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Most of the functionality that dracut implements are actually implemented
by dracut modules. Dracut modules live in modules.d, and have the following
structure:
dracut_install_dir/modules.d/
00modname/
install
check
<other files as needed by the hook>
00modname: The name of the module prefixed by a two-digit numeric sort code.
The numeric code must be present and in the range of 00 - 99.
Modules with lower numbers are installed first. This is important
because the dracut install functions (which install files onto
the initrd) refuse to overwrite already installed files. This makes
it easy for an earlier module to override the functionality of a
later module, so that you can have a distro or system specific
module override or modify the functionality of a generic module
without having to patch the more generic module.
install: dracut sources this script to install the functionality that a
module implements onto the initrd. For the most part, this amounts
to copying files from the host system onto the initrd in a controlled
manner. dracut supplies several install functions that are
specialized for different file types. Browse through dracut-functions
fore more details. dracut also provides a $moddir variable if you
need to install a file from the module directory, such as an initrd
hook, a udev rule, or a specialized executable.
check: Dracut calls this program to check and see if a module can be installed
on the initrd.
When called without options, check should check to make sure that
any files it needs to install into the initrd from the host system
are present. It should exit with a 0 if they are, and a 1 if they are
not.
When called with -h, it should perform the same check that it would
without any options, and it should also check to see if the
functionality the module implements is being used on the host system.
For example, if this module handles installing support for LUKS
encrypted volumes, it should return 0 if all the tools to handle
encrpted volumes are available and the host system has the root
partition on an encrypted volume, 1 otherwise.
When called with -d, it should output a list of dracut modules
that it relies upon. An example would be the nfs and iscsi modules,
which rely on the network module to detect and configure network
interfaces.
Check may take additional options in the future.
Any other files in the module will not be touched by dracut directly.
You are encouraged to provide a README that describes what the module is for.
HOOKS
=====
init has the following hook points to inject scripts:
/cmdline/*.sh
scripts for command line parsing
/pre-udev/*.sh
scripts to run before udev is started
/pre-trigger/*.sh
scripts to run before the main udev trigger is pulled
/initqueue/*.sh
runs in parallel to the udev trigger
Udev events can add scripts here with /sbin/initqueue.
If /sbin/initqueue is called with the "--onetime" option, the script
will be removed after it was run.
If /initqueue/work is created and udev >= 143 then this loop can
process the jobs in parallel to the udevtrigger.
If the udev queue is empty and no root device is found or no root
filesystem was mounted, the user will be dropped to a shell after
a timeout.
Scripts can remove themselves from the initqueue by "rm $job".
/pre-mount/*.sh
scripts to run before the root filesystem is mounted
Network filesystems like NFS that do not use device files are an
exception. Root can be mounted already at this point.
/mount/*.sh
scripts to mount the root filesystem
If the udev queue is empty and no root device is found or no root
filesystem was mounted, the user will be dropped to a shell after
a timeout.
/pre-pivot/*.sh
scripts to run before the real init is executed and the initramfs
disappears
All processes started before should be killed here.
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