利用“任务计划”，可以将任何脚本、程序或文档安排在某个最方便的时间运行。通俗的讲就是定时执行某个脚本、程序。
定时关机
定时检查更新
定时对系统配置、数据库、日志进行备份
定时清理垃圾文件
# crontab
-bash: crontab: command not found执行 crontab 命令如果报 command not found，就表明没有安装.
# yum -y install vixie-cron
Loaded plugins: fastestmirror, refresh-packagekit, security
Existing lock /var/run/yum.pid: another copy is running as pid 25960.
Another app is currently holding the yum lock; waiting for it to exit...可能是系统自动升级正在运行，yum在锁定状态中。
可以通过强制关掉yum进程：
#rm -f /var/run/yum.pid
然后就可以使用yum了。
# yum -y install vixie-cron
Loaded plugins: fastestmirror, refresh-packagekit, security
Determining fastest mirrors
* base: mirrors.btte.net
* extras: mirrors.sina.cn
* updates: mirrors.sina.cn
base | 3.7 kB 00:00
extras | 3.4 kB 00:00
extras/primary_db | 32 kB 00:00
updates | 3.4 kB 00:00
updates/primary_db | 1.9 MB 00:07
Setting up Install Process
Resolving Dependencies
--> Running transaction check
---> Package cronie.x86_64 0:1.4.4-15.el6 will be installed
--> Processing Dependency: dailyjobs for package: cronie-1.4.4-15.el6.x86_64
--> Processing Dependency: /usr/sbin/sendmail for package: cronie-1.4.4-15.el6.x86_64
--> Running transaction check
---> Package cronie-anacron.x86_64 0:1.4.4-15.el6 will be installed
--> Processing Dependency: crontabs for package: cronie-anacron-1.4.4-15.el6.x86_64
---> Package sendmail.x86_64 0:8.14.4-9.el6 will be installed
--> Processing Dependency: procmail for package: sendmail-8.14.4-9.el6.x86_64
--> Running transaction check
---> Package crontabs.noarch 0:1.10-33.el6 will be installed
---> Package procmail.x86_64 0:3.22-25.1.el6_5.1 will be installed
--> Finished Dependency Resolution
Dependencies Resolved
===============================================================================================================================================================================================
Package Arch Version Repository Size
===============================================================================================================================================================================================
Installing:
cronie x86_64 1.4.4-15.el6 base 74 k
Installing for dependencies:
cronie-anacron x86_64 1.4.4-15.el6 base 31 k
crontabs noarch 1.10-33.el6 base 10 k
procmail x86_64 3.22-25.1.el6_5.1 base 162 k
sendmail x86_64 8.14.4-9.el6 base 716 k
Transaction Summary
===============================================================================================================================================================================================
Install 5 Package(s)
Total download size: 994 k
Installed size: 2.1 M
Downloading Packages:
(1/5): cronie-1.4.4-15.el6.x86_64.rpm | 74 kB 00:00
(2/5): cronie-anacron-1.4.4-15.el6.x86_64.rpm | 31 kB 00:00
(3/5): crontabs-1.10-33.el6.noarch.rpm | 10 kB 00:00
(4/5): procmail-3.22-25.1.el6_5.1.x86_64.rpm | 162 kB 00:00
(5/5): sendmail-8.14.4-9.el6.x86_64.rpm | 716 kB 00:00
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Total 1.3 MB/s | 994 kB 00:00
Running rpm_check_debug
Running Transaction Test
Transaction Test Succeeded
Running Transaction
Installing : procmail-3.22-25.1.el6_5.1.x86_64 1/5
Installing : sendmail-8.14.4-9.el6.x86_64 2/5
Installing : cronie-1.4.4-15.el6.x86_64 3/5
Installing : crontabs-1.10-33.el6.noarch 4/5
Installing : cronie-anacron-1.4.4-15.el6.x86_64 5/5
ERROR:dbus.proxies:Introspect error on :1.2513:/org/freedesktop/PackageKit: dbus.exceptions.DBusException: org.freedesktop.DBus.Error.NoReply: Did not receive a reply. Possible causes include: the remote application did not send a reply, the message bus security policy blocked the reply, the reply timeout expired, or the network connection was broken.
Unable to send message to PackageKit
Verifying : crontabs-1.10-33.el6.noarch 1/5
Verifying : cronie-anacron-1.4.4-15.el6.x86_64 2/5
Verifying : sendmail-8.14.4-9.el6.x86_64 3/5
Verifying : cronie-1.4.4-15.el6.x86_64 4/5
Verifying : procmail-3.22-25.1.el6_5.1.x86_64 5/5
Installed:
cronie.x86_64 0:1.4.4-15.el6
Dependency Installed:
cronie-anacron.x86_64 0:1.4.4-15.el6 crontabs.noarch 0:1.10-33.el6 procmail.x86_64 0:3.22-25.1.el6_5.1 sendmail.x86_64 0:8.14.4-9.el6
Complete!crontab 并不支持-h或–help之类的选项，不过还是可以通过它查看命令帮助
# crontab -h
crontab：无效选项 -- h
crontab: usage error: unrecognized option
usage: crontab [-u user] file
crontab [-u user] [ -e | -l | -r ]
(default operation is replace, per 1003.2)
-e (edit user's crontab)
-l (list user's crontab)
-r (delete user's crontab)
-i (prompt before deleting user's crontab)
-s (selinux context)也可以使用 info crontab 命令查看详细的帮助信息。
cron服务提供crontab命令来设定cron服务的，以下是这个命令的一些参数与说明:
crontab -u //设定某个用户的cron服务，一般root用户在执行这个命令的时候需要此参数
crontab -l //列出某个用户cron服务的详细内容
crontab -r //删除没个用户的cron服务
crontab -e //编辑某个用户的cron服务
比如说root查看自己的cron设置:crontab -u root -l
再例如，root想删除fred的cron设置:crontab -u fred -r
在编辑cron服务时，编辑的内容有一些格式和约定，输入:crontab -u root -e
进入vi编辑模式，编辑的内容一定要符合下面的格式:/1 * * * ls >> /tmp/ls.txt
/sbin/service crond start //启动服务
/sbin/service crond stop //关闭服务
/sbin/service crond restart //重启服务
/sbin/service crond reload //重新载入配置
/sbin/service crond status //查看状态
或者使用
# service crond start
# service crond stop
# service crond restart
# service crond reload
# service crond status
你也可以将这个服务在系统启动的时候自动启动:
在/etc/rc.d/rc.local这个脚本的末尾加上:
/sbin/service crond start现在Cron这个服务已经在进程里面了，我们就可以用这个服务了。