DRACUT.MODULES(7) dracut DRACUT.MODULES(7)
dracut.modules - dracut modules
dracut uses a modular system to build and extend the initramfs image.
All modules are located in /usr/lib/dracut/modules.d or in
<git-src>/modules.d. The most basic dracut module is 99base. In 99base
the initial shell script init is defined, which gets run by the kernel
after initramfs loading. Although you can replace init with your own
version of 99base, this is not encouraged. Instead you should use, if
possible, the hooks of dracut. All hooks, and the point of time in
which they are executed, are described in the section called “BOOT
The main script, which creates the initramfs is dracut itself. It
parses all arguments and sets up the directory, in which everything is
installed. It then executes all check, install, installkernel scripts
found in the modules, which are to be processed. After everything is
installed, the install directory is archived and compressed to the
final initramfs image. All helper functions used by check, install and
installkernel are found in in the file dracut-functions. These shell
functions are available to all module installer (install,
installkernel) scripts, without the need to source dracut-functions.
A module can check the preconditions for install and installkernel with
the check script. Also dependencies can be expressed with check. If a
module passed check, install and installkernel will be called to
install all of the necessary files for the module. To split between
kernel and non-kernel parts of the installation, all kernel module
related parts have to be in installkernel. All other files found in a
module directory are module specific and mostly are hook scripts and
dracut modules can insert custom script at various points, to control
the boot process. These hooks are plain directories containing shell
scripts ending with ".sh", which are sourced by init. Common used
functions are in dracut-lib.sh, which can be sourced by any script.
The cmdline hook is a place to insert scripts to parse the kernel
command line and prepare the later actions, like setting up udev rules
and configuration files.
In this hook the most important environment variable is defined: root.
The second one is rootok, which indicates, that a module claimed to be
able to parse the root defined. So for example, root=iscsi:.... will be
claimed by the iscsi dracut module, which then sets rootok.
This hook is executed right after the cmdline hook and a check if root
and rootok were set. Here modules can take action with the final root,
and before udev has been run.
Now udev is started and the logging for udev is setup.
In this hook, you can set udev environment variables with udevadm
control --property=KEY=value or control the further execution of udev
udev is triggered by calling udevadm trigger, which sends add events
for all devices and subsystems.
In the main loop of dracut loops until udev has settled and all scripts
in initqueue/finished returned true. In this loop there are three
hooks, where scripts can be inserted by calling /sbin/initqueue.
This hook gets executed every time a script is inserted here,
regardless of the udev state.
This hooks (initqueue/settled) gets executed every time udev has
This hooks (initqueue/timeout) gets executed, when the main loop
counter becomes half of the rd.retry counter.
This hook (initqueue/finished) is called after udev has settled and
if all scripts herein return 0 the main loop will be ended.
Abritary scripts can be added here, to loop in the initqueue until
something happens, which a dracut module wants to wait for.
Before the root device is mounted all scripts in the hook pre-mount are
executed. In some cases (e.g. NFS) the real root device is already
This hook is mainly to mount the real root device.
This hook is called before cleanup hook, This is a good place for
actions other than cleanups which need to be called before pivot.
This hook is the last hook and is called before init finally switches
root to the real root device. This is a good place to clean up and kill
processes not needed anymore.
Cleanup and switch_root
Init (or systemd) kills all udev processes, cleans up the environment,
sets up the arguments for the real init process and finally calls
switch_root. switch_root removes the whole filesystem hierarchy of the
initramfs, chroot()s to the real root device and calls /sbin/init with
the specified arguments.
To ensure all files in the initramfs hierarchy can be removed, all
processes still running from the initramfs should not have any open
file descriptors left.
A simple example module is 96insmodpost, which modprobes a kernel
module after udev has settled and the basic device drivers have been
All module installation information is in the file module-setup.sh.
First we create a check() function, which just exits with 0 indicating
that this module should be included by default.
The we create the install() function, which installs a cmdline hook
with priority number 20 called parse-insmodpost.sh. It also installs
the insmodpost.sh script in /sbin.
inst_hook cmdline 20 "$moddir/parse-insmodpost.sh"
inst_simple "$moddir/insmodpost.sh" /sbin/insmodpost.sh
The parse-instmodpost.sh parses the kernel command line for a argument
rd.driver.post, blacklists the module from being autoloaded and
installs the hook insmodpost.sh in the initqueue/settled.
for p in $(getargs rd.driver.post=); do
echo "blacklist $p" >> /etc/modprobe.d/initramfsblacklist.conf
[ -n "$_do_insmodpost" ] && /sbin/initqueue --settled --unique --onetime /sbin/insmodpost.sh
insmodpost.sh, which is called in the initqueue/settled hook will just
modprobe the kernel modules specified in all rd.driver.post kernel
command line parameters. It runs after udev has settled and is only
called once (--onetime).
for p in $(getargs rd.driver.post=); do
check() is called by dracut to evaluate the inclusion of a dracut
module in the initramfs.
If the $hostonly variable is set, then the module check() function
should be in "hostonly" mode, which means, that the check() should
only return 0, if the module is really needed to boot this specific
check() should return with:
Include the dracut module in the initramfs.
Do not include the dracut module. The requirements are not
fulfilled (missing tools, etc.)
Only include the dracut module, if another module requires it or if
explicitly specified in the config file or on the argument list.
The function depends() should echo all other dracut module names the
module depends on.
This function should print the kernel command line options needed to
boot the current machine setup. It should start with a space and should
not print a newline.
The install() function is called to install everything non-kernel
related. To install binaries, scripts, and other files, you can use the
functions mentioned in [creation].
To address a file in the current module directory, use the variable
In installkernel() all kernel related files should be installed. You
can use all of the functions mentioned in [creation] to install files.
inst_multiple [-o] <file> [ <file> ...]
installs multiple binaries and files. If executables are specified
without a path, dracut will search the path
PATH=/usr/sbin:/sbin:/usr/bin:/bin for the binary. If the option
"-o" is given as the first parameter, a missing file does not lead
to an error.
inst <src> [<dst>]
installs one file <src> either to the same place in the initramfs
or to an optional <dst>. inst with more than two arguments is
treated the same as inst_multiple, all arguments are treated as
files to install and none as install destinations.
inst_hook <hookdir> <prio> <src>
installs an executable/script <src> in the dracut hook <hookdir>
with priority <prio>.
inst_rules <udevrule> [ <udevrule> ...]
installs one ore more udev rules. Non-existant udev rules are
reported, but do not let dracut fail.
instmods <kernelmodule> [ <kernelmodule> ... ]
instmods should be used only in the installkernel() function.
instmods installs one or more kernel modules in the initramfs.
<kernelmodule> can also be a whole subsystem, if prefixed with a
"=", like "=drivers/net/team".
instmods will not install the kernel module, if $hostonly is set
and the kernel module is not currently needed by any
/sys/.../uevent MODALIAS. To install a kernel module regardless of
the hostonly mode use the form:
hostonly='' instmods <kernelmodule>
dracut 10/09/2018 DRACUT.MODULES(7)