一. Crontab 介紹
crontab命令的功能是在一定的時間間隔調度一些命令的執行。
1.1 /etc/crontab 文件
在/etc目錄下有一個crontab文件，這裡存放有系統運行的一些調度程序。每個用戶可以建立自己的調度crontab。
如：
[root@ www.linuxidc.com~]# cat /etc/crontab
SHELL=/bin/bash
PATH=/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=root
HOME=/
# run-parts
01 * * * * root run-parts /etc/cron.hourly
02 4 * * * root run-parts /etc/cron.daily
22 4 * * 0 root run-parts /etc/cron.weekly
42 4 1 * * root run-parts /etc/cron.monthly
1.2 /etc/cron.deny 和 /etc/cron.allow 文件
/etc/cron.deny 表示不能使用crontab 命令的用戶
/etc/cron.allow 表示能使用crontab的用戶。
如果兩個文件同時存在，那麼/etc/cron.allow 優先。
如果兩個文件都不存在，那麼只有超級用戶可以安排作業。
每個用戶都會生成一個自己的crontab 文件。這些文件在/var/spool/cron目錄下：
如：
[root@ www.linuxidc.com~]# cd /var/spool/cron
[root@ www.linuxidc.comcron]# ls
Oracle root
我們直接查看這個文件，裡面的內容和對應用戶顯示的crontab -l 一致。
[root@ www.linuxidc.comcron]# cat oracle
00 6 * * * /u02/scripts/del_st_archive.sh >/u02/scripts/del_st_arch.log 2>&1
[root@ www.linuxidc.comcron]# cat root
0 12 * * * /root/bin/sync-clock.sh
[root@ www.linuxidc.comcron]#
二. Crontab 使用說明
2.1 Crontab語法
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)
其中，file是命令文件的名字。如果在命令行中指定了這個文件，那麼執行crontab命令，則將這個文件拷貝到crontabs目錄下；如果在命令行中沒有制定這個文件，crontab命令將接受標准輸入（鍵盤）上鍵入的命令，並將他們也存放在crontab目錄下。
幫助：
[root@ www.linuxidc.com~]# man crontab
CRONTAB(1) CRONTAB(1)
NAME
crontab - maintain crontab files for individual users (ISC Cron V4.1)
SYNOPSIS
crontab [-u user] file
crontab [-u user] [-l | -r | -e] [-i] [-s]
DESCRIPTION
Crontab is the program used to install, deinstall or list the tables used to drive the cron(8) daemon in ISC Cron. Each user can have their own crontab, and though these are files in /var/spool/ , they are not intended to be edited directly. For SELinux in mls mode can be even more crontabs - for each range. For more see selinux(8).
If the cron.allow file exists, then you must be listed therein in order to be allowed to use this command. If the cron.allow file does not exist but the cron.deny file does exist, then you must not be listed in the cron.deny file in order to use this command. If neither of these files exists, only the super user will be allowed to use this command.
OPTIONS
-u It specifies the name of the user whose crontab is to be tweaked. If this option is not given, crontab examines "your" crontab, i.e., the crontab of the person executing the command. Note that su(8) can confuse crontab and that if you are running inside of su(8) you should always use the -u option for safety¡¯s sake. The first form of this command is used to install a new crontab from some named file or standard input if the pseudo-filename "-" is given.
-l The current crontab will be displayed on standard output.
-r The current crontab will be be removed.
-e This option is used to edit the current crontab using the editor specified by the VISUAL or EDITOR environment variables. After you exit from the edi-tor, the modified crontab will be installed automatically.
-i This option modifies the -r option to prompt the user for a ¡¯y/Y¡¯ response before actually removing the crontab.
-s It will append the current SELinux security context string as an MLS_LEVEL setting to the crontab file before editing / replacement occurs - see the documentation of MLS_LEVEL in crontab(5).
SEE ALSO
crontab(5), cron(8)
FILES
/etc/cron.allow
/etc/cron.deny
STANDARDS
The crontab command conforms to IEEE Std1003.2-1992 (¡®¡®POSIX¡¯¡¯). This new command syntax differs from previous versions of Vixie Cron, as well as from the classic
SVR3 syntax.
DIAGNOSTICS
A fairly informative usage message appears if you run it with a bad command line.
AUTHOR
Paul Vixie <[email protected]>
4th Berkeley Distribution 16 Januar 2007 CRONTAB(1)
