INITRAMFS上的init脚本

INITRAMFS是Linux系统启动中的一个关键环节，这一步如果失败，那么就看不见桌面，出现黑屏故障。
格友公众号的文章《抽丝十万米：苦寻黑屏背后的元凶》介绍了一个案例，下面是对应的脚本文件。

`#!/bin/sh

# Default PATH differs between shells, and is not automatically exported
# by klibc dash.  Make it consistent.
# Furthermore, this PATH ends up being used by the init, set it to the
# Standard PATH, without /snap/bin as documented in
# https://wiki.ubuntu.com/PATH
# This also matches /etc/environment, but without games path
export PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin

[ -d /dev ] || mkdir -m 0755 /dev
[ -d /root ] || mkdir -m 0700 /root
[ -d /sys ] || mkdir /sys
[ -d /proc ] || mkdir /proc
[ -d /tmp ] || mkdir /tmp
mkdir -p /var/lock
mount -t sysfs -o nodev,noexec,nosuid sysfs /sys
mount -t proc -o nodev,noexec,nosuid proc /proc

# shellcheck disable=SC2013
for x in $(cat /proc/cmdline); do
        case $x in
        initramfs.clear)
                clear
                ;;
        quiet)
                quiet=y
                ;;
        esac
done

if [ "$quiet" != "y" ]; then
        quiet=n
        echo "Loading, please wait..."
fi
export quiet

# Note that this only becomes /dev on the real filesystem if udev's scripts
# are used; which they will be, but it's worth pointing out
mount -t devtmpfs -o nosuid,mode=0755 udev /dev

# Prepare the /dev directory
[ ! -h /dev/fd ] && ln -s /proc/self/fd /dev/fd
[ ! -h /dev/stdin ] && ln -s /proc/self/fd/0 /dev/stdin
[ ! -h /dev/stdout ] && ln -s /proc/self/fd/1 /dev/stdout
[ ! -h /dev/stderr ] && ln -s /proc/self/fd/2 /dev/stderr

mkdir /dev/pts
mount -t devpts -o noexec,nosuid,gid=5,mode=0620 devpts /dev/pts || true

# Export the dpkg architecture
export DPKG_ARCH=
. /conf/arch.conf

# Set modprobe env
export MODPROBE_OPTIONS="-qb"

# Export relevant variables
export ROOT=
export ROOTDELAY=
export ROOTFLAGS=
export ROOTFSTYPE=
export IP=
export IP6=
export VLAN=
export DEVICE=
export BOOT=
export BOOTIF=
export UBIMTD=
export NETWORK_SKIP_ENSLAVED=
export break=
export init=/sbin/init
export readonly=y
export rootmnt=/root
export debug=
export panic=
export blacklist=
export resume=
export resume_offset=
export noresume=
export drop_caps=
export fastboot=n
export forcefsck=n
export fsckfix=


# mdadm needs hostname to be set. This has to be done before the udev rules are called!
if [ -f "/etc/hostname" ]; then
        /bin/hostname -F /etc/hostname >/dev/null 2>&1
fi

# Bring in the main config
. /conf/initramfs.conf
for conf in conf/conf.d/*; do
        [ -f "${conf}" ] && . "${conf}"
done
. /scripts/functions

# Parse command line options
# shellcheck disable=SC2013
for x in $(cat /proc/cmdline); do
        case $x in
        init=*)
                init=${x#init=}
                ;;
        root=*)
                ROOT=${x#root=}
                if [ -z "${BOOT}" ] && [ "$ROOT" = "/dev/nfs" ]; then
                        BOOT=nfs
                fi
                ;;
        rootflags=*)
                ROOTFLAGS="-o ${x#rootflags=}"
                ;;
        rootfstype=*)
                ROOTFSTYPE="${x#rootfstype=}"
                ;;
        rootdelay=*)
                ROOTDELAY="${x#rootdelay=}"
                case ${ROOTDELAY} in
                *[![:digit:].]*)
                        ROOTDELAY=
                        ;;
                esac
                ;;
        roottimeout=*)
                ROOTDELAY="${x#roottimeout=}"
                case ${ROOTDELAY} in
                *[![:digit:].]*)
                        ROOTDELAY=
                        ;;
                esac
                ;;
        loop=*)
                # shellcheck disable=SC2034
                LOOP="${x#loop=}"
                ;;
        loopflags=*)
                # shellcheck disable=SC2034
                LOOPFLAGS="-o ${x#loopflags=}"
                ;;
        loopfstype=*)
                # shellcheck disable=SC2034
                LOOPFSTYPE="${x#loopfstype=}"
                ;;
        nfsroot=*)
                # shellcheck disable=SC2034
                NFSROOT="${x#nfsroot=}"
                ;;
        initramfs.runsize=*)
                RUNSIZE="${x#initramfs.runsize=}"
                ;;
        ip=*)
                IP="${x#ip=}"
                ;;
        ip6=*)
                IP6="${x#ip6=}"
                ;;
        vlan=*)
                VLAN="${x#vlan=}"
                ;;
        boot=*)
                BOOT=${x#boot=}
                ;;
        ubi.mtd=*)
                UBIMTD=${x#ubi.mtd=}
                ;;
        resume=*)
                RESUME="${x#resume=}"
                case $RESUME in
                UUID=*)
                        RESUME="/dev/disk/by-uuid/${RESUME#UUID=}"
                esac
                ;;
        resume_offset=*)
                resume_offset="${x#resume_offset=}"
                ;;
        noresume)
                noresume=y
                ;;
        drop_capabilities=*)
                drop_caps="-d ${x#drop_capabilities=}"
                ;;
        panic=*)
                panic="${x#panic=}"
                ;;
        ro)
                readonly=y
                ;;
        rw)
                readonly=n
                ;;
        debug)
                debug=y
                quiet=n
                if [ -n "${netconsole}" ]; then
                        log_output=/dev/kmsg
                else
                        log_output=/run/initramfs/initramfs.debug
                fi
                set -x
                ;;
        debug=*)
                debug=y
                quiet=n
                set -x
                ;;
        break=*)
                break=${x#break=}
                ;;
        break)
                break=premount
                ;;
        blacklist=*)
                blacklist=${x#blacklist=}
                ;;
        netconsole=*)
                netconsole=${x#netconsole=}
                [ "$debug" = "y" ] && log_output=/dev/kmsg
                ;;
        BOOTIF=*)
                BOOTIF=${x#BOOTIF=}
                ;;
        hwaddr=*)
                BOOTIF=${x#hwaddr=}
                ;;
        fastboot|fsck.mode=skip)
                fastboot=y
                ;;
        forcefsck|fsck.mode=force)
                forcefsck=y
                ;;
        fsckfix|fsck.repair=yes)
                fsckfix=y
                ;;
        fsck.repair=no)
                fsckfix=n
                ;;
        esac
done

# Default to BOOT=local if no boot script defined.
if [ -z "${BOOT}" ]; then
        BOOT=local
fi

if [ -n "${noresume}" ] || [ "$RESUME" = none ]; then
        noresume=y
else
        resume=${RESUME:-}
fi

mount -t tmpfs -o "nodev,noexec,nosuid,size=${RUNSIZE:-10%},mode=0755" tmpfs /run
mkdir -m 0700 /run/initramfs

if [ -n "$log_output" ]; then
        exec >"$log_output" 2>&1
        unset log_output
fi

maybe_break top

# Don't do log messages here to avoid confusing graphical boots
run_scripts /scripts/init-top

maybe_break modules
[ "$quiet" != "y" ] && log_begin_msg "Loading essential drivers"
[ -n "${netconsole}" ] && /sbin/modprobe netconsole netconsole="${netconsole}"
load_modules
[ "$quiet" != "y" ] && log_end_msg

starttime="$(_uptime)"
starttime=$((starttime + 1)) # round up
export starttime

maybe_break premount
[ "$quiet" != "y" ] && log_begin_msg "Running /scripts/init-premount"
run_scripts /scripts/init-premount
[ "$quiet" != "y" ] && log_end_msg

maybe_break mount
log_begin_msg "Mounting root file system"
# Always load local and nfs (since these might be needed for /etc or
# /usr, irrespective of the boot script used to mount the rootfs).
. /scripts/local
. /scripts/nfs
. "/scripts/${BOOT}"
parse_numeric "${ROOT}"
maybe_break mountroot
mount_top
mount_premount
mountroot
log_end_msg

if read_fstab_entry /usr; then
        log_begin_msg "Mounting /usr file system"
        mountfs /usr
        log_end_msg
fi

# Mount cleanup
mount_bottom
nfs_bottom
local_bottom

case "$IP" in
""|none|off)
        case "$IP6" in
        ""|none|off) ;; # Do nothing
        *)
                configure_networking
        esac
        ;;
*)
        configure_networking
esac

maybe_break bottom
[ "$quiet" != "y" ] && log_begin_msg "Running /scripts/init-bottom"
# We expect udev's init-bottom script to move /dev to ${rootmnt}/dev
run_scripts /scripts/init-bottom
[ "$quiet" != "y" ] && log_end_msg

# Move /run to the root
mount -n -o move /run ${rootmnt}/run

validate_init() {
        run-init -n "${rootmnt}" "${1}"
}

# Check init is really there
if ! validate_init "$init"; then
        echo "Target filesystem doesn't have requested ${init}."
        init=
        for inittest in /sbin/init /etc/init /bin/init /bin/sh; do
                if validate_init "${inittest}"; then
                        init="$inittest"
                        break
                fi
        done
fi

# No init on rootmount
if ! validate_init "${init}" ; then
        panic "No init found. Try passing init= bootarg."
fi

maybe_break init

# don't leak too much of env - some init(8) don't clear it
# (keep init, rootmnt, drop_caps)
unset debug
unset MODPROBE_OPTIONS
unset DPKG_ARCH
unset ROOTFLAGS
unset ROOTFSTYPE
unset ROOTDELAY
unset ROOT
unset IP
unset IP6
unset VLAN
unset BOOT
unset BOOTIF
unset DEVICE
unset UBIMTD
unset blacklist
unset break
unset noresume
unset panic
unset quiet
unset readonly
unset resume
unset resume_offset
unset noresume
unset fastboot
unset forcefsck
unset fsckfix
unset starttime

# Move virtual filesystems over to the real filesystem
mount -n -o move /sys ${rootmnt}/sys
mount -n -o move /proc ${rootmnt}/proc

# Chain to real filesystem
# shellcheck disable=SC2086,SC2094
exec run-init ${drop_caps} "${rootmnt}" "${init}" "$@" <"${rootmnt}/dev/console" >"${rootmnt}/dev/console" 2>&1
echo "Something went badly wrong in the initramfs."
panic "Please file a bug on initramfs-tools."