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INITRAMFS-TOOLS(8) mkinitramfs script overview INITRAMFS-TOOLS(8)
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 parame-
ters 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. The recommended usage is to
specify the UUID as followed "root=UUID=xxx". As normal device names are not sta-
ble and may change depending on the boot order.
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.
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 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 "Subdirecto-
ries" 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 vari-
able 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 bootpa-
rameter.
panic sets an timeout on panic. panic=<sec> is a documented security feature: it dis-
ables 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, 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.
all_generic_ide
loads generic IDE/ATA chipset support on boot.
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 responsi-
ble 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
. /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 argu-
ments 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 uncondition-
ally 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"
Subdirectories
Both /usr/share/initramfs-tools/scripts and /etc/initramfs-tools/scripts contains the fol-
lowing 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 net-
work 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 $1 in
prereqs)
prereqs
exit 0
;;
esac
. /usr/share/initramfs-tools/hook-functions
# Begin real processing below this line
if [ ! -x "/sbin/frobnicate" ]; then
exit 0
fi
force_load frobnicator interval=10
cp /sbin/frobnicate "${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
# 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
AUTHOR
The initramfs-tools are written by Maximilian Attems <maks AT debian.org>, Jeff Bailey <jbai-
ley AT raspberryginger.com> and numerous others.
This manual was written by David Hardeman <david AT hardeman.nu>, updated by Maximilian
Attems <maks AT debian.org>.
SEE ALSO
initramfs.conf(5), mkinitramfs(8), update-initramfs(8).
2008/12/18 INITRAMFS-TOOLS(8)
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On Apache
Under GNU General Public License
2012-05-26 00:28 @38.107.179.238 Crawled by CCBot/1.0 (+http://www.commoncrawl.org/bot.html)