dracut.modules(7) — Linux manual page


DRACUT.MODULES(7)                  dracut                  DRACUT.MODULES(7)

NAME         top

       dracut.modules - dracut modules

DESCRIPTION         top

       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
       udev rules.


       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.

   Hook: cmdline
       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.

   Hook: pre-udev
       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.

   Start Udev
       Now udev is started and the logging for udev is setup.

   Hook: pre-trigger
       In this hook, you can set udev environment variables with udevadm
       control --property=KEY=value or control the further execution of udev
       with udevadm.

   Trigger Udev
       udev is triggered by calling udevadm trigger, which sends add events
       for all devices and subsystems.

   Main Loop
       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.

       Initqueue settled
           This hooks (initqueue/settled) gets executed every time udev has

       Initqueue timeout
           This hooks (initqueue/timeout) gets executed, when the main loop
           counter becomes half of the rd.retry counter.

       Initqueue finished
           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.

   Hook: pre-mount
       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.

   Hook: mount
       This hook is mainly to mount the real root device.

   Hook: pre-pivot
       This hook is called before cleanup hook, This is a good place for
       actions other than cleanups which need to be called before pivot.

   Hook: cleanup
       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.



WRITING A MODULE         top

       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.


           return 0

       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
           unset _do_insmodpost

       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).


           . /lib/dracut-lib.sh

           for p in $(getargs rd.driver.post=); do
               modprobe $p

   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 argument

   module-setup.sh: depends()
       The function depends() should echo all other dracut module names the
       module depends on.

   module-setup.sh: cmdline()
       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.

   module-setup.sh: install()
       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

   module-setup.sh: installkernel()
       In installkernel() all kernel related files should be installed. You
       can use all of the functions mentioned in [creation] to install

   Creation Functions
       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 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>

   Initramfs Functions

   Network Modules

AUTHOR         top

       Harald Hoyer

SEE ALSO         top


COLOPHON         top

       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
       initramfs@vger.kernel.org.  This page was obtained from the project's
       upstream Git repository
       ⟨git://git.kernel.org/pub/scm/boot/dracut/dracut.git⟩ on 2020-09-18.
       (At that time, the date of the most recent commit that was found in
       the repository was 2020-03-04.)  If you discover any rendering prob‐
       lems in this HTML version of the page, or you believe there is a bet‐
       ter 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 man-pages@man7.org

dracut 050-2-g9e68789d           05/16/2020                DRACUT.MODULES(7)