323 lines
10 KiB
Markdown
323 lines
10 KiB
Markdown
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Blends
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======
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Introduction
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------------
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In the Devuan SDK, a _blend_ is the preferred way we use to make
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customizations to the _vanilla_ image. Using blends we can very easily
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create different flavors of our image, by easily including/excluding
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certain software packages, files, or anything we wish to do. Blends
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can become a very quick way of creating entire new derivatives of the
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original _vanilla_ distribution we are building.
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This time, we will take the DECODE OS as a _blend_ example. In DECODE
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OS we provide a blend called _decode_ which is the blend we use to
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create a production release of DECODE OS. The blend's files are
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contained within their own directory in the _decode-os_ git
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repository.
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Configuration
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-------------
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Any SDK requires a single file to act as a _blend_. This file is also a
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_zsh_ script, and, at the very least, it must contain two functions
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called:
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```
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blend_preinst()
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blend_postinst()
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```
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These functions are your pathway to expanding your blend into whatever
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you would like it to do. The _preinst_ function is usually called
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right after bootstrapping the _vanilla_ root filesystem, and the
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_postinst_ function is called near the very end, just before packing
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or compressing the image. These two strategic places should be enough
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to do changes within the image. If this is not enough, blends also
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allow you to simply **override any variable or function** contained
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within _libdevuansdk_ or the sdk you are using.
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Our _decode_ blend is such an example. It is a somewhat expanded blend,
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not contained within a single file, but rather a directory. This allows
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easier maintenance and makes the scripts clearer and cleaner.
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### Adding and removing packages
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When we want to add or remove specific packages to our build, we have
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to override or append to _libdevuansdk_'s arrays. The array for
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packages we want installed is called *extra_packages*, and the array
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for packages we want purged is called *purge_packages*. In the Decode
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blend, these can be found in the _config_ file located inside the
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_decode-os_ blend directory. Keep in mind that these arrays could
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already contain specific packages, so you are advised to rather append
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to them, than overriding them.
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If the packages you want to install are not available in the
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repositories, you still have a way of automatically installing
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them. All you have to do is copy your corresponding .deb files to the
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following directory of the blend:
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```
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$R/extra/custom-packages/
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```
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And when that is done, just call the function *install-custdebs*
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Creating a blend
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----------------
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Rather than explaining the following in theory, you are best off
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viewing the blend files that are provided with _decode-os_. It is a
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fairly simple blend and should give you enough insight on how to
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create your own blend. Here are some important guidelines for creating
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a blend:
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* The blend should always contain at least two functions
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This means you must provide *blend_preinst* and *blend_postinst* in your
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blend. They don't even have to do anything, but they should be there.
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These two functions open the path for you to call any other functions
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you created for your blend.
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* When overriding functions, make sure they provide a result that
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doesn't break the API
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Breaking the API may result in unwanted behavior. You should always
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study well the functions you are planning to override and figure out if
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it is safe to override them in the way you want. The same goes for any
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variables as well.
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* Any arguments used after the blend name when loading from the SDK are
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free for you to use in the blend.
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This means you can use anything after the fourth argument (**$4** in
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_zsh_) inside your blend if you require passing arguments to it.
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These are some of the more important guidelines. There is plenty more
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tricks and quirks, but it's easy to find out how to tweak the
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configuration files and the blend in general once you read through a
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blend or two on your own.
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### Enable the blend
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To use your blend in the first place, you need to make the SDK know
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about it. Thus you should append the path to your new blend inside
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the **blend_map** of the _sdk_ file:
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```
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blend_map=(
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"devuan-live" "$R/blends/devuan-live/devuan-live.blend"
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"decode" "$R/../decode.blend"
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"heads" "$R/../heads.blend"
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"ournewblend" "$R/blends/newblend/new-blend.blend"
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)
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```
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As you can see, the map is a key-value storage. So you can have an alias
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(name) for your blend, and just use that to point to the path of the
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blend. The blend file will be sourced by the SDK once it is told to do
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so.
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### A configuration file
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For having a finer-grained control of what goes into our build, we can
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create a config file for our blend. From here we can easily control
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any configurable aspect of it, such as packages that go in or out, the
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blend name, and much more. **Make sure you source this file from your
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blend.**
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Adding and removing packages was abstractly mentioned earlier: it goes
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into two separate arrays holding package names. To add packages, we
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append to the `extra_packages` array, which would look like this:
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```
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extra_packages+=(
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my_new_package
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foo
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bar
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baz
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)
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```
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This would install the four packages `my_new_package`, `foo`, `bar`,
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and `baz` along with the ones predefined in either _libdevuansdk_ or
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the SDK you are using. You may also want to see which those are in
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case you wish to exclude them, but they are sane and useful utilities
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which should be included in your build if possible. Overriding all
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those packages, you would need to reset the whole array, so you would
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simply issue this:
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```
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extra_packages=(
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my_new_package
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foo
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bar
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baz
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)
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```
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As you can see, we no longer have the `+=`, but rather only `=`, which
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means we are not appending to the array, but rather redefining it.
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All of the above applies as well for removing packages, but in this case
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the array is called `purge_packages`.
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#### Custom packages
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If you want to install deb packages that aren't in any repositories, put
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them in the blend directory and simply add them to another array in the
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configuration file. The contents of the arrays are the paths to the
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debs, relative to this configuration file:
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```
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custom_deb_packages=(
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yad_0.27.0-1_amd64.deb
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palemoon_27.2.0~repack-1_amd64.deb
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)
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```
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To trigger the installation of these packages, you will need to copy
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them to `$R/extra/custom_packages`, and then call the
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`install_custdebs` function somewhere from your blend.
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### Custom files
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Any files you want to add to the system to override what's there by
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default you can add using a *rootfs overlay*. Create a directory
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inside your blend directory called *rootfs-overlay* and simply put
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files inside it. The directory structure is absolute to the image we
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are building. For example what's in "rootfs-overlay/etc/" would end
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up in the "/etc" of our final image. See _hier(7)_ in the Linux
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manpages for more explanation on this directory hierarchy.
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If you end up with any files here, to actually copy them, you will need
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to either run `cp -f` it, or `rsync` the directory if you prefer.
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### The .blend file
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We listed a path to the .blend file in our first step. We need to create
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this file now.
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Start your blend file with the following, so the sdk is aware of the
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environment:
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```
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BLENDPATH="${BLENDPATH:-$(dirname $0)}"
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source $BLENDPATH/config
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```
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The minimum blend should contain two functions: `blend_preinst` and
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`blend_postinst`. These functions are called at specific points in the
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build, where they give the most power: just after bootstrapping the
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_vanilla_ system, and just before packaging the final build,
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respectively.
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#### blend_preinst
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A preinst function can look like this:
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```
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blend_preinst() {
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fn blend_preinst
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req=(BLENDPATH R)
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ckreq || return 1
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notice "executing blend preinst"
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add-user "user" "pass"
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cp -fv "$BLENDPATH"/*.deb "$R/extra/custom-packages" || zerr
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install-custdebs || zerr
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}
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```
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As you can see, the pre-install function will add a new user with the
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credentials `user:pass`, it will copy our custom debs where they can
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be used, and finally it will trigger their installation.
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The `fn, req, ckreq` part on the top of the function is a safety check
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for the function that is enabled by _zuper_. It allows us to check if
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variables are defined when the function is called and fail if it is
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wrong. You should utilize this as much as possible. The `zerr` calls
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are used to exit if the function fails.
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#### blend_postinst
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A post-install function can look like the following:
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```
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blend_postinst() {
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fn blend_postinst
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req=(BLENDPATH strapdir)
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ckreq || return 1
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notice "executing blend postinst"
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sudo cp -vf "$BLENDPATH"/rootfs-overlay/* $strapdir || zerr
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blend_finalize || zerr
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}
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```
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This function would copy the `rootfs-overlay` to the `strapdir` (which
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holds our image's filesystem) and it would call the `blend_finalize`
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function. By default this function doesn't exist, we quote it as an
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example for you to see how it is possible to call your own functions
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as well. You can define them within the blend file.
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Using a blend
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-------------
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As previously explained, you can use your blends through the SDK's
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interactive shell. In _decode-os_ the blend is placed in the root of
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the git repository, and the sdk wrappers are located within. Therefore
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an SDK would have to source it with such a path:
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```
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$R/../decode.blend
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```
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If you take a look at _vm-sdk_'s `sdk` file, you will see the
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`blend_map` array. Using a new blend requires you to add it to this
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map in the same manner. The map is key-value formatted, and on the
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left you have an alias of your blend, and on the right you have a
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script you have to write. It can either be the blend itself or any
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helper file you might need to initialize your blend.
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After you've added it to the blend map, you simply initialize the SDK,
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and use the same *load* command we learned earlier, while appending
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the blend alias and any optional argument.
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```
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$ zsh -f
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$ source sdk
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$ load devuan decode <these> <arguments> <we> <can> <use> <in> <the> <blend>
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```
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With this, we've initialized our *decode* blend. It's always good to add a
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*notice()* call to your blend to signal it's been loaded successfully.
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Once this is done, we simply build the image the same way we have
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learned before:
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```
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$ build_vagrant_dist
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```
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Consult the _vm-sdk_ chapter for this.
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