append file or a
configuration file. Use the following in an append file::

   hostname = "myhostname"

Use the following in a configuration file::

   hostname:pn-base-files = "myhostname"

Changing the default value of the variable "hostname" can be useful in
certain situations. For example, suppose you need to do extensive
testing on an image and you would like to easily identify the image
under test from existing images with typical default hostnames. In this
situation, you could change the default hostname to "testme", which
results in all the images using the name "testme". Once testing is
complete and you do not need to rebuild the image for test any longer,
you can easily reset the default hostname.

Another point of interest is that if you unset the variable, the image
will have no default hostname in the filesystem. Here is an example that
unsets the variable in a configuration file::

  hostname:pn-base-files = ""

Having no default hostname in the filesystem is suitable for
environments that use dynamic hostnames such as virtual machines.