kernel-install - Add and remove kernel and initrd images to and from /boot
kernel-install [OPTIONS...] COMMAND KERNEL-VERSION KERNEL-IMAGE [INITRD-FILE...]
kernel-install is used to install and remove kernel and initrd images  to and from the boot loader partition, referred to as $BOOT here. It will usually be one of /boot/, /efi/, or /boot/efi/, see below.
kernel-install will run the executable files ("plugins") located in the directory /usr/lib/kernel/install.d/ and the local administration directory /etc/kernel/install.d/. All files are collectively sorted and executed in lexical order, regardless of the directory in which they live. However, files with identical filenames replace each other. Files in /etc/kernel/install.d/ take precedence over files with the same name in /usr/lib/kernel/install.d/. This can be used to override a system-supplied executables with a local file if needed; a symbolic link in /etc/kernel/install.d/ with the same name as an executable in /usr/lib/kernel/install.d/, pointing to /dev/null, disables the executable entirely. Executables must have the extension ".install"; other extensions are ignored.
An executable placed in these directories should return 0 on success. It may also return 77 to cause the whole operation to terminate (executables later in lexical order will be skipped).
The following commands are understood:
add KERNEL-VERSION KERNEL-IMAGE [INITRD-FILE ...]
add KERNEL-VERSION $BOOT/ENTRY-TOKEN/KERNEL-VERSION/ KERNEL-IMAGE [INITRD-FILE ...]
The third argument directly refers to the path where to place kernel images, initrd images and other resources for Boot Loader Specification Type #1 entries (the "entry directory"). If other boot loader schemes are used the parameter may be ignored. The ENTRY-TOKEN string is typically the machine ID and is supposed to identify the local installation on the system. For details see below.
Two default plugins execute the following operations in this case:
If $KERNEL_INSTALL_LAYOUT is not "bls", this plugin does nothing.
remove KERNEL-VERSION $BOOT/ENTRY-TOKEN/KERNEL-VERSION/
Afterwards, kernel-install removes the entry directory $BOOT/ENTRY-TOKEN/KERNEL-VERSION/ and its contents, if it exists.
Two default plugins execute the following operations in this case:
THE $BOOT PARTITION¶
The partition where the kernels and Boot Loader Specification snippets are located is called $BOOT. kernel-install determines the location of this partition by checking /efi/, /boot/, and /boot/efi/ in turn. The first location where $BOOT/loader/entries/ or $BOOT/ENTRY-TOKEN/ exists is used.
The following options are understood:
Environment variables exported for plugins¶
If --verbose is used, $KERNEL_INSTALL_VERBOSE=1 will be exported for plugins. They may output additional logs in this case.
$KERNEL_INSTALL_MACHINE_ID is set for the plugins to the desired machine-id to use. It's always a 128-bit ID. Normally it's read from /etc/machine-id, but it can also be overridden via $MACHINE_ID (see below). If not specified via these methods a fallback value will generated by kernel-install, and used only for a single invocation.
$KERNEL_INSTALL_ENTRY_TOKEN is set for the plugins to the desired entry "token" to use. It's an identifier that shall be used to identify the local installation, and is often the machine ID, i.e. same as $KERNEL_INSTALL_MACHINE_ID, but might also be a different type of identifier, for example a fixed string or the ID=, IMAGE_ID= values from /etc/os-release. The string passed here will be used to name Boot Loader Specification entries, or the directories the kernel image and initial RAM disk images are placed into.
Note that while $KERNEL_INSTALL_ENTRY_TOKEN and $KERNEL_INSTALL_MACHINE_ID are often set to the same value, the latter is guaranteed to be a valid 32 character ID in lowercase hexadecimals while the former can be any short string. The entry token to use is read from /etc/kernel/entry-token, if it exists. Otherwise a few possible candidates below $BOOT are checked for Boot Loader Specification Type 1 entry directories, and if found the entry token is derived from that. If that is not successful, $KERNEL_INSTALL_MACHINE_ID is used as fallback.
$KERNEL_INSTALL_BOOT_ROOT is set for the plugins to the absolute path of the root directory (mount point, usually) of the hierarchy where boot loader entries, kernel images, and associated resources should be placed. This usually is the path where the XBOOTLDR partition or the ESP (EFI System Partition) are mounted, and also conceptually referred to as $BOOT. Can be overridden by setting $BOOT_ROOT (see below).
$KERNEL_INSTALL_LAYOUT=bls|other|... is set for the plugins to specify the installation layout. Defaults to bls if $BOOT/ENTRY-TOKEN exists, or other otherwise. Additional layout names may be defined by convention. If a plugin uses a special layout, it's encouraged to declare its own layout name and configure layout= in install.conf upon initial installation. The following values are currently understood:
Implemented by 90-loaderentry.install.
$KERNEL_INSTALL_INITRD_GENERATOR is set for plugins to select the initrd generator. This may be configured as initrd_generator= in install.conf, see below.
$KERNEL_INSTALL_STAGING_AREA is set for plugins to a path to a directory. Plugins may drop files in that directory, and they will be installed as part of the loader entry, based on the file name and extension.
Environment variables understood by kernel-install¶
$KERNEL_INSTALL_CONF_ROOT can be set to override the location of the configuration files read by kernel-install. When set, install.conf, entry-token, and other files will be read from this directory.
$KERNEL_INSTALL_PLUGINS can be set to override the list of plugins executed by kernel-install. The argument is a whitespace-separated list of paths. "KERNEL_INSTALL_PLUGINS=:" may be used to prevent any plugins from running.
$MACHINE_ID can be set for kernel-install to override $KERNEL_INSTALL_MACHINE_ID, the machine ID.
$BOOT_ROOT can be set for kernel-install to override $KERNEL_INSTALL_BOOT_ROOT, the installation location for boot entries.
The last two variables may also be set in install.conf. Variables set in the environment take precedence over the values specified in the config file.
If every executable returns 0 or 77, 0 is returned, and a non-zero failure code otherwise.
/usr/lib/kernel/cmdline /etc/kernel/cmdline /proc/cmdline
Currently, the following keys are supported: MACHINE_ID=, BOOT_ROOT=, layout=, initrd_generator=. See the Environment variables section above for details.
- Nowadays actually CPIO archives used as an "initramfs", rather than "initrd". See bootup(7) for an explanation.
- Boot Loader Specification