1 When you configure crontab scheduled tasks, record what format?
Minute hour date month week executable statement
2 Configuration crontab scheduled task instance.
Add the root user program tasks, 7:30 am start sshd service, 23:30 every day shut down sshd service.
crontab -e // configure crontab scheduled tasks
crontab -l // View crontab plan tasks
3 licensing program tasks.
Prohibit zhangsan user cron cycle scheduled tasks, simply write to the user name (/etc/cron.deny) configuration file can be.
4 What is the role of system service cron and anacron Yes.
Where is the main difference between the two?
cron is used to control the execution of the routine work cycle, the cycle time can be minutes, hours, weekly, monthly, yearly, etc. The anacron is used to handle 24 hours of non-Linux system has started the implementation of cron service! anacron not specify when to perform a task, but in days or anacron action is immediately after boot, he would go to detect the cron service should be carried out but did not during the shutdown, if it is the task perform again, and then automatically stop.
5 top use command.
The implementation of "dd if = / dev / sda of = / dev / null &" command, and then find out the system CPU usage highest process, this the process.
6 scheduling process.
若要新启动一条后台运行的任务，可以在命令行末尾添加（ & ）符号；若要将正在前台运行的任务暂停并挂起到后台，可按快捷键（ Ctrl + z ）；使用（ jobs ）命令可以直接列出后台有哪些任务，使用（ bg ）命令可以启用在后台暂停的某个任务，而使用（ fg ）命令可以将后台的某个任务调度到前台执行。
7 请描述RHEL6.x系统的引导过程？
加载BIOS，检查硬件信息
读取并执行第一个开机设备内MBR
运行grub引导加载 kernel
内核启动/sbin/init程序
init 系统初始化
确定默认的运行级别
触发runlevel事件，运行/etc/rc.d/rc
最后执行/etc/rc.d/rc.local
加载终端或X-Window接口
8 运行级别间切换。
执行（ runlevel ）可查看当前的运行级别，执行（ init 5 ）可将运行级别切换为图形模式，执行（ init 3 ）可切换为完整的文本模式，执行（ init 0 ）表示关机模式、（ init 6 ）表示重启模式。
9 开机引导常用配置文件。
通过修改（ /etc/inittab ）配置文件，可以更改默认运行级别；若要添加用户自定义的开机后自动运行任务，可以修改（ /etc/rc.local ）文件。