Provided by: initramfs-tools-core_0.122ubuntu8_all bug


       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 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=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 done.

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

              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 "Subdirectories" section under boot scripts).

              The  resume  hook  tries  to autodetect the resume partition and
              uses the first swap partition as valid guess. It is possible  to
              set  the  RESUME variable in /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,module2,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
              example "debug=vc".

        break spawns a shell in the initramfs image at the chosen phase  (top,
              modules,   premount,  mount,  mountroot,  bottom,  init)  before
              actually executing the  corresponding  scripts  (see  the  "Boot
              scripts"  section) or action.  Multiple phases may be specified,
              delimited by commas.  The default, if no phase is specified,  is
              "premount".   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

       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
       setup  in the script.  Please notice that PREREQ is only honored inside
       a single directory.  So first the scripts in /usr/share/initramfs-tools
       are  ordered  according  to  their PREREQ values and executed. Then all
       scripts in /etc/initramfs-tools are ordered according to  their  PREREQ
       values  and  executed. This mean that currently there is no possibility
       to have a local script (/etc/initramfs-tools) get executed  before  one
       from the package (/usr/share/initramfs-tools).

       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

   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 modules dir.

              is the $(uname -r) linux 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
              network interface is expected to be usable (NFS).

              local-block These scripts are called with the name  of  a  local
              block  device.   After  these  scripts  have been executed, that
              device node should be present.  If the local-top or  local-block
              scripts  fail  to create the wanted device node, the local-block
              scripts will be called periodically to try again.

              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 /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
       usually     be     placed     in    /etc/initramfs-tools/scripts/local-

              # 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


       initramfs-tools  includes  hook  scripts  that  are  called  by  kernel
       packages  on  installation  and  removal,  so  that  an  initramfs   is
       automatically  created,  updated  or  deleted  as  necessary.  The hook
       scripts do nothing if the environment variable INITRD  is  set  to  No.
       This  will  be the case for kernel packages built with make deb-pkg and
       with CONFIG_BLK_DEV_INITRD not set in the kernel config, or built  with
       make-kpkg and not using the --initrd option.


       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
              lsinitramfs /boot/initrd.img-3.16-3-amd64


              Log  of  fsck  commands  run  within  the  initramfs, with their

              Exists only if fsck ran successfully for the root filesystem.

              Exists only if fsck ran successfully for the /usr filesystem.


       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),