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Linux 2.6 - man page for initramfs-tools (linux section 8)

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 subscripts which will be
       used during different phases of execution. Each of  these  will	be  discussed  separately
       below  with the help of an imaginary 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 tra-
       ditional  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=uuidnumber
       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

	root= "<path to blockdevice>"
	      the  device  node  to  mount  as the root file system.  The recommended 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:NFSPATH: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  Documentation/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 "Subdirecto-
	      ries" section under boot scripts).

	      On install initramfs-tools tries to autodetect the resume partition. On success the
	      RESUME  variable	is written to /etc/initramfs-tools/conf.d/resume.  The boot vari-
	      able 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,module2,module3 bootpa-

	panic sets an timeout on panic.  panic=<sec> is a documented security  feature:  it  dis-
	      ables 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 example "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,  numerics,  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 mounted.

       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	responsi-
       ble 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  setup
       in the script.

       In order to support prereqs, each script should begin with the following 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 functions 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 executables, unless compiled with klibc,  will
       automatically  include  glibc in the image which will increase its size by several hundred

   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 fol-
       lowing 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 network interface is expected to be usable

	      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 variables 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 usually be placed in

	      # 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  usually  be  placed  in

	      # 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 option.

	      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 script.

	      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 investigation.

       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 modules:
	      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 <maks@debian.org>, Jeff Bailey <jbai-
       ley@raspberryginger.com> and numerous others.

       This manual was written by David   Hardeman  <david@hardeman.nu>,  updated  by  Maximilian
       Attems <maks@debian.org>.

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

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

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