The work that the system does when the Linux system's operating level changes

#  actually the shutdown of the computer is the switch of the running level, the # init process monitors whether the runlevel changes. #  If the runlevel changes, the INIT process will trigger the  /etc/rc.d/rc  script to run. # rc  script function is: #    1, if the current computer is running there, at the current run level #    (correctrunlevel )/ The service that starts with K in the etc/rc.d/rc$runlevel.d/directory. is closed. #     2, turn on the current runlevel (correct runlevel )/etc/rc.d/rc$runlevel.d/directory #     Services that start with S # This enables the control of services that are allowed to start at different operating levels, and that prohibit startup. The # Service script that starts with s in the correct runlevel /etc/rc.d/rc$runlevel.d/directory with a:half  will shut down the computer #如果当前运行级别. #[[email protected] rc.d]# ll rc0.d/s*lrwxrwxrwx 1 root root 17  Apr  5 00:13 rc0.d/s00killall -&gt, .... /init.d/killalllrwxrwxrwx 1 root root 14 apr  5 00:13 rc0.d/ S01HALT&NBSP;-&GT;&NBSP, .... /init.d/halt#  executes the kill script before executing the halt script. #  actually the system administrator issued the "shutdown" command, that is, the notification  init  process, the operational level changed. [[Email protected] ~]# man shutdown.....shutdown does its job by signalling the init process, asking it to change the  runlevel.......#  Process init configuration file/etc/inittab  define level switch action [[Email protected] ~]# vim  /etc/inittabl0:0:wait:/etc/rc.d/rc 0  #  performs L1:1:WAIT:/ETC/RC.D/RC at boot time or when changing the operating level  1l2:2:wait:/etc/rc.d/rc 2l3:3:wait:/etc/rc.d/rc 3l4:4:wait:/etc/rc.d/rc 4l5:5:wait:/etc/ rc.d/rc 5l6:6:wait:/etc/rc.d/rc 6#  These rows indicate that the RC program (/ETC/RC.D/RC script) is run once every time the system's RunLevel is changed,#  Also, Init should pass a single-character argument that contains a run-level number (0~~6) to the RC program. #  will monitor changes in the operating level of the host, such as the change in operational level, will invoke the:/etc/rc.d/rc  script #  if the current host is running at: [[email protected] bin]#  runlevelN 3#  function Command init  issue shutdown program signal using: [[email protected] bin]# init  The 0  # init process discovers that a change in the run level will follow the  l0:0:wait:/etc/rc.d/rc # 0 in profile Inittab.   [[email protected] ~]# vim /etc/rc.d/rc#! /bin/bash## rc   &nBsp;         this file is responsible for  starting/stopping#                services when the runlevel changes.## Original Author:#                miquel van smoorenburg,  <[email protected]> #set  -m############################################################## ##### #该函数是检查当前运行级别的脚本条件:#                      1, the script must have permissions to run;#                      2, not part of a function is_ignored_file  The type specified in # check a file to be a correct runlevel scriptcheck_ runlevel  () {        # check if the file exists at all.         [ -x  "$  ] |" |  return 1        is_ignored_file  "$"  &&  return 1 #  calls the function to check the file type.         return 0}# now find out what the  current and what the previous runlevel are.argv1= "$"    # Record: When the runlevel changes, the INIT process triggers the arguments passed when the RC script is run. set  '/sbin/runlevel '   #  records the running level of the current host runlevel=$2previous=$1export runlevel  Previous. /etc/init.d/functions# see if we want to be in user  confirmation modeif [  "$previous"  =  "N"  ]; then         if [ -f /var/run/confirm ]; then                echo $ "Entering  Interactive startup "        else                 echo $ "Entering non-interactive  startup "        fifi# Get first argument.  Set new runlevel to this argument. [ -n  "$ARGV 1"  ] && runlevel= "$ARGV 1"     #如果进程init监控到运行级别改变时, The parameter,   is passed when the RC script is triggered #就把该参数当作当前主机的运行级别 # is there an rc directory for  this new runlevel?     #判断运行级别在 The directory under the/etc/directory, and if not, exit directly.     #这也可判断传递的运行参数是否正确 [ -d /etc/rc$runlevel.d ] | |  exit 0#################################################################### #给/etc/rc.d/rc$ Script delivery at the beginning of K in the RUNLEVEL.D directory: Stop  To enable the shutdown of this runlevel does not start the service # first, run the kill scripts.           for i in /etc/rc$runlevel.d/K* ; do         check_runlevel  "$i"  | |  continue          #调用函数check_runlevel检查脚本, if the startup script for a service goes down,          #否则退出当前循环执行下一次循环 ####################### #如果   $i   is a service startup script that judges the script (a script starting with K) #在运行级别切换之前是否已经启动. #因为通过脚本启动服务后 (such as:service httpd start  or  httpd start), #都会在/var/lock/subsys/ Directory to create a file named after the startup script # It is also by judging the/var/lock/subsys/directory that there is no file corresponding to the service script, #来确定该服务是否启动的.         # Check if the subsystem is  Already up.        subsys=${i#/etc/rc$runlevel.d/k??}         [ -f /var/lock/subsys/$subsys &NBSP;-O&NBsp;-f /var/lock/subsys/$subsys .init ]                  | |  continue        # bring the subsystem down.           #关闭当前运行级别设置不启动的服务 (Service,     started before run-level switchover      #但是该服务在当前运行级别是设置不启动的),         if lc_all=c  egrep -q  "^.. *init.d/functions "  $i  ; then                  $i  stop                       else                 action $ "stopping  $subsys: "  $ i stop        fidone###############  start the current run level setting start the service ################################################################### ### now run the start scripts.for i in /etc/rc$runlevel.d/s* ;  do        check_runlevel  "$i"  | |  continue        # check if the subsystem  is already up.        subsys=${i#/etc/rc$runlevel.d/s??}         [ -f /var/lock/subsys/$subsys  -o -f / var/lock/subsys/$subsys .init ]                  && continue                   #如果运行级别切换之前, the service has been started and exited directly.                  #防止一个服务启动两次. Directory/var/lock/subsys/The role of the lock file.         # if we ' Re in confirmation mode,  get user confirmation        if [ -f / var/run/confirm ]; then                 confirm  $subsys                  test $? = 1 && continue         fi           #如果当前运行级别下 (/etc/rc.d/rc$ RUNLEVEL.D) Half script is the start of S            #就会运行该脚本, the script is to turn off the computer,         update_boot_stage  "$subsys"          # Bring the subsystem up.        if [  "$subsys"  =  "Halt"  -o  "$subsys"  =  " Reboot " ]; then                 export LC_ALL=C                       exec  $i  start         fi        if LC_ALL=C egrep -q  " ^.. *init.d/functions "  $i                           | |  [  "$subsys"  =  "single"  -o  "$subsys"  =  "local"  ]; then                  $i  start         else                action $ "Starting   $subsys:  "  $i  start        fidonerm -f  /var/run/confirmif [ -x /usr/bin/rhgb-client ] && /usr/bin/rhgb-client  --ping ; then  /usr/bin/rhgb-client --quitfi

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