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 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
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.
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
/dev/.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.
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.
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.
Exported variables
mkinitramfs sets several variables for the hook scripts environment.
MODULESDIR
corresponds to the linux-2.6 modules dir.
version
is the $(uname -r) linux-2.6 version against mkinitramfs is run.
CONFDIR
is the path of the used initramfs-tools configurations.
DESTDIR
is the root path of the newly build initramfs.
DPKG_ARCH
allows arch specific hook additions.
verbose
corresponds to the verbosity of the update-initramfs run.
MODULES
specifies which kind of modules should land on initramfs. This
setting shouldn't be overridden by hook script, but can guide
them on how much they need to include on initramfs.
BOOT SCRIPTS
Similarly to hook scripts, boot scripts can be found in two places
/usr/share/initramfs-tools/scripts/ and /etc/initramfs-tools/scripts/.
There are a number of subdirectories to these two directories which
control the boot stage at which the scripts are executed.
Header
Like for hook scripts, there are no guarantees as to the order in which
the different scripts in one subdirectory (see "Subdirectories" below)
are executed. In order to define a certain order, a similar header as
for hook scripts should be used:
#!/bin/sh
PREREQ=""
prereqs()
{
echo "$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 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 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. 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
. /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.
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@debian.org>,
Jeff Bailey <jbailey@raspberryginger.com> and numerous others.
This manual was written by David Hardeman <david@hardeman.nu>, updated
by Maximilian Attems <maks@debian.org>.
SEE ALSO
initramfs.conf(5), mkinitramfs(8), update-initramfs(8).