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 PROCESS STAGES”.
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 udevadmcontrol --property=KEY=value or control the further execution of
udev with udevadm.
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
This hook gets executed every time a script is inserted here,
regardless of the udev state.
This hook (initqueue/settled) gets executed every time udev
This hook (initqueue/timeout) gets executed, when the main
loop counter becomes half of the rd.retry counter.
This hook (initqueue/online) gets executed whenever a network
interface comes online (that is, once it is up and configured
by the configured network module).
This hook (initqueue/finished) is called after udev has
settled and if all scripts herein return 0 the main loop will
be ended. Arbitrary 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 mounted, though.
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 90kernel-modules, which modprobes a
kernel module after udev has settled and the basic device drivers
have been loaded.
All module installation information is in the file
First we create a check() function, which just exits with 0
indicating that this module should be included by default.
Then 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
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
module-setup.sh: check()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 host.
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
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
In installkernel() all kernel related files should be installed.
You can use all of the functions mentioned in [creation] to
Creation Functionsinst_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
inst_hook <hookdir> <prio> <src>
installs an executable/script <src> in the dracut hook
<hookdir> with priority <prio>.
inst_rules <udevrule> [ <udevrule> ...]
installs one or 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>
This page is part of the dracut (event driven initramfs
infrastructure) project. Information about the project can be
found at ⟨https://dracut.wiki.kernel.org/index.php/Main_Page⟩.
If you have a bug report for this manual page, send it to
email@example.com. This page was obtained from the
project's upstream Git repository
2020-12-18. (At that time, the date of the most recent commit
that was found in the repository was 2020-12-14.) If you
discover any rendering problems in this HTML version of the page,
or you believe there is a better or more up-to-date source for
the page, or you have corrections or improvements to the
information in this COLOPHON (which is not part of the original
manual page), send a mail to firstname.lastname@example.org
dracut 050-166-g811c8146 10/25/2020 DRACUT.MODULES(7)