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INITRAMFS-TOOLS(8)	  mkinitramfs script overview	    INITRAMFS-TOOLS(8)

       initramfs-tools - an introduction to writing scripts for mkinitramfs

       initramfs-tools	has  one  main	script	and two different sets of sub‐
       scripts which will be used during different phases of  execution.  Each
       of  these will be discussed separately below with the help of an imagi‐
       nary tool which performs a frobnication of a  lvm  partition  prior  to
       mounting the root partition.

Kernel Command Line
       The  root filesystem used by the kernel is specified by the boot loader
       as always. The traditional root=/dev/sda1 style device specification is
       allowed.	 If a label is used, as in root=LABEL=rootPart the initrd will
       search all available devices for	 a  filesystem	with  the  appropriate
       label,  and  mount that device as the root filesystem.  root=UUID=uuid‐
       number will mount the partition with that UUID as the root filesystem.

	init= "<path to real init>"
	      the binary to hand over execution to on the root	fs  after  the
	      initramfs scripts are done.

	root= "<path to blockdevice>"
	      the  device  node	 to mount as the root file system.  The recom‐
	      mended usage is to specify the UUID as followed "root=UUID=xxx".

	      set the root file system type.

	      set delay in seconds. Determines how long	 mountroot  waits  for
	      root to appear.  The default is 180 seconds.

	      set the file system mount option string.

	loop= "<path to image>"
	      path  within the original root file system to loop-mount and use
	      as the real root file system.

	      set the loop file system type, if applicable.

	      set the loop file system mount option string, if applicable.

	      can be either "auto" to try to get the relevant information from
	      DHCP or a string of the form NFSSERVER:NFSPATH or NFSSERVER:NFS‐
	      PATH:NFSOPTS.  Use root=/dev/nfs for NFS to kick to in.  NFSOPTS
	      can be looked up in nfs(5).

	ip    tells  how to configure the ip address. Allows one to specify an
	      different NFS  server  than  the	DHCP  server.  See  Documenta‐
	      tion/filesystems/nfsroot.txt  in	any  recent  Linux  source for
	      details. Optional parameter for NFS root.

	      is a mac address in pxelinux format with leading "01-"  and  "-"
	      as  separations.	 pxelinux  passes  mac address of network card
	      used to PXE boot on with this bootarg.

	boot  either local or NFS (affects which initramfs  scripts  are  run,
	      see the "Subdirectories" section under boot scripts).

	      On install initramfs-tools tries to autodetect the resume parti‐
	      tion.  On	 success   the	 RESUME	  variable   is	  written   to
	      /etc/initramfs-tools/conf.d/resume.   The boot variable noresume
	      overrides it.

	      Specify the offset from the  partition  given  by	 "resume="  at
	      which the swap header of the swap file is located.

	quiet reduces the amount of text output to the console during boot.

	ro    mounts the rootfs read-only.

	rw    mounts the rootfs read-write.

	      disables	load  of specific modules.  Use blacklist=module1,mod‐
	      ule2,module3 bootparameter.

	panic sets an timeout on panic.	 panic=<sec> is a documented  security
	      feature: it disables the debug shell.

	debug generates	   lots	   of	 output.    It	 writes	  a   log   to
	      /run/initramfs/initramfs.debug.  Instead when  invoked  with  an
	      arbitrary	 argument output is written to console.	 Use for exam‐
	      ple "debug=vc".

	break spawns a shell in the initramfs image at chosen  run-time	 (top,
	      modules, premount, mount, mountroot, bottom, init).  The default
	      is premount without any arg.  Beware that if  both  "panic"  and
	      "break"  are  present,  initramfs	 will not spawn any shells but
	      reboot instead.

	      loads netconsole linux modules with the chosen args.

	      loads generic IDE/ATA chipset support on boot.

       Valid boot and hook scripts names consist solely of alphabetics, numer‐
       ics, dashes and underscores. Other scripts are discarded.

   Hook scripts
       These  are  used when an initramfs image is created and not included in
       the image itself. They can however cause files to be  included  in  the
       image.  Hook scripts are executed under errexit. Thus a hook script can
       abort the mkinitramfs build on possible errors (exitcode != 0).

   Boot scripts
       These are included in the initramfs image and normally executed	during
       kernel  boot in the early user-space before the root partition has been

       Hooks can be found in two places: /usr/share/initramfs-tools/hooks  and
       /etc/initramfs-tools/hooks.  They are executed during generation of the
       initramfs-image and are responsible for	including  all	the  necessary
       components  in the image itself. No guarantees are made as to the order
       in which the different scripts are executed unless the prereqs are set‐
       up in the script.

       In  order to support prereqs, each script should begin with the follow‐
       ing lines:

		   echo "$PREREQ"

	      case $1 in
		   exit 0

	      . /usr/share/initramfs-tools/hook-functions
	      # Begin real processing below this line

       For example, if you are writing a new hook script which relies on  lvm,
       the  line  starting with PREREQ should be changed to PREREQ="lvm" which
       will ensure that the lvm hook script is run before your custom script.

   Help functions
       /usr/share/initramfs-tools/hook-functions contains a  number  of	 func‐
       tions which deal with some common tasks in a hook script:

	      manual_add_modules  adds	a  module  (and	 any  modules which it
	      depends on) to the initramfs image.

	      Example: manual_add_modules isofs

	      add_modules_from_file reads a file containing a list of  modules
	      (one  per line) to be added to the initramfs image. The file can
	      contain comments (lines starting with #) and  arguments  to  the
	      modules by writing the arguments on the same line as the name of
	      the module.

	      Example: add_modules_from_file /tmp/modlist

	      force_load adds a module (and its dependencies) to the initramfs
	      image  and  also	unconditionally	 loads the module during boot.
	      Also supports passing arguments to the module  by	 listing  them
	      after the module name.

	      Example: force_load cdrom debug=1

	      copy_modules_dir	 copies	  an   entire  module  directory  from
	      /lib/modules/KERNELVERSION/ into the initramfs image.

	      Example: copy_modules_dir kernel/drivers/ata

   Including binaries
       If you need to copy binaries to the initramfs module,  a	 command  like
       this should be used:

	      copy_exec /sbin/mdadm /sbin

       mkinitramfs  will  automatically	 detect which libraries the executable
       depends on and copy them to the initramfs. This means  that  most  exe‐
       cutables,  unless compiled with klibc, will automatically include glibc
       in the image which will increase its size by several hundred kilobytes.

   Including a system firmware preimage (early initramfs)
       If you need to prepend data to the initramfs image, you need to prepare
       it  in  a file, and call the prepend_earlyinitramfs function.  The file
       can be disposed of as soon as the function returns.

       TEMP_FILE=$(mktemp ...)
       prepend_earlyinitramfs ${TEMP_FILE}
       rm -f ${TEMP_FILE}

   Exported variables
       mkinitramfs sets several variables for the hook scripts environment.

	      corresponds to the linux-2.6 modules dir.

	      is the $(uname -r) linux-2.6 version against mkinitramfs is run.

	      is the path of the used initramfs-tools configurations.

	      is the root path of the newly build initramfs.

	      allows arch specific hook additions.

	      corresponds to the verbosity of the update-initramfs run.

	      specifies which kind of modules should land on initramfs.	  This
	      setting  shouldn't  be  overridden by hook script, but can guide
	      them on how much they need to include on initramfs.

       Similarly to hook scripts, boot scripts can  be	found  in  two	places
       /usr/share/initramfs-tools/scripts/  and /etc/initramfs-tools/scripts/.
       There are a number of subdirectories to	these  two  directories	 which
       control the boot stage at which the scripts are executed.

       Like for hook scripts, there are no guarantees as to the order in which
       the different scripts in one subdirectory (see "Subdirectories"	below)
       are  executed.  In order to define a certain order, a similar header as
       for hook scripts should be used:

		   echo "$PREREQ"

	      case $1 in
		   exit 0

       Where PREREQ is modified to list other scripts in the same subdirectory
       if necessary.

   Help functions
       A number of functions (mostly dealing with output) are provided to boot
       scripts in /scripts/functions :

	      log_success_msg Logs a success message

	      Example: log_success_msg "Frobnication successful"

	      log_failure_msg Logs a failure message

	      Example: log_failure_msg "Frobnication component froobz missing"

	      log_warning_msg Logs a warning message

	      Example: log_warning_msg "Only partial frobnication possible"

	      log_begin_msg Logs a message that some processing step has begun

	      log_end_msg Logs a message that some processing step is finished


		     log_begin_msg "Frobnication begun"
		     # Do something

	      panic Logs  an  error  message  and  executes  a	shell  in  the
	      initramfs image to allow the user to investigate the situation.

	      Example: panic "Frobnication failed"

	      add_mountroot_fail_hook  Registers the script as able to provide
	      possible further information, in the event that the root	device
	      cannot  be  found. See the example script in the initramfs-tools
	      examples directory for more information.

	      Example: add_mountroot_fail_hook

       Both	/usr/share/initramfs-tools/scripts     and     /etc/initramfs-
       tools/scripts contains the following subdirectories.

	      init-top	the scripts in this directory are the first scripts to
	      be executed after sysfs and procfs have been mounted.   It  also
	      runs  the udev hook for populating the /dev tree (udev will keep
	      running until init-bottom).

	      init-premount happens  after  modules  specified	by  hooks  and
	      /etc/initramfs-tools/modules have been loaded.

	      local-top OR nfs-top After these scripts have been executed, the
	      root device node is expected to be present (local) or  the  net‐
	      work interface is expected to be usable (NFS).

	      local-premount  OR  nfs-premount are run after the sanity of the
	      root device has been verified (local) or the  network  interface
	      has  been	 brought  up  (NFS), but before the actual root fs has
	      been mounted.

	      local-bottom OR nfs-bottom are run after	the  rootfs  has  been
	      mounted (local) or the NFS root share has been mounted.

	      init-bottom  are	the  last scripts to be executed before procfs
	      and sysfs are moved to the real rootfs and execution  is	turned
	      over to the init binary which should now be found in the mounted
	      rootfs. udev is stopped.

   Boot parameters
	      /conf/param.conf allows boot scripts to  change  exported	 vari‐
	      ables  that  are	listed on top of init. Write the new values to
	      it. It will be sourced after an boot script run if it exists.

   Hook script
       An example hook script would look something like this (and  would  usu‐
       ally be placed in /etc/initramfs-tools/hooks/frobnicate):

	      # Example frobnication hook script

		   echo "$PREREQ"

	      case $1 in
		   exit 0

	      . /usr/share/initramfs-tools/hook-functions
	      # Begin real processing below this line

	      if [ ! -x "/sbin/frobnicate" ]; then
		   exit 0

	      force_load frobnicator interval=10
	      cp /sbin/frobnicate "${DESTDIR}/sbin"
	      exit 0

   Boot script
       An  example  boot script would look something like this (and would usu‐
       ally be placed in /etc/initramfs-tools/scripts/local-top/frobnicate):

	      # Example frobnication boot script

		   echo "$PREREQ"

	      case $1 in
		   exit 0

	      . /scripts/functions
	      # Begin real processing below this line
	      if [ ! -x "/sbin/frobnicate" ]; then
		   panic "Frobnication executable not found"

	      if [ ! -e "/dev/mapper/frobb" ]; then
		   panic "Frobnication device not found"

	      log_begin_msg "Starting frobnication"
	      /sbin/frobnicate "/dev/mapper/frobb" || panic "Frobnication failed"

	      exit 0

   Exported variables
       init sets several variables for the boot scripts environment.

	ROOT  corresponds to the root boot option.  Advanced boot scripts like
	      cryptsetup  or  live-initramfs  need  to play tricks.  Otherwise
	      keep it alone.

	      corresponds to the rootdelay, rootflags, rootfstype or  ip  boot

	      allows arch specific boot actions.

	blacklist, panic, quiet, resume, noresume, resume_offset
	      set according relevant boot option.

	break Useful  for  manual intervention during setup and coding an boot

	      Argument passed to the panic helper function.  Use to  find  out
	      why you landed in the initramfs shell.

	init  passes the path to init(8) usually /sbin/init.

	      is  the  default	for  mounting  the  root corresponds to the ro
	      bootarg.	Overridden by rw bootarg.

	      is the path where root gets mounted usually /root.

	debug indicates that a debug log is captured  for  further  investiga‐

       It  is  easy  to check the generated initramfs for its content. One may
       need to double-check if it contains the relevant binaries, libs or mod‐
	      mkdir tmp/initramfs
	      cd tmp/initramfs
	      gunzip -c /boot/initrd.img-2.6.18-1-686 | \
	      cpio -i -d -H newc --no-absolute-filenames

       The initramfs-tools are written by Maximilian Attems <>,
       Jeff Bailey <> and numerous others.

       This manual was written by David	 Härdeman <>, updated
       by Maximilian Attems <>.

	initramfs.conf(5), mkinitramfs(8), update-initramfs(8).

Linux				  2010/09/23		    INITRAMFS-TOOLS(8)

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