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参考链接

1. https://wiki.debian.org/Initrd

2. https://blog.csdn.net/zhaominyong/article/details/119546892

initrd.img的作用

1. 用于在启动时临时挂载根文件系统

2. 加载关键驱动

3. 切换到真正的根文件系统

initrd.img所处的位置

ls -lh /boot/initrd.img-*

可以看到有多个initrd.img文件

查看当前内核版本确定使用的initrd.img文件

uname -r

解压initrd.img

1. 查看压缩方式

file /boot/initrd.img-$(uname -r)

• gzip compressed data → 压缩的CPIO归档

• ASCII cpio archive → 未压缩的CPIO归档。

• Linux rev 1.0 ext2 filesystem data → 旧式RAM磁盘映像

压缩方式为未压缩的cpio归档

2. 解压

cd /boot
sudo mkdir temp_initrd
cd temp_initrd
sudo cpio -idmv < /initrd.img-$(uname -r)

此时你会发现这并不是实际的initrd.img文件:(

因为Debian系统使用了另一套方式（initramfs-tools）来管理initrd.img文件

sudo unmkinitramfs /boot/initrd.img-$(uname -r) /boot/temp_initrd

使用这条命令才能正确地解压

之后打开文件夹，目录如下

/tmp/initrd/
├── main/      # 主 initramfs 内容（包含 init 脚本和核心文件）
│   ├── bin
│   ├── dev
│   ├── etc
│   ├── init    # 初始化脚本
│   ├── lib
│   └── ...
└── early/     # 早期加载的微码文件（如 CPU 微码更新）└── kernel/x86/microcode/AuthenticAMD.bin

查看initrd.img内部的执行命令

initrd.img内部执行操作由init脚本决定

使用cat命令打开init脚本后可以看到内部操作

cat /boot/temp_initrd/main/init

#!/bin/sh# Default PATH differs between shells, and is not automatically exported
# by klibc dash.  Make it consistent.
export PATH=/sbin:/usr/sbin:/bin:/usr/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); docase $x ininitramfs.clear)clear;;quiet)quiet=y;;esac
doneif [ "$quiet" != "y" ]; thenquiet=necho "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/stderrmkdir /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 DEVICE=
export BOOT=
export BOOTIF=
export UBIMTD=
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=# 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); docase $x ininit=*)init=${x#init=};;root=*)ROOT=${x#root=}if [ -z "${BOOT}" ] && [ "$ROOT" = "/dev/nfs" ]; thenBOOT=nfsfi;;rootflags=*)ROOTFLAGS="-o ${x#rootflags=}";;rootfstype=*)ROOTFSTYPE="${x#rootfstype=}";;rootdelay=*)ROOTDELAY="${x#rootdelay=}"case ${ROOTDELAY} in*[![:digit:].]*)ROOTDELAY=;;esac;;nfsroot=*)# shellcheck disable=SC2034NFSROOT="${x#nfsroot=}";;initramfs.runsize=*)RUNSIZE="${x#initramfs.runsize=}";;ip=*)IP="${x#ip=}";;boot=*)BOOT=${x#boot=};;ubi.mtd=*)UBIMTD=${x#ubi.mtd=};;resume=*)RESUME="${x#resume=}";;resume_offset=*)resume_offset="${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=yquiet=nif [ -n "${netconsole}" ]; thenlog_output=/dev/kmsgelselog_output=/run/initramfs/initramfs.debugfiset -x;;debug=*)debug=yquiet=nset -x;;break=*)break=${x#break=};;break)break=premount;;blacklist=*)blacklist=${x#blacklist=};;netconsole=*)netconsole=${x#netconsole=}[ "$debug" = "y" ] && log_output=/dev/kmsg;;BOOTIF=*)BOOTIF=${x#BOOTIF=};;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 "${BOOT}" ]; thenBOOT=local
fiif [ -n "${noresume}" ] || [ "$RESUME" = none ]; thennoresume=y
elseresume=${RESUME:-}
fimount -t tmpfs -o "nodev,noexec,nosuid,size=${RUNSIZE:-10%},mode=0755" tmpfs /run
mkdir -m 0700 /run/initramfsif [ -n "$log_output" ]; thenexec >"$log_output" 2>&1unset log_output
fimaybe_break top# Don't do log messages here to avoid confusing graphical boots
run_scripts /scripts/init-topmaybe_break modules
[ "$quiet" != "y" ] && log_begin_msg "Loading essential drivers"
[ -n "${netconsole}" ] && /sbin/modprobe netconsole netconsole="${netconsole}"
load_modules
[ "$quiet" != "y" ] && log_end_msgstarttime="$(_uptime)"
starttime=$((starttime + 1)) # round up
export starttimeif [ "$ROOTDELAY" ]; thensleep "$ROOTDELAY"
fimaybe_break premount
[ "$quiet" != "y" ] && log_begin_msg "Running /scripts/init-premount"
run_scripts /scripts/init-premount
[ "$quiet" != "y" ] && log_end_msgmaybe_break mount
log_begin_msg "Mounting root file system"
# Always load local and nfs (since these might be needed for /etc or
# /usr, irrespective of the boot script used to mount the rootfs).
. /scripts/local
. /scripts/nfs
. "/scripts/${BOOT}"
parse_numeric "${ROOT}"
maybe_break mountroot
mount_top
mount_premount
mountroot
log_end_msgif read_fstab_entry /usr; thenlog_begin_msg "Mounting /usr file system"mountfs /usrlog_end_msg
fi# Mount cleanup
mount_bottom
nfs_bottom
local_bottommaybe_break bottom
[ "$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 ${rootmnt}/runvalidate_init() {run-init -n "${rootmnt}" "${1}"
}# Check init is really there
if ! validate_init "$init"; thenecho "Target filesystem doesn't have requested ${init}."init=for inittest in /sbin/init /etc/init /bin/init /bin/sh; doif validate_init "${inittest}"; theninit="$inittest"breakfidone
fi# No init on rootmount
if ! validate_init "${init}" ; thenpanic "No init found. Try passing init= bootarg."
fimaybe_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 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."