ntrepid (8) initramfs-tools.8.gz
 Provided by: initramfs-tools_0.92bubuntu15_all

NAME
       initramfs-tools - an introduction to writing scripts for mkinitramfs

DESCRIPTION
       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.

       Valid  boot  and  hook  scripts  names  consist  solely of alphabetics,
       numerics 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.

   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.

INIT SCRIPT
       The script which is executed first and is  in  charge  of  running  all
       other scripts can be found in /usr/share/initramfs-tools/init. It takes
       a number of arguments which influence the boot procedure:

   Boot options
       The init and root are usually passed by the boot loader for local boot.
       The other parameters are optional.

         init the  binary  to  hand over execution to on the root fs after the
              initramfs scripts are done.

         root the device node to mount as the root file system.

         rootdelay
              set delay in seconds. Determines how long  mountroot  waits  for
              root to appear.

         rootflags
              set the file system mount option string.

         rootfstype
              set the root file system type.

         loop path  within the original root file system to loop-mount and use
              as the real root file system.

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

         loopfstype
              set the loop file system type, if applicable.

         nfsroot
              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.

         ip   tells  how  to  configure  the  ip address. Allows to specify an
              different   NFS   server   than    the    DHCP    server.    See
              Documentation/nfsroot.txt   in   any  recent  Linux  source  for
              details. Optional paramater for NFS root.

         cryptopts
              passes the args for  cryptoroot.  Set  by  the  cryptsetup  boot
              hooks.

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

         resume
              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 variable  noresume
              overrides it.

         resume_offset
              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.

         blacklist
              disables      load      of      specific      modules.       Use
              blacklist=module1,module2,module3 bootparameter.

         panic
              sets  an  timeout  on  panic.   panic=0 is a documented security
              feature: it disables the debug shell.

         debug
              generates   lots   of   output.   It    writes    a    log    to
              /tmp/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,  bottom,  init).   The  default   is
              premount without any arg.

HOOK SCRIPTS
       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.

   Header
       In order  to  support  prereqs,  each  script  should  begin  with  the
       following lines:

              #!/bin/sh
              PREREQ=""
              prereqs()
              {
                   echo "$PREREQ"
              }

              case $1 in
              prereqs)
                   prereqs
                   exit 0
                   ;;
              esac

              # 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
       kilobytes.

BOOT SCRIPTS
       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.

   Header
       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:

              #!/bin/sh
              PREREQ=""
              prereqs()
              {
                   echo "$PREREQ"
              }

              case $1 in
              prereqs)
                   prereqs
                   exit 0
                   ;;
              esac

       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:

       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

              Example:

                     log_begin_msg "Frobnication begun"
                     # Do something
                     log_end_msg

       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

   Subdirectories
       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  and
              /dev/console and /dev/null have been created.  No  other  device
              files are present yet.

       init-premount
              runs the udev hooks for populating the /dev tree (udev will keep
              running until init-bottom) 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-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. udev is stopped.

       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.

   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.

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

              #!/bin/sh
              # Example frobnication hook script

              PREREQ="lvm"
              prereqs()
              {
                   echo "$PREREQ"
              }

              case $1 in
              prereqs)
                   prereqs
                   exit 0
                   ;;
              esac

              # Begin real processing below this line

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

              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-
       top/frobnicate):

              #!/bin/sh
              # Example frobnication boot script

              PREREQ="lvm"
              prereqs()
              {
                   echo "$PREREQ"
              }

              case $1 in
              prereqs)
                   prereqs
                   exit 0
                   ;;
              esac

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

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

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

              exit 0

DEBUG
       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