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用cron定时执行任务

galeki posted @ 2007年5月27日 23:28 in Linux with tags linux 定期执行 cron crontab , 6948 阅读

假如你有一些任务要定期执行,比如清理磁盘、删除过期文件、发送邮件和提醒,这个时候可以用cron来帮你。

首先是安装,大多数发行版都预装了,Gentoo下的cron有许多种实现,可以用emerge --search cron来查看,这里用vixie-cron,因为功能比较完整,命令语法也比较通用。

# emerge vixie-cron

之后要启动cron服务:

# /etc/init.d/vixie-cron start

别忘了把cron加入到启动脚本中:

# rc-update add vixie-cron default

完成安装。

系统cron设定:/etc/crontab

通过 /etc/crontab 文件,可以设定系统定期执行的任务,当然,要想编辑这个文件,得有root权限。

让我们打开 /etc/crontab,看看cron的语法:

  1. # for vixie cron
  2. #
  3. # $Header: /var/cvsroot/gentoo-x86/sys-process/vixie-cron/files/crontab-3.0.1-r4,v 1.1 2005/03/04 23:59:48 ciaranm Exp $
  4. #
  5. #
  6.  
  7. # Global variables
  8. SHELL=/bin/bash
  9. PATH=/sbin:/bin:/usr/sbin:/usr/bin
  10. MAILTO=root
  11. HOME=/
  12.  
  13. # check scripts in cron.hourly, cron.daily, cron.weekly and cron.monthly
  14. 0  *  * * *     root    rm -f /var/spool/cron/lastrun/cron.hourly
  15. 1  3  * * *     root    rm -f /var/spool/cron/lastrun/cron.daily
  16. 15 4  * * 6     root    rm -f /var/spool/cron/lastrun/cron.weekly
  17. 30 5  1 * *     root    rm -f /var/spool/cron/lastrun/cron.monthly
  18. */10  *  * * *  root    test -x /usr/sbin/run-crons && /usr/sbin/run-crons 

以上是一个默认的/etc/crontab文件,7~11行设定了执行任务时特定环境变量的值,MAILTO=root 指出出错信息发给root。

真正描述执行的任务在14~18行,每行代表了一个定期执行的任务,从前到后,分为3部分:

第一部分,一堆奇怪的数字和星号,这部分描述任务的执行周期和时间,后面我们将详细解释。

第二部分,上面的文件全部是"root",描述了执行任务时的用户。

第三部分,为需要执行的完整命令。

第一部分分为5个小部分,没部分可以是数字或者星号,分别代表了分钟、小时、日期、月份、周,让我们来看一个例子:

0   7   *    *   *    root    mpg123 ~/wakeup.mp3
分 时 日 月 周

星号,即代表不限制,上面的例子就是:每天的7点0分,都将执行mpg123 ~/wakeup.mp3,放首歌,把自己从睡梦中叫醒~

以下是各部分的取之范围:

 分钟 0~59
 小时 0~23
 日期 1~31
 月份 1~12
0~6

其中周的范围0~6,0代表星期天,1~6代表周一~周六。

让我们来看更多的例子:

0  4  * * 0     root    emerge --sync && emerge -uD world              #每周日凌晨4点,更新系统
0  2  1 * *     root    rm -f /tmp/*                                                    #每月1号凌晨2点,清理/tmp下的文件
0  8  6  5 *   root     mail  robin < /home/galeki/happy.txt             #每年56日给robin发信祝他生日快乐

假如,我想每隔2分钟就要执行某个命令,或者我想在每天的6点、12点、18点执行命令,诸如此类的周期,可以通过 “ / ” 和 “ , ” 来设置:

*/2   *   *   *   *           root      ...............      #每两分钟就执行........
0  6,12,18   *   *   *    root      ...............      #每天6点、12点、18点执行........

用户的cron: crontab命令

上面我们提到的 /etc/crontab 文件,只能指定系统定期执行的命令,如果用户想设定自己定期需要执行的命令的话,就要用到crontab命令,编辑自己的crontab文件。

首先,用户必须隶属于cron组才可以用crontab命令,可以用下面的命令把用户加入到cron组中:

# gpasswd -a galeki cron

crontab命令主要有3个参数:

-e : 编辑用户的crontab。

-l  : 列出用户的crontab的内容。

-r  : 删除用户的crontab的内容。

执行crontab -e,将自动打开编辑器,你可以编辑自己的crontab文件,语法和 /etc/crontab 文件一样,不同的只是,不必再指出执行的用户,编辑后保存即可。

crontab -l用来查看自己的crontab文件内能,crontab -r删除自己的crontab。

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