Linux 下 Crontab 命令使用详解定时任务

2023-08-30 13:25:28

一. Crontab 介绍

crontab命令的功能是在一定的时间间隔调度一些命令的运行。

1.1 /etc/crontab 文件

在/etc文件夹下有一个crontab文件，这里存放有系统运行的一些调度程序。每一个用户能够建立自己的调度crontab。

如：

[root@dave ~]# 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 命令的用户

/etc/cron.allow 表示能使用crontab的用户。

假设两个文件同一时候存在，那么/etc/cron.allow 优先。

假设两个文件都不存在，那么唯独超级用户能够安排作业。

每一个用户都会生成一个自己的crontab 文件。这些文件在/var/spool/cron文件夹下：

如：

[root@dave ~]# cd /var/spool/cron

[root@dave cron]# ls

oracle root

我们直接查看这个文件，里面的内容和相应用户显示的crontab -l 一致。

[root@dave cron]# cat oracle

00 6 * * * /u02/scripts/del_st_archive.sh >/u02/scripts/del_st_arch.log 2>&1

[root@dave cron]# cat root

0 12 * * * /root/bin/sync-clock.sh

[root@dave cron]#

二. Crontab 使用说明

2.1 Crontab语法

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)

当中，file是命令文件的名字。假设在命令行中指定了这个文件，那么运行crontab命令，则将这个文件复制到crontabs文件夹下；假设在命令行中沒有制定这个文件，crontab命令将接受标准输入（键盘）上键入的命令，并将他们也存放在crontab文件夹下。

帮助：

[root@dave ~]# 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).

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