INITRAMFS-TOOLS(7) Linux Programmer's Manual INITRAMFS-TOOLS(7)
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 la-
bel, 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.
initramfs.clear
clear screen at the beginning
initramfs.runsize
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
https://www.kernel.org/doc/Documentation/filesystems/tmpfs.txt
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".
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.
nfsroot
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.
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,mod-
ule2,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 ar-
bitrary 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 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 "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, 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 im-
age. 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 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.
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 com-
ponents 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 or-
dered 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 lo-
cal script (/etc/initramfs-tools) get executed before one from the pack-
age (/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.
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 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/mod-
ules/KERNELVERSION/ into the initramfs image.
Example: copy_modules_dir kernel/drivers/ata
Including binaries
If you need to copy an executable or shared library to the initramfs
module, use a command like this:
copy_exec /sbin/mdadm /sbin
mkinitramfs will automatically detect which libraries it 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 $(uname -r) linux version against mkinitramfs is run.
CONFDIR
is the path of the used initramfs-tools configurations.
DESTDIR
is the root path of the newly build initramfs.
DPKG_ARCH
allows arch specific hook additions.
verbose
corresponds to the verbosity of the update-initramfs run.
BUSYBOX, KEYMAP, MODULES
are as described in initramfs.conf(5).
BUSYBOXDIR
is the directory where busybox utilities should be installed
from, or empty if busybox is not being used.
BOOT SCRIPTS
Similarly to hook scripts, boot scripts can be found in two places
/usr/share/initramfs-tools/scripts/ and /etc/initramfs-tools/scripts/.
There are a number of subdirectories to these two directories which con-
trol the boot stage at which the scripts are executed.
Header
Like for hook scripts, there are no guarantees as to the order in which
the different scripts in one subdirectory (see "Subdirectories" below)
are executed. In order to define a certain order, a similar header as
for hook scripts should be used:
#!/bin/sh
PREREQ=""
prereqs()
{
echo "$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"
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 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 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 $1 in
prereqs)
prereqs
exit 0
;;
esac
. /usr/share/initramfs-tools/hook-functions
# Begin real processing below this line
if [ ! -x "/sbin/frobnicate" ]; then
exit 0
fi
force_load frobnicator interval=10
copy_exec /sbin/frobnicate /sbin
exit 0
Boot script
An example boot script would look something like this (and would usually
be placed in /etc/initramfs-tools/scripts/local-top/frobnicate):
#!/bin/sh
# Example frobnication boot script
PREREQ="lvm"
prereqs()
{
echo "$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.
UPDATING THE INITRAMFS FROM ANOTHER PACKAGE
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
KERNEL HOOKS
initramfs-tools includes hook scripts that are called by kernel packages
on installation and removal, so that an initramfs is automatically cre-
ated, 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 mod-
ules:
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).
initramfs-tools 2018/07/18 INITRAMFS-TOOLS(7)
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