电脑开机后，init.d脚本如何启动系统服务揭秘

在Linux系统中，init.d（初始化目录）是一个非常重要的目录，它包含了启动和停止系统服务的脚本。当你打开电脑，操作系统会按照一定的顺序启动一系列服务，以确保系统的正常运行。下面，我们就来揭秘init.d脚本是如何启动系统服务的。

初始化进程（init进程）

在Linux系统中，init进程是系统启动的第一个进程，其进程ID（PID）为1。init进程负责初始化系统，并启动其他系统服务。在早期的Linux系统中，init进程通常是一个名为init的shell脚本。

init.d目录结构

init.d目录通常位于/etc目录下，里面包含了各种系统服务的启动脚本。这些脚本通常以服务名称命名，并遵循一定的命名规则。例如，httpd服务对应的启动脚本可能命名为/etc/init.d/httpd。

脚本结构

init.d脚本通常包含以下几个部分：

• 描述信息：脚本顶部通常包含一些描述信息，如脚本作者、版本、描述等。
• 功能定义：定义脚本执行的主要功能，如启动、停止、重启、状态查询等。
• 配置变量：设置一些变量，如服务名称、日志文件路径等。
• 实际功能实现：根据功能定义，实现具体的启动、停止、重启、状态查询等功能。

以下是一个简单的init.d脚本示例：

#!/bin/sh
#
# httpd - Apache HTTP Server
#

# Source function library.
. /etc/init.d/functions

# Define your variables here
HTTPD=/usr/sbin/httpd
HTTPD_CGIBIN=/usr/lib/cgi-bin
HTTPD_PIDFILE=/var/run/httpd.pid
DESC="Apache HTTP Server"

# Start/Stop/Restart/Status functions
start() {
    echo -n $"Starting $DESC: "
    $HTTPD &
    echo $"done."
}

stop() {
    echo -n $"Stopping $DESC: "
    killproc $HTTPD
    echo $"done."
}

restart() {
    stop
    start
}

status() {
    echo -n $"Checking $DESC status: "
    status_of_proc $HTTPD $DESC && echo $"running" || echo $"stopped"
}

case "$1" in
  start)
    start
    ;;
  stop)
    stop
    ;;
  restart)
    restart
    ;;
  status)
    status
    ;;
  *)
    echo $"Usage: $0 {start|stop|restart|status}"
    exit 1
esac

init.d脚本启动过程

当系统启动时，init进程会读取/etc/inittab文件，该文件定义了init进程要启动的脚本。在早期的Linux系统中，/etc/inittab文件使用表格形式定义了各种运行级别（runlevel）和对应的脚本。

以下是一个/etc/inittab文件示例：

”` ::sysinit:/etc/init.d/hwclock.sh ::sysinit:/etc/init.d/local_fs ::sysinit:/etc/init.d/swap ::sysinit:/etc/init.d/networking ::sysinit:/etc/init.d/securelevel ::sysinit:/etc/init.d/auditd ::sysinit:/etc/init.d/urandom ::sysinit:/etc/init.d/hostname.sh ::sysinit:/etc/init.d/ypbind ::sysinit:/etc/init.d/ntpd ::sysinit:/etc/init.d/iscsi ::sysinit:/etc/init.d/iscsid ::sysinit:/etc/init.d/firstboot ::sysinit:/etc/init.d/NetworkManager ::sysinit:/etc/init.d/NetworkManager-dispatcher ::sysinit:/etc/init.d/NetworkManager-system-ctls ::sysinit:/etc/init.d/NetworkManager-wait-online ::sysinit:/etc/init.d/dbus ::sysinit:/etc/init.d/xdg-user-sessions ::sysinit:/etc/init.d/xdg-display-manager ::sysinit:/etc/init.d/xdg-screensaver ::sysinit:/etc/init.d/systemd-firstboot ::sysinit:/etc/init.d/plymouth ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-boot-progress ::sysinit:/etc/init.d/plymouth-quit-wait ::sysinit:/etc/init.d/plymouth-quit-early ::sysinit:/etc/init.d/plymouth-start ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit:/etc/init.d/plymouth-display-manager ::sysinit