#概述

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。

##常见用途
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

#检测是否已安装

# crontab
-bash: crontab: command not found

执行 crontab 命令如果报 command not found，就表明没有安装.

#开始安装

# 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 ResolutionDependencies 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 kcrontabs                                         noarch                                   1.10-33.el6                                            base                                    10 kprocmail                                         x86_64                                   3.22-25.1.el6_5.1                                      base                                   162 ksendmail                                         x86_64                                   8.14.4-9.el6                                           base                                   716 kTransaction 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 TransactionInstalling : 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 PackageKitVerifying  : 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!

#查看命令帮助

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] filecrontab [-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

#服务状态控制

/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

#开机启动

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。

相关参考：
Crontab使用语法格式