linux system init

linux system init (by hanlray(at)gmail.com)

unix類的系統通常都有多個runlevel，一個runlevel是一組服務的運行狀態的配置：在該runlevel下哪些服務應該運行，哪些服務應該停止。一個服務的啟動、停止等動作是由一個init script(通常包括在該服務程式的package中)控制的，LSB對init script做了標準化，一個標準的init script應該支援如下action：

startstart the service stopstop the service restartstop and restart the service if the service is already running, otherwise start the service try-restartrestart the service if the service is already running reloadcause the configuration of the service to be reloaded without actually stopping and restarting the service force-reloadcause the configuration to be reloaded if the service supports this, otherwise restart the service if it is running statusprint the current status of the service 

每個runlevel都對應一個目錄，裡面是一些到這些init script的symbol link，形如SNNxxx，KNNxxx，NN是一個序號，當改變到某個runlevel時(包括系統啟動進入一個runlevel時)，對該runlevel目錄下的symbol links，先按序號順序執行K打頭的，執行的action為stop，再按序號順序執行S打頭的，執行的action為start。

 

手工管理各個runlevel是一件費力的事，LSB可以簡化這一工作，其原理是在init script裡寫入該服務的meta資訊：它依賴的服務、在哪些runlevel下啟動等，然後由工具利用這些資訊自動設定各個runlevel。下面是為xvnc寫的一個init script(/etc/init.d/vncserver):

#! /bin/sh # # Author: Willem van der Mark <wvdmark@computer.org>  # # /etc/init.d/xvncserverthis Script # /usr/sbin/rcvncserverRoot-Link to this Script # /usr/X11R6/bin/vncserverProgram # # System startup script for vnc server # # LSB compatible service control script; see http://www.linuxbase.org/spec/ # # Note: This template uses functions rc_XXX defined in /etc/rc.status on # UnitedLinux (UL) based Linux distributions. If you want to base your # script on this template and ensure that it works on non UL based LSB # compliant Linux distributions, you either have to provide the rc.status # functions from UL or change the script to work without them. # # ### BEGIN INIT INFO # Provides:       xvncserver # Required-Start: $remote_fs $syslog $network xdm # X-UnitedLinux-Should-Start: $network xdm # Required-Stop:  $remote_fs $syslog $network xdm # X-UnitedLinux-Should-Stop: $network xdm # Default-Start:  3 5 # Default-Stop:   0 1 2 6 # Description:    Start vncserver for remote control # ### END INIT INFO   # Check for missing binaries VNC_WRAPPER=/usr/X11R6/bin/vncserver test -x $VNC_WRAPPER || exit 5 VNC_MASTER=/usr/X11R6/bin/Xvnc test -x $VNC_MASTER || exit 5 VNC_CONFIG=/etc/sysconfig/vncservers test -r $VNC_CONFIG || exit 5  # Shell functions sourced from /etc/rc.status: #      rc_check         check and set local and overall rc status #      rc_status        check and set local and overall rc status #      rc_status -v     ditto but be verbose in local rc status #      rc_status -v -r  ditto and clear the local rc status #      rc_failed        set local and overall rc status to failed #      rc_failed <num>  set local and overall rc status to <num><num> #      rc_reset         clear local rc status (overall remains) #      rc_exit          exit appropriate to overall rc status #      rc_activechecks whether a service is activated by symlinks . /etc/rc.status  # First reset status of this service rc_reset  # Return values acc. to LSB for all commands but status: # 0 - success # 1 - generic or unspecified error # 2 - invalid or excess argument(s) # 3 - unimplemented feature (e.g. "reload") # 4 - insufficient privilege # 5 - program is not installed # 6 - program is not configured # 7 - program is not running #  VNCSERVERS="" VNCARGS="" [ -f $VNC_CONFIG ] && . $VNC_CONFIG  prog="VNC server"  case "$1" in     start) for display in ${VNCSERVERS} do if test -f /home/${display##*:}/.vnc/passwd ; then rm -fv /tmp/.X${display%%:*}-lock rm -fv /tmp/.X11-unix/X${display%%:*} echo -n "Starting $prog on: ${display} -- " su ${display##*:} -l -c "cd && vncserver :${display%%:*} ${VNCARGS}" [ $? -eq 0 ] && echo $rc_done_up || echo $rc_failed_up else echo -n "Vnc not initialised for user: ${display##*:}"  echo $rc_failed ; fi done     ;;     stop) for display in ${VNCSERVERS} do echo -n "Shutting down $prog: ${display} " su ${display##*:} -c "vncserver -kill :${display%%:*}" >/dev/null 2>&1 [ $? -eq 0 ] && echo $rc_done || echo $rc_failed done     ;;     restart) $0 stop $0 start rc_status     ;;     status) echo -n "Checking for service Vnc-Server " ## Check status with checkproc(8), if process is running ## checkproc will return with exit status 0.  # Return value is slightly different for the status command: # 0 - service running # 1 - service dead, but /var/run/  pid  file exists # 2 - service dead, but /var/lock/ lock file exists # 3 - service not running  # NOTE: checkproc returns LSB compliant status values. checkproc $VNC_MASTER rc_status -v     ;;     *) echo "Usage: $0 {start|stop|status|restart}"  exit 1     ;; esac rc_exit 

# BEGIN INIT INFO和### END INIT INFO之間的文本就是該服務的meta資訊，用install_initd/remove_initd來activate/deactivate符合LSB標準的init script:

/usr/lib/lsb/install_initd /etc/init.d/vncserver
install_initd根據這些meta資訊在相應的runlevel目錄下產生symbol link，並保證其依賴關係。