Provided by: initramfs-tools-core_0.122ubuntu8.17_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

        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
              "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

        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 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
       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

              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

              # 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.


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


              Log of fsck commands run within the initramfs, with their output.

              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), lsinitramfs(8).