Ubuntu Manpage: initramfs-tools - an introduction to writing scripts for mkinitramfs

Provided by: initramfs-tools-core_0.130ubuntu3.13_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.

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.

SCRIPTS

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

CONFIGURATION HOOK SCRIPTS

       Configuration  hook scripts can be found in /usr/share/initramfs-tools/conf-hooks.d.  They are sourced by
       mkinitramfs after the configuration 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 package's boot script requires commands not provided by klibc-utils, it
       should also install a configuration hook that sets BUSYBOX=y.

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. 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 separate from the /etc/initramfs-tools directory. It must not read
initramfs-tools configuration files directly.

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.

BUSYBOX, MODULES
are as described in initramfs.conf(5).

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

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. Use of ROOTDELAY is
deprecated; you should implement a local-block boot script rather than delaying or polling.

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>.

NAME

DESCRIPTION

Kernel Command Line

SCRIPTS

CONFIGURATION HOOK SCRIPTS

HOOK SCRIPTS

BOOT SCRIPTS

EXAMPLES

KERNEL HOOKS

DEBUG

FILES

AUTHOR

SEE ALSO