[转载]Linux crontab命令解析

名称 : crontab

crontab 是用来让使用者在固定时间或固定间隔执行程序之用，换句话说，也就是类似使用者的时程表。-u user 是指设定指定

user 的时程表，这个前提是你必须要有其权限(比如说是 root)才能够指定他人的时程表。如果不使用 -u user 的话，就是表示设

定自己的时程表。

使用权限 : 所有使用者

使用方式 :

crontab [-u user] file-用指定的文件替代目前的crontab。

crontab-[-u user] -用标准输入替代目前的crontab.

crontab -l [-u user] -列出用户目前的crontab.

crontab -e [-u user] -编辑用户目前的crontab.

crontab -d [-u user] -删除用户目前的crontab.

crontab -c dir- 指定crontab的目录。

crontab文件的格式：M H D m d cmd.

M: 分钟（0-59）。

H：小时（0-23）。

D：天（1-31）。

m: 月（1-12）。

d: 一星期内的天（0~6，0为星期天）。

cmd要运行的程序，程序被送入sh执行，这个shell只有USER,HOME,SHELL这三个环境变量。

例子：

基本格式 :
*　　*　　*　　*　　*　　command
分　 时　  日　  月　  周　  命令

第1列表示分钟1～59 每分钟用*或者 */1表示
第2列表示小时1～23（0表示0点）
第3列表示日期1～31
第4列表示月份1～12
第5列标识号星期0～6（0表示星期天）
第6列要运行的命令

crontab的一些例子：

30 21 * * * /usr/local/etc/rc.d/lighttpd restart
上面的例子表示每晚的21:30重启apache。

45 4 1,10,22 * * /usr/local/etc/rc.d/lighttpd restart
上面的例子表示每月1、10、22日的4 : 45重启apache。

10 1 * * 6,0 /usr/local/etc/rc.d/lighttpd restart
上面的例子表示每周六、周日的1 : 10重启apache。

0,30 18-23 * * * /usr/local/etc/rc.d/lighttpd restart
上面的例子表示在每天18 : 00至23 : 00之间每隔30分钟重启apache。

0 23 * * 6 /usr/local/etc/rc.d/lighttpd restart
上面的例子表示每星期六的11 : 00 pm重启apache。

* */1 * * * /usr/local/etc/rc.d/lighttpd restart
每一小时重启apache

* 23-7/1 * * * /usr/local/etc/rc.d/lighttpd restart
晚上11点到早上7点之间，每隔一小时重启apache

0 11 4 * mon-wed /usr/local/etc/rc.d/lighttpd restart
每月的4号与每周一到周三的11点重启apache

0 4 1 jan * /usr/local/etc/rc.d/lighttpd restart
一月一号的4点重启apache

Manual文档：

CRONTAB(1) General Commands Manual CRONTAB(1)

NAME
crontab - maintain crontab files for individual users (Vixie Cron)

SYNOPSIS
crontab [ -u user ] file
crontab [ -u user ] [ -i ] { -e | -l | -r }

DESCRIPTION
crontab is the program used to install, deinstall or list the tables used to drive the cron(8) daemon in Vixie Cron. Each user can
have their own crontab, and though these are files in /var/spool/cron/crontabs, they are not intended to be edited directly.

If the /etc/cron.allow file exists, then you must be listed (one user per line) therein in order to be allowed to use this command. If
the /etc/cron.allow file does not exist but the /etc/cron.deny file does exist, then you must not be listed in the /etc/cron.deny file
in order to use this command.

If neither of these files exists, then depending on site-dependent configuration parameters, only the super user will be allowed to use
this command, or all users will be able to use this command.

If both files exist then /etc/cron.allow takes precedence. Which means that /etc/cron.deny is not considered and your user must be
listed in /etc/cron.allow in order to be able to use the crontab.

Regardless of the existance of any of these files, the root administrative user is always allowed to setup a crontab. For standard
Debian systems, all users may use this command.

If the -u option is given, it specifies the name of the user whose crontab is to be used (when listing) or modified (when editing). 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.

The -l option causes the current crontab to be displayed on standard output. See the note under DEBIAN SPECIFIC below.

The -r option causes the current crontab to be removed.

The -e option is used to edit the current crontab using the editor specified by the VISUAL or EDITOR environment variables. After you
exit from the editor, the modified crontab will be installed automatically. If neither of the environment variables is defined, then
the default editor /usr/bin/editor is used.

The -i option modifies the -r option to prompt the user for a 'y/Y' response before actually removing the crontab.

DEBIAN SPECIFIC
The "out-of-the-box" behaviour for crontab -l is to display the three line "DO NOT EDIT THIS FILE" header that is placed at the begin‐
ning of the crontab when it is installed. The problem is that it makes the sequence

crontab -l | crontab -

non-idempotent -- you keep adding copies of the header. This causes pain to scripts that use sed to edit a crontab. Therefore, the
default behaviour of the -l option has been changed to not output such header. You may obtain the original behaviour by setting the
environment variable CRONTAB_NOHEADER to 'N', which will cause the crontab -l command to emit the extraneous header.

SEE ALSO
crontab(5), cron(8)

FILES
/etc/cron.allow
/etc/cron.deny
/var/spool/cron/crontabs

There is one file for each user's crontab under the /var/spool/cron/crontabs directory. Users are not allowed to edit the files under
that directory directly to ensure that only users allowed by the system to run periodic tasks can add them, and only syntactically cor‐
rect crontabs will be written there. This is enforced by having the directory writable only by the crontab group and configuring
crontab command with the setgid bid set for that specific group.

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.

cron requires that each entry in a crontab end in a newline character. If the last entry in a crontab is missing the newline, cron will
consider the crontab (at least partially) broken and refuse to install it.

AUTHOR
Paul Vixie <paul@vix.com> is the author of cron and original creator of this manual page. This page has also been modified for Debian
by Steve Greenland, Javier Fernandez-Sanguino and Christian Kastner.

4th Berkeley Distribution 19 April 2010 CRONTAB(1)