INITRAMFS-TOOLS(8) mkinitramfs script overview INITRAMFS-TOOLS(8)NAMEinitramfs-tools - an introduction to writing scripts for mkinitramfs
DESCRIPTIONinitramfs-tools has one main script and two different sets of sub‐
scripts which will be used during different phases of execution. Each
of these will be discussed separately below with the help of an imagi‐
nary 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, numer‐
ics 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 parameters 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.
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.
loop path within the original root file system to loop-mount and use
as the real root file system.
loopflags
set the loop file system mount option string, if applicable.
loopfstype
set the loop file system type, 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:NFS‐
PATH:NFSOPTS. Use root=/dev/nfs for NFS to kick to in.
ip tells how to configure the ip address. Allows to specify an dif‐
ferent NFS server than the DHCP server. See Documentation/nfs‐
root.txt in any recent Linux source for details. Optional para‐
mater 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 "Subdirectories" section under boot scripts).
resume
On install initramfs-tools tries to autodetect the resume parti‐
tion. 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,mod‐
ule2,module3 bootparameter.
panic
sets an timeout on panic. panic=0 is a documented security fea‐
ture: 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 exam‐
ple "debug=vc".
break
spawns a shell in the initramfs image at chosen run-time (top,
modules, premount, mount, bottom, init). The default is pre‐
mount without any arg.
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 set‐
up in the script.
Header
In order to support prereqs, each script should begin with the follow‐
ing 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 func‐
tions 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/KERNELVER‐
SION/ 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 exe‐
cutables, 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"
add_mountroot_fail_hook
Registers the script as able to provide possible further infor‐
mation, 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 exe‐
cuted 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 usu‐
ally 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 usu‐
ally 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 mod‐
ules:
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 Härdeman <david@hardeman.nu>, updated
by Maximilian Attems <maks@debian.org>.
SEE ALSOinitramfs.conf(5), mkinitramfs(8), update-initramfs(8).
Date: 2007/07/07 INITRAMFS-TOOLS(8)
