xenial (8) initramfs-tools.8.gz

Provided by: initramfs-tools-core_0.122ubuntu8.17_all bug

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.

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.

   Standard
        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 "root=UUID=xxx".

        rootfstype
              set the root file system type.

        rootdelay
              set delay in seconds. Determines how long mountroot waits for root to appear.  The default is  180
              seconds.

        rootflags
              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.

        loopfstype
              set the loop file system type, if applicable.

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

        BOOTIF
              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).

        resume
              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.

        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.

   Debug
        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.

        netconsole
              loads netconsole linux modules with the chosen args.

        all_generic_ide
              loads generic IDE/ATA chipset support on boot.

HOOK SCRIPTS

       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).

   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

              . /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 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.

       Example:
       TEMP_FILE=$(mktemp ...)
         ...
       prepend_earlyinitramfs ${TEMP_FILE}
       rm -f ${TEMP_FILE}

   Exported variables
       mkinitramfs sets several variables for the hook scripts environment.

        MODULESDIR
              corresponds to the linux modules dir.

        version
              is the $(uname -r) linux version against mkinitramfs is run.

        CONFDIR
              is the path of the used initramfs-tools configurations.

        DESTDIR
              is the root path of the newly build initramfs.

        DPKG_ARCH
              allows arch specific hook additions.

        verbose
              corresponds to the verbosity of the update-initramfs run.

        MODULES
              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.

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 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

              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.  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.

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

              . /usr/share/initramfs-tools/hook-functions
              # 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

              . /scripts/functions
              # 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

   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.

        ROOTDELAY, ROOTFLAGS, ROOTFSTYPE, IP
              corresponds to the rootdelay, rootflags, rootfstype or ip boot option.

        DPKG_ARCH
              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.

        REASON
              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.

        readonly
              is the default for mounting the root corresponds to the ro bootarg.  Overridden by rw bootarg.

        rootmnt
              is the path where root gets mounted usually /root.

        debug indicates that a debug log is captured for further investigation.

KERNEL HOOKS

       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.

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

FILES

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

       /run/initramfs/fsck-root
              Exists only if fsck ran successfully for the root filesystem.

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

AUTHOR

       The    initramfs-tools    are    written    by   Maximilian   Attems   <maks@debian.org>,   Jeff   Bailey
       <jbailey@raspberryginger.com> and numerous others.

       This  manual  was  written  by  David   Härdeman  <david@hardeman.nu>,  updated  by   Maximilian   Attems
       <maks@debian.org>.

SEE ALSO

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