实现

解析选项参数

# parse options
while [ -n "$1" ]; do
    case "$1" in
        -i) export INTERACTIVE=1;;
        -d) export DELAY="$2"; shift;;
        -v) export VERBOSE="$(($VERBOSE + 1))";;
        -q) export VERBOSE="$(($VERBOSE - 1))";;
        -n) export SAVE_CONFIG=0;;
        -c) export SAVE_OVERLAY=1;;
        -b|--create-backup) export CONF_BACKUP="$2" NEED_IMAGE=1; shift;;
        -r|--restore-backup) export CONF_RESTORE="$2" NEED_IMAGE=1; shift;;
        -l|--list-backup) export CONF_BACKUP_LIST=1; break;;
        -f) export CONF_IMAGE="$2"; shift;;
        -F|--force) export FORCE=1;;
        -T|--test) export TEST=1;;
        -h|--help) export HELP=1; break;;
        -*)
            echo "Invalid option: $1"
            exit 1
        ;;
        *) break;;
    esac
    shift;
done
 
export CONFFILES=/tmp/sysupgrade.conffiles
export CONF_TAR=/tmp/sysupgrade.tgz
 
export ARGV="$*"
export ARGC="$#"

解析选项。ARGC 是参数个数。
选项中有 - d, -b, -r, -f 时，由于这些选项都要带一个参数，所以使用了 shift 去减少 ARGC 的值。

CONFFILES 和 CONF_TAR 是两个临时文件，后面会用到。

sysupgrade openwrt.bin --> ARGV="openwrt.bin", ARGC=1
sysupgrade -b config.backup --> ARGV 为空，ARGC=0

判断参数合法

[ -n "$ARGV" -a -n "$NEED_IMAGE" ] && {
    cat <<-EOF
        -b|--create-backup and -r|--restore-backup do not perform a firmware upgrade.
        Do not specify both -b|-r and a firmware image.
    EOF
    exit 1
}

如果 sysupgrade 附带参数 - b 或 - r 时，则$NEED_IMAGE=1，否则为空
当$NEED_IMAGE=1时，我们希望 ARGV 是空的，否则就是出错，则输出帮助信息，并退出。
例如:

• sysupgrade -b config.backup，此时$NEED_IMAGE=1, ARGV 为空，合法
• sysupgrade -b config.backup openwrt.bin，此时$NEED_IMAGE=1, ARGV 为 "openwrt.bin"，参数多了，错误。

# hooks
sysupgrade_image_check="platform_check_image"
[ $SAVE_OVERLAY = 0 -o ! -d /overlay/etc ] && \
    sysupgrade_init_conffiles="add_uci_conffiles" || \
    sysupgrade_init_conffiles="add_overlayfiles"

带 - c 参数，且 "/overlay/etc" 目录存在 --> sysupgrade_init_conffiles="add_overlayfiles"
否则 --> sysupgrade_init_conffiles="add_uci_conffiles"

这里会影响要备份的配置文件

include /lib/upgrade

包含 lib/upgrade 目录下的所有文件

[ "$1" = "nand" ] && nand_upgrade_stage2 $@

命令指定 nand 时，则调用 nand_upgrade_stage2 函数，例如sysupgrade nand openwrt.bin。 暂时使用 spi flash，不讨论这里。

backup list

add_uci_conffiles() {
    local file="$1"
    ( find $(sed -ne '/^[[:space:]]*$/d; /^#/d; p' \
        /etc/sysupgrade.conf /lib/upgrade/keep.d/* 2>/dev/null) \
        -type f 2>/dev/null;
      opkg list-changed-conffiles ) | sort -u > "$file"
    return 0
}
 
if [ $CONF_BACKUP_LIST -eq 1 ]; then
    add_uci_conffiles "$CONFFILES"
    cat "$CONFFILES"
    rm -f "$CONFFILES"
    exit 0
fi

列出一份文件列表，放入 / tmp/sysupgrade.conffiles，打印出来，然后删掉。文件列表：

find $(sed -ne '/^[[:space:]]*$/d; /^#/d; p' /etc/sysupgrade.conf /lib/upgrade/keep.d/* 2>/dev/null) -type f 2>/dev/null
 
opkg list-changed-conffiles

create backup

do_save_conffiles() {
    local conf_tar="${1:-$CONF_TAR}"
 
    [ -z "$(rootfs_type)" ] && {
        echo "Cannot save config while running from ramdisk."
        ask_bool 0 "Abort" && exit
        return 0
    }
    run_hooks "$CONFFILES" $sysupgrade_init_conffiles
    ask_bool 0 "Edit config file list" && vi "$CONFFILES"
 
    v "Saving config files..."
    [ "$VERBOSE" -gt 1 ] && TAR_V="v" || TAR_V=""
    tar c${TAR_V}zf "$conf_tar" -T "$CONFFILES" 2>/dev/null
 
    rm -f "$CONFFILES"
}
 
if [ -n "$CONF_BACKUP" ]; then
    do_save_conffiles "$CONF_BACKUP"
    exit $?
fi

-b 如果指定打包文件时，$CONF_BACKUP 为那个文件名。则此时按如下流程来生成备份文件：

do_save_conffiles
  -> sysupgrade_init_conffiles
     -> add_uci_conffiles
        -> tar czf /tmp/backup.tgz -T /tmp/sysupgrade.conffiles
    -> rm -f /tmp/sysupgrade.conffiles

restore

if [ -n "$CONF_RESTORE" ]; then
    if [ "$CONF_RESTORE" != "-" ] && [ ! -f "$CONF_RESTORE" ]; then
        echo "Backup archive '$CONF_RESTORE' not found."
        exit 1
    fi
 
    [ "$VERBOSE" -gt 1 ] && TAR_V="v" || TAR_V=""
    tar -C / -x${TAR_V}zf "$CONF_RESTORE"
    exit $?
fi

sysupgrade -r config.tgz --> tar -C / -xzf config.tgz
解压，覆盖到 / 目录下

image check

for check in $sysupgrade_image_check; do
    ( eval "$check \"\$ARGV\"" ) || {
        if [ $FORCE -eq 1 ]; then
            echo "Image check '$check' failed but --force given - will update anyway!"
            break
        else
            echo "Image check '$check' failed."
            exit 1
        fi
    }
done

sysupgrade openwrt.bin --> 检查 bin 文件：platform_check_image openwrt.bin
lib/upgrade/platform.sh 中定义 platform_check_image 函数，取. bin 文件的头部，检查 magic number

upgrade backup

if [ -n "$CONF_IMAGE" ]; then
    case "$(get_magic_word $CONF_IMAGE cat)" in
        # .gz files
        1f8b) ;;
        *)
            echo "Invalid config file. Please use only .tar.gz files"
            exit 1
        ;;
    esac
    get_image "$CONF_IMAGE" "cat" > "$CONF_TAR"
    export SAVE_CONFIG=1
elif ask_bool $SAVE_CONFIG "Keep config files over reflash"; then
    [ $TEST -eq 1 ] || do_save_conffiles
    export SAVE_CONFIG=1
else
    export SAVE_CONFIG=0
fi

在升级时会先保存配置文件到 / tmp/sysupgrade.tgz

1. -f 指定配置文件
2. 交互模式 ask_bool 可以获取输入值，如果不在交互模式，则 ask_bool 的第一个参数就是默认值。
   $SAVE_CONFIG 默认为 1, 这里调用 do_save_conffiles 保存当前系统的配置文件。

upgrade

if [ -n "$(rootfs_type)" ]; then
    v "Switching to ramdisk..."
    run_ramfs '. /lib/functions.sh; include /lib/upgrade; do_upgrade'
else
    do_upgrade
fi

1. rootfs_type = "overlayfs" ，执行第一个逻辑
2. run_ramfs, 在 / tmp/root 下安装一个临时 ramdisk，最后再执行 do_upgrade
3. do_upgrade -> platform_do_upgrade -> get_image "$1" | mtd -j "$CONF_TAR" write - "firmware"

mtd 工具在写入时，会把 $CONF_TAR 文件整合进入 jffs2 分区，可以看到打印信息：

Appending jffs2 data from /tmp/sysupgrade.tgz to firmware...