An in-depth analysis of the system task settings of Linux study notes

This article brings you relevant knowledge about system task settings in Linux, including issues related to the system startup process, system services, self-starting services and scheduled tasks. I hope it will be helpful to everyone.

System startup process

• Start the computer's hardware (BIOS)
  • Read time
  • Select Corresponding startup mode (USB HDD EFI)
• If it is a Linux system, go back to the /boot directory and boot the system to start
• The computer system starts to start and reads the initialization Configuration file
  • vi /etc/inittab
  • Controls the running level of the computer when starting runlevel
  • ##3Full multiuser mode (multi-user full version mode) 4unused (reserved mode) 5X11 (User Interface Mode) 6reboot (restart mode)
    id:3initdefault: The default runlevel is 3
    Start starting the corresponding services and components with runlevel=3
    Start to boot the public components by default or Service

    • vi /etc/rc.d/rc.sysinit
    Start loading the service corresponding to runlevel

    • vi /etc/rc3.d
      • K: Services that need to be turned off when shutting down
      • S: Services that need to be turned on when starting up
      • The numbers represent the order of turning on or off

      • All The files are all soft links (shortcuts), and the link address is /etc/init.

    

    • ##When the startup is completed, all services

    

    ##System services

    ---

    You can use the chkconfig command to view the services of the current virtual machine.
    • By viewing, you can know that different levels correspond to each service and confirm that it will start automatically during this boot.
    • After the boot is completed, you need Use the service (CentOS6) Systemctl (CentOS7) command to control the opening or closing of the service
    Auto-start service at boot

    ---

    rc.local
    • (When this file has execution permission, it will be executed automatically after booting) First create the folder where the script is stored
      • mkdir -p /usr/local/scripts
        Create a script file in the folder
      • vi hello.sh
        • Give execution permission
        • 
        • 
        Go to the /etc/rc.d/rc.local file and add the absolute path to the script
      • Give rc.local execution permissions

    

    • • • 

    

    ##chkconfig

    • First cancel the execution permission of rc.local, and then start testing

      chmod  a-x rc.local

      • • Create automatic startup at boot Script file

        vi schoolntpdate.sh

      • #!/bin/bash
        #chkconfig：2345 88 99
        #description：schoolntpdate.sh
        
        ##  开机自启动同步时间
        yum info ntp && ntpdate cn.ntp.org.cn

      • Set execution permissions

      • chmod a+x schoolnptdate.sh

      • • Copy the script to /etc/init.d directory

        cp schoolntpdate.sh /etc/init.d/

      • • Add to service

        chkconfig --add /etc/init.d/schoolntpdate.sh

      • • Restart Server
        reboot
    ## Scheduled task

    

    In the system service center, crond is responsible for periodic tasks

    ---

    systemctl status crond.service

    • Add tasks and edit the current user’s task list

      crontab -e

    Edit task

    星 星 星 星 星  command
    分 时 日 月 周 命令
    第一列表示分钟1~59  每分钟用*或者*/1表示
    第二列表示小时1~23（0表示0点）
    第三列表示日期1~31
    第四列表示月份1~12
    第五列标识号星期0~6（0表示星期天）
    第六列表示要运行的命令
    
    *： 表示任何时间都，实际上就是“每”的意思。可以代表00-23小时或者00-12每月或者00-59分
    -： 表示区间，是一个范围，00 17-19 * * * cmd，就是每天17，18，19的整点执行命令
    ,： 是分割时段，30 3,19,21 * * * cmd，就是每天凌晨3和晚上19，21的半点时刻执行命令
    /n： 表示分割，可以看成除法，*/5 * * * * cmd，每隔五分钟执行一次

    30 21 * * * /usr/local/rc.d/Lighttpd restart
    #上面的例子表示每晚21:30重启apache
    
    45 4 1,10,22 * * /usr/local/rc.d/Lighttpd restart
    #上面的例子表示每月1，10，22日的4:45重启apache
    
    10 1 * * 6,0 /usr/local/rc.d/Lighttpd restart
    #上面的例子表示周六、周日的1：10重启apache
    
    0,30 18-23 * * * /usr/local/rc.d/Lighttpd restart
    #上面的例子表示每天的18：00至23:0每隔30分钟重启apache
    
    0 23 * * 6 /usr/local/rc.d/Lighttpd restart
    #上面的例子表示每星期六的11:00重启apache
    
    * */2 * * * /usr/local/rc.d/Lighttpd restart
    #上面的例子表示每两小时重启apache
    
    * 23-7/1 * * * /usr/local/rc.d/Lighttpd restart
    #上面的例子表示晚上的11点到早上的7点之间，每隔一小时重启apache
    
    0 21 4 * mon-wed /usr/local/rc.d/Lighttpd restart
    #上面的例子表示每月的4号与每周一到周三的11点重启apache
    
    0 4 1 jan * /usr/local/rc.d/Lighttpd restart
    #上面的例子表示一月一号的4：00重启apache
    
    --（功能描述：显示年月日时分秒）
    date "+%Y %m %d %H %M %S"

    Restart crontab to make the configuration take effect

    systemctl restart crond.service

    View the current scheduled task through crontab -l
    View the history of the task

    vi /var/spool/mail/root

    Clear task

    crontab -r

    Test
    • Create a script that creates a folder

    #!/bin/bash
    
    dname=`date "+%Y%m%d%H%M%S"`
    mkdir -p ~/$dname

    • • Add a scheduled task and create a folder every minute

      crontab -e #编辑定时任务

    

    • • #

        systemctl restart crond.service  #重启生效

        • crontab -l  #查看当前的定时任务

          • Check if created
          • 
      • • • Clear tasks
            • 

      Related recommendations: "Linux Video Tutorial"
      

    0	##halt (shut down)
    1	Single user mode
    2	Multiuser, without NFS (multi-user mode, but no network status) FS-->FileSystem

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