Provided by: initramfs-tools_0.103ubuntu4_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).

              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.

              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 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
       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.

       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-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
       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-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  <>, Jeff Bailey
       <> and numerous others.

       This manual was written by David   Härdeman  <>,  updated  by  Maximilian
       Attems <>.


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