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。
之后要启动 cron 服务:
别忘了把 cron 加入到启动脚本中:
完成安装。
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:
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:
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 |
周 | 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:
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:
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:
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。
is-Programmer 是一个面向 Linux 爱好者与程序员的 Blog 系统,具有简洁的风格,方便的代码高亮和数学公式插入功能,适合撰写学习笔记和技术文档。
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