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 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:NFSPATH:NFSOPTS. Use root=/dev/nfs for NFS to kick to in.

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 "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.

panic
    sets an timeout on panic. panic=0 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, bottom, init). The default is premount 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 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.

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 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
    # 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@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).