INITRAMFS-TOOLS(7)         Linux Programmer's Manual        INITRAMFS-TOOLS(7)

       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  la-
       bel,  and mount that device as the root filesystem.  root=UUID=uuidnum-
       ber 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.

              clear screen at the beginning

              The  size  of  the /run tmpfs mount point in bytes (suffixes are
              supported) or as percentage of your physical RAM. This parameter
              is  used  as  the  value  of the size mount option to tmpfs. See
              for details. The default is 10%.

        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 timeout in seconds. Determines how long mountroot waits  for
              root to appear.  The default is 30 seconds.

              alias for roottimeout.

              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 de-
              tails. Optional parameter for NFS root.

        vlan  tells to create a VLAN tagged device. Allows  one  to  configure
              one    or    multiple    VLAN    tagged    devices   using   the
              "vlan=$name.$id:$parent"  syntax.  E.g.  "vlan=eth0.1:eth0"  Op-
              tional 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,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 the chosen phase (top,
              modules, premount, mount, mountroot, bottom, init) before  actu-
              ally 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 "pre-
              mount".  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.

   Configuration hook scripts
       These are used to override the user configuration where necessary,  for
       example to force use of busybox instead of klibc utilities.

   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

       Configuration  hook  scripts  can  be  found  in  /usr/share/initramfs-
       tools/conf-hooks.d.  They are sourced by mkinitramfs after the configu-
       ration files in /etc and before running any  hook  scripts.   They  can
       override any of the variables documented in initramfs.conf(5), but this
       should be done only if absolutely necessary.  For example, if  a  pack-
       age's  boot  script  requires  commands not provided by klibc-utils, it
       should also install a configuration hook that sets BUSYBOX=y.

       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).

       If  a  hook script requires configuration beyond the exported variables
       listed below, it should read a private configuration file that is sepa-
       rate  from  the  /etc/initramfs-tools  directory.   It  must  not  read
       initramfs-tools configuration files directly.

       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  de-
              pends 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 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.

              are as described in initramfs.conf(5).

              is  the  directory  where  busybox utilities should be installed
              from, or empty if busybox is not being used.

       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  NN-name 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 NN-name

       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-block  These  scripts  are called with the name of a local
              block device.  After these scripts have been executed, that  de-
              vice  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  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
              copy_exec /sbin/frobnicate /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
              option.   Use of ROOTDELAY is deprecated; you should implement a
              local-block boot script rather than delaying or polling.

              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-

       Package maintainer scripts should not run update-initramfs directly.  A
       package  that installs hooks for initramfs-tools should include a trig-
       gers file containing:
              activate-noawait update-initramfs

       Kernel packages must call the kernel hooks as documented in the  Debian
       Kernel Handbook.

       A  package  that requires an initramfs to function, but is not a kernel
       package, should include a triggers file containing:
              activate-await update-initramfs

       initramfs-tools includes hook scripts that are called by  kernel  pack-
       ages 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   CON-
       FIG_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 mod-
              lsinitramfs /boot/initrd.img-3.16-3-amd64

              Log of fsck commands run within the initramfs, with  their  out-

              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  Hardeman <>, updated
       by Maximilian Attems <>.

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

initramfs-tools                   2018/07/18                INITRAMFS-TOOLS(7)
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