制作initrd(2):update-initramfs和mkinitramfs腳本分析

前一篇文章<制作initrd(1):向initrd内部更新驅動子產品>提到更新initrd.img鏡像時需要運作update-initramfs指令。起初以為是二進制檔案，網上胡亂搜尋一通發現update-initramfs和mkinitramfs兩個指令同為腳本檔案，既然是shell腳本那必須得分析内容并備忘。

root@ubuntu:~# file `which update-initramfs`
/usr/sbin/update-initramfs: POSIX shell script, ASCII text executable
root@ubuntu:~# file `which mkinitramfs`
/usr/sbin/mkinitramfs: POSIX shell script, ASCII text executable

總的來說，編譯核心的最後一步執行make install時會調用update-initramfs，update-initramfs繼而調用mkinitramfs生成initrd.img。是以，mkinitramfs是核心腳本，他的作用是啥？如果你有手工做過initrd.img的經曆，一定記得這是一個往臨時initrd目錄copy檔案的繁瑣過程，mkinitramfs則用腳本替代了手工操作(真偉大，還把人逼失業了！)：1).在臨時initrd目錄下建構FHS規定的檔案系統;2).按/etc/initramfs-tools/module和/etc/modules檔案的配置，往lib/modules/目錄拷貝子產品，同時生成子產品依賴檔案modules.dep，以後核心啟動後會從initramfs中(initrd.img被解壓到記憶體中)按子產品依賴關系modprobe子產品;3).拷貝/etc/initramfs-tools/scripts和/usr/share/initramfs-tools/scripts下的配置檔案到conf/目錄下,以後核心啟動，建立第一個程序init(initrd.img根目錄下init.sh檔案)會從conf/*讀取配置，按一定的順序加載子產品/執行程式;4).子產品的加載離不開modprobe工具集，是以需要拷貝modprobe工具集及其他工具到initrd目錄結構下，同時解決這些工具的依賴關系(依賴的so檔案的路徑);5).所有步驟完成，調用cpio和gzip工具打包壓縮臨時initrd目錄結構。

上面是mkinitramfs整體流程的概括，下面直接貼注釋後腳本，如注釋有誤請在留言欄告訴我，謝謝~

首先是update-initramfs腳本，僅注釋了執行update-initramfs -u更新initrd.img時的腳本執行流。腳本的開始定義了一些變量，然後跳去執行L400+之後的while getopts... do--按指令參數設定不同的運作模式。之後，按運作模式，進入不同的函數，以執行update-initramfs -u為例，進入update函數。

#!/bin/sh

STATEDIR=/var/lib/initramfs-tools
BOOTDIR=/boot
CONF=/etc/initramfs-tools/update-initramfs.conf
USETRIGGERS=true
mode=""
version=""
update_initramfs=yes
backup_initramfs=no

set -e
#CONF存在且可讀 則source ${CONF}中的設定 即source <span style="font-family: Arial, Helvetica, sans-serif;">/etc/initramfs-tools/update-initramfs.conf檔案裡的變量</span>

[ -r ${CONF} ] && . ${CONF}

case "$DPKG_MAINTSCRIPT_PACKAGE" in
linux-image-*)
  #INITRAMFS_TOOLS_KERNEL_HOOK 長度為0
  if [ -z "$INITRAMFS_TOOLS_KERNEL_HOOK" ]; then
    # kernel maintainer script called us directly; ignore
    # it and let the hook script handle it instead
    echo "update-initramfs: deferring update (hook will be called later)"
    exit 0
  fi
  ;;
?*)
  if     $USETRIGGERS           \ #USETRIGGERS=true
    && [ $# = 1 ]           \ #參數數量==1
    && [ x"$1" = x-u ]          \ #第一個參數是-u
    && dpkg-trigger --check-supported 2>/dev/null
  then
    if dpkg-trigger --no-await update-initramfs; then
      echo "update-initramfs: deferring update (trigger activated)"
      exit 0
    fi
  fi
  ;;
esac
#開始定義一些函數 先搜尋getopts，定位到那繼續
usage()
{
  if [ -n "${1:-}" ]; then
    printf "${*}\n\n" >&2
  fi
  cat >&2 << EOF
Usage: ${0} [OPTION]...

Options:
 -k [version] Specify kernel version or 'all'
 -c   Create a new initramfs
 -u   Update an existing initramfs
 -d   Remove an existing initramfs
 -t   Take over a custom initramfs with this one
 -b   Set alternate boot directory
 -v   Be verbose
 -h   This message

EOF
  exit 1
}

# chroot check
chrooted()
{
  # borrowed from udev's postinst
  if [ "$(stat -c %d/%i /)" = "$(stat -Lc %d/%i /proc/1/root 2>/dev/null)" ]; then
    # the devicenumber/inode pair of / is the same as that of
    # /sbin/init's root, so we're *not* in a chroot and hence
    # return false.
    return 1
  fi
return 0
}

mild_panic()
{
  if [ -n "${1:-}" ]; then
    printf "${*}\n" >&2
  fi
  exit 0
}

panic()
{
  if [ -n "${1:-}" ]; then
    printf "${*}\n" >&2
  fi
  exit 1
}

verbose()
{
  if [ "${verbose}" = 1 ]; then
    printf "${*}\n"
  fi
}

version_exists()
{
  [ -e "${STATEDIR}/${1}" ] && [ -e "${initramfs}" ]
  return $?
}

set_initramfs()
{
  initramfs="${BOOTDIR}/initrd.img-${version}"
}


# backup initramfs while running
backup_initramfs()
{
  [ ! -r "${initramfs}" ] && return 0
  initramfs_bak="${initramfs}.dpkg-bak"
  [ -r "${initramfs_bak}" ] && rm -f "${initramfs_bak}"
  ln -f "${initramfs}" "${initramfs_bak}" \
    || cp -a "${initramfs}" "${initramfs_bak}"
  verbose "Keeping ${initramfs_bak}"
}

# keep booted initramfs
backup_booted_initramfs()
{
  initramfs_bak="${initramfs}.dpkg-bak"

  # first time run thus no backup
  [ ! -r "${initramfs_bak}" ] && return 0

  # chroot with no /proc
  [ ! -r /proc/uptime ] && rm -f "${initramfs_bak}" && return 0

  # no kept backup wanted
  [ "${backup_initramfs}" = "no" ] && rm -f "${initramfs_bak}" && return 0

  # no backup yet
  if [ ! -r "${initramfs}.bak" ]; then
    mv -f ${initramfs_bak} "${initramfs}.bak"
    verbose "Backup ${initramfs}.bak"
    return 0
  fi

  # keep booted initramfs
  boot_initramfs=
  uptime_days=$(awk '{printf "%d", $1 / 3600 / 24}' /proc/uptime)
  if [ -n "$uptime_days" ]; then
    boot_initramfs=$(find "${initramfs}.bak" -mtime +${uptime_days})
  fi
  if [ -n "${boot_initramfs}" ]; then
    mv -f "${initramfs_bak}" "${initramfs}.bak"
    verbose "Backup ${initramfs}.bak"
    return 0
  fi
  verbose "Removing current backup ${initramfs_bak}"
  rm -f ${initramfs_bak}
}

# nuke generated copy
remove_initramfs_bak()
{
  [ -z "${initramfs_bak:-}" ] && return 0
  rm -f "${initramfs_bak}"
  verbose "Removing ${initramfs_bak}"
}


generate_initramfs()
{
  echo "update-initramfs: Generating ${initramfs}"
  OPTS="-o"
  if [ "${verbose}" = 1 ]; then
    OPTS="-v ${OPTS}"
  fi
  #if分支其實做這兩件事
  #mkinitramfs -o /boot/initrd.img-`uname -r`.new  ${version}
  #mv /boot/initrd.img-`uname -r`.new /boot/initrd.img-`uname -r`
  
  if mkinitramfs ${OPTS} "${initramfs}.new" "${version}"; then
    mv -f "${initramfs}.new" "${initramfs}"
    set_sha1
  else
    mkinitramfs_return="$?"
    remove_initramfs_bak
    rm -f "${initramfs}.new"
    if [ "$mkinitramfs_return" = "2" ]; then
      # minversion wasn't met, exit 0
      exit 0
    fi
    echo "update-initramfs: failed for ${initramfs} with $mkinitramfs_return." >&2
    exit $mkinitramfs_return
  fi
}

# lilo call
run_lilo()
{
  # show lilo errors on failure
  if ! lilo -t  > /dev/null 2>&1 ; then
    echo "ERROR lilo fails for new ${initramfs}:" >&2
    echo
    lilo -t
  fi
  lilo
}

# Invoke bootloader
run_bootloader()
{
  # invoke policy conformant bootloader hooks
  if [ -d /etc/initramfs/post-update.d/ ]; then
    run-parts --arg=${version} --arg=${initramfs} \
      /etc/initramfs/post-update.d/
    return 0
  fi
}

compare_sha1()
{
  sha1sum "${initramfs}" | diff "${STATEDIR}/${version}" - >/dev/null 2>&1
  return $?
}

# Note that this must overwrite so that updates work.
set_sha1()
{
  sha1sum "${initramfs}" > "${STATEDIR}/${version}"
}

delete_sha1()
{
  rm -f "${STATEDIR}/${version}"
}

# ro /boot is not modified
ro_boot_check()
{
  # check irrelevant inside of a chroot
  if [ ! -r /proc/mounts ] || chrooted; then
    return 0
  fi

  boot_opts=$(awk '/boot/{if ((match($4, /^ro/) || match($4, /,ro/)) \
    && $2 == "/boot") print "ro"}' /proc/mounts)
  if [ -n "${boot_opts}" ]; then
    echo "WARNING: /boot is ro mounted."
    echo "update-initramfs: Not updating ${initramfs}"
    exit 0
  fi
}

get_sorted_versions()
{
  version_list=""

  for gsv_x in "${STATEDIR}"/*; do
    gsv_x="$(basename "${gsv_x}")"
    if [ "${gsv_x}" = '*' ]; then
      return 0
    fi
    worklist=""
    for gsv_i in $version_list; do
      if dpkg --compare-versions "${gsv_x}" '>' "${gsv_i}"; then
        worklist="${worklist} ${gsv_x} ${gsv_i}"
        gsv_x=""
      else
        worklist="${worklist} ${gsv_i}"
      fi
    done
    if [ "${gsv_x}" != "" ]; then
      worklist="${worklist} ${gsv_x}"
    fi
    version_list="${worklist}"
  done

  verbose "Available versions: ${version_list}"
}

set_current_version()
{
  if [ -f /boot/initrd.img-`uname -r` ]; then
    version=`uname -r`
  fi
}

set_linked_version()
{
  linktarget=
  if [ -e /initrd.img ] && [ -L /initrd.img ]; then
    linktarget="$(basename "$(readlink /initrd.img)")"
  fi

  if [ -e /boot/initrd.img ] && [ -L /boot/initrd.img ]; then
    linktarget="$(basename "$(readlink /boot/initrd.img)")"
  fi

  if [ -z "${linktarget}" ]; then
    return
  fi

  version="${linktarget##initrd.img-}"
}

set_highest_version()
{
  get_sorted_versions
  if [ -z "${version_list}" ]; then
    version=
    return
  fi
  set -- ${version_list}
  version=${1}
}

create()
{
  if [ -z "${version}" ]; then
    usage "Create mode requires a version argument"
  fi

  set_initramfs

  if [ "${takeover}" = 0 ]; then
    if version_exists "${version}"; then
      panic "Cannot create version ${version}: already exists"
    fi

    if [ -e "${initramfs}" ]; then
      panic "${initramfs} already exists, cannot create."
    fi
  fi

  generate_initramfs
}

update()
{
  #update_initramfs定義在/etc/initramfs-tools/update_initramfs.conf檔案中
  #在本檔案開始處被包含進來，值為update_initramfs=yes
  if [ "${update_initramfs}" = "no" ]; then
    echo "update-initramfs: Not updating initramfs."
    exit 0
  fi

  if [ -z "${version}" ]; then
    set_highest_version
  fi

  if [ -z "${version}" ]; then
    set_linked_version
  fi

  if [ -z "${version}" ]; then
    #等效version=`uname -r`
    set_current_version
  fi

  if [ -z "${version}" ]; then
    verbose "Nothing to do, exiting."
    exit 0
  fi
  #initramfs="/boot/initrd.img-`uname -r`"
  set_initramfs

  ro_boot_check

  altered_check

  backup_initramfs
  #準備調用mkinitramfs,進入函數generate_initramfs
  generate_initramfs
  #如果系統中存在/etc/initramfs/post-update.d/，就調用函數run_bootloader,ubuntu12.04并不存在這個目錄
  run_bootloader

  backup_booted_initramfs
}

delete()
{
  if [ -z "${version}" ]; then
    usage "Delete mode requires a version argument"
  fi

  set_initramfs

  if [ "${takeover}" = 0 ]; then
    if [ ! -e "${initramfs}" ]; then
      panic "Cannot delete ${initramfs}, doesn't exist."
    fi

    if ! version_exists "${version}"; then
      panic "Cannot delete version ${version}: Not created by this utility."
    fi
  fi

  altered_check

  echo "update-initramfs: Deleting ${initramfs}"

  delete_sha1

  rm -f "${initramfs}" "${initramfs}.bak"
}

# Check for update mode on existing and modified initramfs
altered_check()
{
  # No check on takeover
  [ "${takeover}" = 1 ] && return 0
  if [ ! -e "${initramfs}" ]; then
    mild_panic "${initramfs} does not exist. Cannot update."
  fi
  if ! compare_sha1; then
    echo "update-initramfs: ${initramfs} has been altered." >&2
    mild_panic "update-initramfs: Cannot update. Override with -t option."
  fi
}

# Defaults
verbose=0
yes=0
# We default to takeover=1 in Ubuntu, but not Debian
takeover=1

##
#可選參數-k和-b後面帶其他參數
#參數存放到flag中，供case判斷
while getopts "k:cudyvtb:h?" flag; do
  case "${flag}" in
  k)
    #-k 後面有值，則儲存在OPTARG(-k kernel version)
    version="${OPTARG}"
    ;;
  c)
    mode="c"
    ;;
  d)
    mode="d"
    ;;
  u)
    mode="u"
    ;;
  v)
    verbose="1"
    ;;
  y)
    yes="1"
    ;;
  t)
    takeover="1"
    ;;
  b)
    BOOTDIR="${OPTARG}"
    if [ ! -d "${BOOTDIR}" ]; then
      echo "Error: ${BOOTDIR} is not a directory." >&2
      exit 1
    fi
    ;;
  h|?)
    usage
    ;;
  esac
done
#常見的擷取參數的最後一項的方法
shift $((${OPTIND} - 1))

if [ $# -ne 0 ]; then
  echo "Invalid argument for option -k." >&2
  usage
fi

# Validate arguments
#${mode}為空
if [ -z "${mode}" ]; then
  usage "You must specify at least one of -c, -u, or -d."
fi

#如果輸入-k all 或者/etc/initramfs-tools/update-initramfs中的update_initramfs的值為all
if [ "${version}" = "all" ] \
  || ( [ "${update_initramfs}" = "all" ] && [ -z "${version}" ] ); then
  : FIXME check for --yes, and if not ask are you sure
  get_sorted_versions
  if [ -z "${version_list}" ]; then
    verbose "Nothing to do, exiting."
    exit 0
  fi

  OPTS="-b ${BOOTDIR}"
  if [ "${verbose}" = "1" ]; then
    OPTS="${OPTS} -v"
  fi
  if [ "${takeover}" = "1" ]; then
    OPTS="${OPTS} -t"
  fi
  if [ "${yes}" = "1" ]; then
    OPTS="${OPTS} -y"
  fi
  for u_version in ${version_list}; do
    verbose "Execute: ${0} -${mode} -k \"${u_version}\" ${OPTS}"
    "${0}" -${mode} -k "${u_version}" ${OPTS}
  done
  exit 0
fi

#隻讨論update-initramfs -u的情況
case "${mode}" in
  c)
    create
    ;;
  d)
    delete
    ;;
  u)
    #-u 則調用update函數
    update
    ;;
esac

如果你耐心夠好，看到這，差不多也該跟着執行流進入到mkinitramfs腳本。update函數的結尾依次執行generate_initramfs函數和mkinitramfs指令。就此進入mkinitramfs腳本。mkinitramfs腳本開始處也是定義一些變量，然後include兩個重要的輔助腳本/usr/share/initramfs-tools/scripts/functions和/usr/share/initramfs-tools/hook-functions。一些重要的函數定義在這兩個腳本中。mkinitramfs在這兩個腳本的輔助下完成了前述臨時initrd目錄的制作

#!/bin/sh

umask 0022
export PATH='/usr/bin:/sbin:/bin'

# Defaults
keep="n"
CONFDIR="/etc/initramfs-tools"
verbose="n"
#如果/bin/busybox檔案夾存在 變量BUSYBOXDIR=/bin
test -e /bin/busybox && BUSYBOXDIR=/bin
test -e /usr/lib/initramfs-tools/bin/busybox && BUSYBOXDIR=/usr/lib/initramfs-tools/bin
export BUSYBOXDIR

OPTIONS=`getopt -o c:d:ko:r:v -n "$0" -- "$@"`

# Check for non-GNU getopt
if [ $? != 0 ] ; then echo "W: non-GNU getopt" >&2 ; exit 1 ; fi

eval set -- "$OPTIONS"
#update-initramfs調用mkinitramfs時指令為
#mkinitramfs -o /boot/initrd.img-`uname -r`.new ${version},是以進入-o分支
while true; do
  case "$1" in
  -c)
    compress="$2"
    shift 2
    ;;
  -d)
    CONFDIR="$2"
    shift 2
    if [ ! -d "${CONFDIR}" ]; then
      echo "${0}: ${CONFDIR}: Not a directory" >&2
      exit 1
    fi
    ;;
  -o)
    #outfile=/boot/initrd.img-`uname -r`.new
    outfile="$2"
    shift 2
    ;;
  -k)
    keep="y"
    shift
    ;;
  -r)
    ROOT="$2"
    shift 2
    ;;
  -v)
    verbose="y"
    shift
    ;;
  --)
    shift
    break
    ;;
  *)
    echo "Internal error!" >&2
    exit 1
    ;;
  esac
done

# For dependency ordered mkinitramfs hook scripts.
#引用這兩處的檔案 這是一些幫助函數
. /usr/share/initramfs-tools/scripts/functions
. /usr/share/initramfs-tools/hook-functions

#引用/etc/initramfs-tools/initramfs.conf 這個檔案有關于本地啟動或者nfs啟動的資訊
. "${CONFDIR}/initramfs.conf"
EXTRA_CONF=''
#/usr/share/initramfs-tools/conf.d/目錄下為空
#/etc/initramfs-tools/initramfs.conf/conf.d/ 目錄下隻有resume檔案
for i in /usr/share/initramfs-tools/conf.d/* ${CONFDIR}/conf.d/*; do
  [ -e $i ] && EXTRA_CONF="${EXTRA_CONF} $(basename $i \
    | grep '^[[:alnum:]][[:alnum:]\._-]*$' | grep -v '\.dpkg-.*$')";
done
# FIXME: deprecated those settings on mkinitramfs run
#    these conf dirs are for boot scripts and land on initramfs
for i in ${EXTRA_CONF}; do
  if [ -e  ${CONFDIR}/conf.d/${i} ]; then
    . ${CONFDIR}/conf.d/${i}
  elif [ -e  /usr/share/initramfs-tools/conf.d/${i} ]; then
    . /usr/share/initramfs-tools/conf.d/${i}
  fi
done

# source package confs
for i in /usr/share/initramfs-tools/conf-hooks.d/*; do
  if [ -e "${i}" ]; then
    . "${i}"
  fi
done
#UMASK 非空 不過沒找到何處設定了這個變量
if [ -n "${UMASK:-}" ]; then
  umask "${UMASK}"
fi
#outfile為空 也不成立
if [ -z "${outfile}" ]; then
  usage
fi

#生成新的空的initrd.img-`uname -r`檔案
touch "$outfile"
outfile="$(readlink -f "$outfile")"

# And by "version" we really mean path to kernel modules
# This is braindead, and exists to preserve the interface with mkinitrd
if [ ${#} -ne 1 ]; then
  version="$(uname -r)"
else
  version="${1}"
fi

# Check that we're using a new enough kernel version, first for ourselves,
# then for each of the hooks, which can have a MINKVER variable defined
check_minkver ${version}
check_minkver ${version} /usr/share/initramfs-tools/hooks
check_minkver ${version} ${CONFDIR}/hooks

case "${version}" in
#/lib/modules/*/目錄下除了/以外的檔案，這是指除了該目錄下除了子目錄以外的所有檔案?
/lib/modules/*/[!/]*)
  ;;
/lib/modules/[!/]*)
  version="${version#/lib/modules/}"
  version="${version%%/*}"
  ;;
esac

case "${version}" in
*/*)
  echo "$PROG: ${version} is not a valid kernel version" >&2
  exit 1
  ;;
esac

# Check userspace and kernel support for compressed initramfs images
if [ -z "${compress:-}" ]; then
  #COMPRESS=gzip gzip壓縮
  compress=${COMPRESS}
else
  COMPRESS=${compress}
fi

if ! command -v "${compress}" >/dev/null 2>&1; then
  compress=gzip
  [ "${verbose}" = y ] && \
    echo "No ${COMPRESS} in ${PATH}, using gzip"
  COMPRESS=gzip
fi

#/boot/config-${version}比對以config_rd_${COMPRESS%p}開頭
if ! `grep -q -i ^config_rd_${COMPRESS%p} /boot/config-${version}` ; then
  compress=gzip
  [ "${verbose}" = y ] && \
    echo "linux-2.6 misses ${COMPRESS} support, using gzip"
fi

[ "${compress}" = lzop ] && compress="lzop -9"
[ "${compress}" = xz ] && compress="xz -8 --check=crc32"

if [ -d "${outfile}" ]; then
  echo "${outfile} is a directory" >&2
  exit 1
fi
#要編譯核心的版本
MODULESDIR="/lib/modules/${version}"

if [ ! -e "${MODULESDIR}" ]; then
  echo "WARNING: missing ${MODULESDIR}"
  echo "Device driver support needs thus be built-in linux image!"
fi
#檢測子產品依賴性 不存在子產品依賴檔案 怎生成
if [ ! -e "${MODULESDIR}/modules.dep" ]; then
  depmod ${version}
fi
#在/tmp下生成名為mkinitramfs_fb9BN這樣的檔案
DESTDIR="$(mktemp -d ${TMPDIR:-/tmp}/mkinitramfs_XXXXXX)" || exit 1
chmod 755 "${DESTDIR}"

# do not execute cache_run_scripts() if mounted with noexec
NOEXEC=""
#找到臨時檔案的挂載點 fs=/
fs=$(df -P $DESTDIR | tail -1 | awk '{print $6}')
if [ -n "$fs" ] && mount | grep -q "on $fs .*noexec" ; then
  NOEXEC=1
fi

__TMPCPIOGZ="$(mktemp ${TMPDIR:-/tmp}/mkinitramfs-OL_XXXXXX)" || exit 1

DPKG_ARCH=`dpkg --print-architecture`

# Export environment for hook scripts.
#
export MODULESDIR
export version
export CONFDIR
export DESTDIR
export DPKG_ARCH
export verbose
export MODULES
export BUSYBOX
export COMPCACHE_SIZE

# Private, used by 'catenate_cpiogz'.
export __TMPCPIOGZ

#在/tmp/mkinitramfs_fb9BNB檔案夾下生成bin conf/conf.d etc lib/modules run sbin scripts /lib/modules/${version}這些檔案夾
#這是在制作根檔案系統
for d in bin conf/conf.d etc lib/modules run sbin scripts ${MODULESDIR}; do
  mkdir -p "${DESTDIR}/${d}"
done

# Copy the modules.order file in
if [ -f "${MODULESDIR}/modules.order" ]; then
  cp -p "${MODULESDIR}/modules.order" \
    "${DESTDIR}${MODULESDIR}/modules.order"
fi

# MODULES=list case.  Always honour.
for x in "${CONFDIR}/modules" /usr/share/initramfs-tools/modules.d/*; do
  if [ -f "${x}" ]; then
    #add_modules_from_file定義在/usr/share/initramfs-tools/hook-functions中
    #這裡就是add_modules_from_file /etc/initramfs-tools/modules
    #/etc/initramfs-tools/modules中含有要添加到initrd鏡像中的modules
    add_modules_from_file "${x}"
  fi
done

# 沒有找到MODULES定義的檔案 按注釋MODULES=most
# MODULES=most is default
case "${MODULES}" in
dep)
  dep_add_modules
  ;;
most)
  #auto_add_modules仍然定義在/usr/share/initramfs-tools/hook-functions中
  auto_add_modules
  ;;
netboot)
  auto_add_modules base
  auto_add_modules net
  ;;
list)
  # nothing to add
  ;;
*)
  echo "W: mkinitramfs: unsupported MODULES setting: ${MODULES}."
  echo "W: mkinitramfs: Falling back to MODULES=most."
  auto_add_modules
  ;;
esac

# Resolve hidden dependencies
hidden_dep_add_modules

# First file executed by linux-2.6
#拷貝init檔案，init是系統啟動後進入根檔案系統之後啟動的第一個程序
cp -p /usr/share/initramfs-tools/init ${DESTDIR}/init

# add existant boot scripts
#遞歸搜尋/usr/share/initramfs-tools/scripts/目錄下所有配置檔案(init-top/all_generic_ide udev...)，如果檔案中不含有"OPTION="字段，
#則在${DESTDIR}/scripts/目錄中建立對應目錄，并把檔案拷貝到該目錄中
for b in $(cd /usr/share/initramfs-tools/scripts/ && find . \
  -regextype posix-extended -regex '.*/[[:alnum:]\._-]+$' -type f); do
  option=$(sed '/^OPTION=/!d;$d;s/^OPTION=//;s/[[:space:]]*$//' "/usr/share/initramfs-tools/scripts/${b}")
  [ -z "${option}" ] || eval test -n \"\${$option}\" -a \"\${$option}\" != \"n\" || continue

  [ -d "${DESTDIR}/scripts/$(dirname "${b}")" ] \
    || mkdir -p "${DESTDIR}/scripts/$(dirname "${b}")"
  cp -p "/usr/share/initramfs-tools/scripts/${b}" \
    "${DESTDIR}/scripts/$(dirname "${b}")/"
done
#對于/etc/initramfs-tools/scripts下的檔案使用同樣的操作
for b in $(cd "${CONFDIR}/scripts" && find . \
  -regextype posix-extended -regex '.*/[[:alnum:]\._-]+$' -type f); do
  option=$(sed '/^OPTION=/!d;$d;s/^OPTION=//;s/[[:space:]]*$//' "${CONFDIR}/scripts/${b}")
  [ -z "${option}" ] || eval test -n \"\${$option}\" -a \"\${$option}\" != \"n\" || continue

  [ -d "${DESTDIR}/scripts/$(dirname "${b}")" ] \
    || mkdir -p "${DESTDIR}/scripts/$(dirname "${b}")"
  cp -p "${CONFDIR}/scripts/${b}" "${DESTDIR}/scripts/$(dirname "${b}")/"
done

echo "DPKG_ARCH=${DPKG_ARCH}" > ${DESTDIR}/conf/arch.conf
#把/etc/initramfs-tools/initramfs.conf拷貝到${DESTDIR}/conf目錄下，
#這個檔案涉及本地啟動和nfs啟動,以及加載的驅動MODULES=most
cp -p "${CONFDIR}/initramfs.conf" ${DESTDIR}/conf
for i in ${EXTRA_CONF}; do
  if [ -e "${CONFDIR}/conf.d/${i}" ]; then
    copy_exec "${CONFDIR}/conf.d/${i}" /conf/conf.d
  elif [ -e "/usr/share/initramfs-tools/conf.d/${i}" ]; then
    copy_exec "/usr/share/initramfs-tools/conf.d/${i}" /conf/conf.d
  fi
done

# ROOT hardcoding
if [ -n "${ROOT:-}" ]; then
  echo "ROOT=${ROOT}" > ${DESTDIR}/conf/conf.d/root
fi

if ! command -v ldd >/dev/null 2>&1 ; then
  echo "WARNING: no ldd around - install libc-bin" >&2
  exit 1
fi
#copy_exec定義在hook-functions中
copy_exec /usr/lib/initramfs-tools/bin/wait-for-root /sbin

# module-init-tools
copy_exec /sbin/modprobe /sbin
copy_exec /sbin/rmmod /sbin
mkdir -p "${DESTDIR}/etc/modprobe.d"
#拷貝/etc/modprobe.d/下檔案到${DESTDIR}/etc/modprobe.d/下
#這些檔案關系到modprobe mod時的行為
cp -a /etc/modprobe.d/* "${DESTDIR}/etc/modprobe.d/"

# util-linux
copy_exec /sbin/blkid /sbin

# workaround: libgcc always needed on old-abi arm
if [ "$DPKG_ARCH" = arm ] || [ "$DPKG_ARCH" = armeb ]; then
  cp -a /lib/libgcc_s.so.1 "${DESTDIR}/lib/"
fi

run_scripts /usr/share/initramfs-tools/hooks optional
run_scripts "${CONFDIR}"/hooks optional

# cache boot run order
if [ -n "$NOEXEC" ]; then
  echo "W: TMPDIR is mounted noexec, will not cache run scripts."
else
  for b in $(cd "${DESTDIR}/scripts" && find . -mindepth 1 -type d); do
    cache_run_scripts "${DESTDIR}" "/scripts/${b#./}"
  done
fi

# generate module deps
#生成子產品依賴檔案
depmod -a -b "${DESTDIR}" ${version}
rm -f "${DESTDIR}/lib/modules/${version}"/modules.*map

# make sure that library search path is up to date
#拷貝/etc/ld.so.conf*到initrd目錄結構中對應位置，可執行程式可能依賴非标準路徑下的so檔案，
#是以在此調用ldconfig -r分析/${DESTDIR}/etc/ld.so.conf*中列出的非标準搜尋路徑，并生成
#緩存檔案加速連結速度
#ldconfig -r ROOT : 此選項改變應用程式的根目錄為ROOT(是調用chroot函數實作的).選擇此項時,系統預設的配置檔案
#/etc/ld.so.conf,實際對應的為 ROOT/etc/ld.so.conf.如用-r /usr/zzz時,打開配置檔案 /etc/ld.so.conf時,實際打開的
#是/usr/zzz/etc/ld.so.conf檔案.用此選項,可以大大增加動态連結庫管理的靈活性
cp -ar /etc/ld.so.conf* "$DESTDIR"/etc/
if ! ldconfig -r "$DESTDIR" ; then
  [ $(id -u) != "0" ] \
  && echo "ldconfig might need uid=0 (root) for chroot()" >&2
fi

# Apply DSDT to initramfs
if [ -e "${CONFDIR}/DSDT.aml" ]; then
  copy_exec "${CONFDIR}/DSDT.aml" /
fi

# Remove any looping or broken symbolic links, since they break cpio.
[ "${verbose}" = y ] && xargs_verbose="-t"
(cd "${DESTDIR}" && find . -type l -printf '%p %Y\n' | sed -n 's/ [LN]$//p' \
  | xargs ${xargs_verbose:-} -rL1 rm -f)

# dirty hack for armhf's double-linker situation; if we have one of
# the two known eglibc linkers, nuke both and re-create sanity
if [ "$DPKG_ARCH" = armhf ]; then
  if [ -e "${DESTDIR}/lib/arm-linux-gnueabihf/ld-linux.so.3" ] || \
     [ -e "${DESTDIR}/lib/ld-linux-armhf.so.3" ]; then
    rm -f "${DESTDIR}/lib/arm-linux-gnueabihf/ld-linux.so.3"
    rm -f "${DESTDIR}/lib/ld-linux-armhf.so.3"
    cp -aL /lib/ld-linux-armhf.so.3 "${DESTDIR}/lib/"
    ln -sf /lib/ld-linux-armhf.so.3 "${DESTDIR}/lib/arm-linux-gnueabihf/ld-linux.so.3"
  fi
fi

[ "${verbose}" = y ] && echo "Building cpio ${outfile} initramfs"
(
# work around lack of "set -o pipefail" for the following pipe:
# cd "${DESTDIR}" && find . | cpio --quiet -R 0:0 -o -H newc | gzip >"${outfile}" || exit 1
#這是建立initramfs的最後一步了 調用cpio和gzip 打包并壓縮initramfs目錄
exec 3>&1
eval `
  # http://cfaj.freeshell.org/shell/cus-faq-2.html
  exec 4>&1 >&3 3>&-
  cd  "${DESTDIR}"
  {
    find . 4>&-; echo "ec1=$?;" >&4
  } | {
    cpio --quiet -R 0:0 -o -H newc 4>&-; echo "ec2=$?;" >&4
  } | ${compress} >"${outfile}"
  echo "ec3=$?;" >&4
`
if [ "$ec1" -ne 0 ]; then
  echo "E: mkinitramfs failure find $ec1 cpio $ec2 $compress $ec3"
  exit "$ec1"
fi
if [ "$ec2" -ne 0 ]; then
  echo "E: mkinitramfs failure cpio $ec2 $compress $ec3"
  exit "$ec2"
fi
if [ "$ec3" -ne 0 ]; then
  echo "E: mkinitramfs failure $compress $ec3"
  exit "$ec3"
fi
) || exit 1

if [ -s "${__TMPCPIOGZ}" ]; then
  cat "${__TMPCPIOGZ}" >>"${outfile}" || exit 1
fi

if [ "${keep}" = "y" ]; then
  echo "Working files in ${DESTDIR} and overlay in ${__TMPCPIOGZ}"
else
  rm -rf "${DESTDIR}"
  rm -rf "${__TMPCPIOGZ}"
fi

exit 0

最後貼出hook-functions中用到的幾個函數的注釋

force_load()
{
    manual_add_modules ${@}
    #調用mkinitramfs時設定DESTDIR=/tmp/mkinitramfs_fb9BNB---一個臨時根檔案系統結構，
    #把/etc/initramfs-tools/modules中讀到的modules加入到/tmp/mkinitramfs_fb9BNB/conf/modules中
    #前面已經說過update-initramfs -u時會将/etc/initramfs-tools/modules檔案中的子產品加入到initrd
    #檔案,作為引導加載modules，看來就是在這實作的
    echo "${@}" >>"${DESTDIR}/conf/modules"
}

add_modules_from_file()
{
# Sanity check
if [ ! -e "${1}" ]; then
echo "W: add_modules_from_file: arg1='${1}' does not exist." >&2
return
fi
#從/etc/initramfs-tools/modules中讀取非注釋的行，子產品名和參數分别讀入module和args
#并調用force_load函數
grep '^[^#]' ${1} | while read module args; do
[ -n "$module" ] || continue
force_load "${module}" "${args}"
done
}

# $1 = file to copy to ramdisk
# $2 (optional) Name for the file on the ramdisk
# Location of the image dir is assumed to be $DESTDIR
# We never overwrite the target if it exists.
#拷貝可執行程式到$DESTDIR對應目錄下，同時解決可執行程式的依賴庫(ldd)
copy_exec() {
local src target x nonoptlib
local libname dirname


src="${1}"
#如果$2(可執行程式所在的目錄)為空 則傳回$1的值 
target="${2:-$1}"
#可執行程式不存在就return，否則往下執行
[ -f "${src}" ] || return 1
if [ -d "${DESTDIR}/${target}" ]; then
# check if already copied
#目錄存在并且exe存在則傳回
[ -e "${DESTDIR}/$target/${src##*/}" ] && return 0
else
#目錄不存在 則建立目錄
[ -e "${DESTDIR}/$target" ] && return 0
#FIXME: inst_dir
mkdir -p "${DESTDIR}/${target%/*}"
fi
#拷貝host系統的exe檔案到initrd目錄結構中
[ "${verbose}" = "y" ] && echo "Adding binary ${src}"
cp -pL "${src}" "${DESTDIR}/${target}"
# Copy the dependant libraries
#ldd的輸出形式為 xxx ==> yyy
#for循環中的第一個sed是提取第3列中的yyy
for x in $(ldd ${src} 2>/dev/null | sed -e '
/\//!d;/linux-gate/d;/ {s/.*=>[[:blank:]]*\([^[:blank:]]*\).*/\1/};s/[[:blank:]]*\([^[:blank:]]*\) (.*)/\1/' 2>/dev/null); do


# Try to use non-optimised libraries where possible.
# We assume that all HWCAP libraries will be in tls,
# sse2, vfp or neon.
nonoptlib=$(echo "${x}" | sed -e 's#/lib/\(tls\|i686\|sse2\|neon\|vfp\).*/\(lib.*\)#/lib/\2#')

if [ -e "${nonoptlib}" ]; then
x="${nonoptlib}"
fi


libname=$(basename "${x}")
dirname=$(dirname "${x}")
# FIXME inst_lib
#從host機上拷貝依賴庫到initrd目錄結構中
mkdir -p "${DESTDIR}/${dirname}"
if [ ! -e "${DESTDIR}/${dirname}/${libname}" ]; then
cp -pL "${x}" "${DESTDIR}/${dirname}"
[ "${verbose}" = "y" ] && echo "Adding library ${x}" || true
fi
done
}

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