linux crontab 實現定時任務非常方便
CRONTAB(1) Cronie Users’ Manual CRONTAB(1)
NAME
crontab - maintain crontab files for individual users
SYNOPSIS
crontab [-u user] file
crontab [-u user] [-l | -r | -e] [-i] [-s]
DESCRIPTION
Crontab is the program used to install, remove or list the tables used to drive the cron(8) daemon. 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).
The cron jobs could be allow or disallow for different users. For classical crontab there exists cron.allow and cron.deny files. If 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. The second option is using PAM authentication, where you set
up users, which could or couldn’t use crontab and also system cron jobs from /etc/cron.d/.
The temporary directory could be set in enviroment variables. If it’s not set by user than /tmp is used.
OPTIONS
-u Append 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 com-
mand. 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 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 editor, 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 (crontab權限問題)
/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.
日志文件一般在 /var/log/cron 文件
基本格式 :
* * * * * command
分 時 日 月 周 命令
第1列表示分鐘1～59 每分鐘用*或者 */1表示
第2列表示小時1～23（0表示0點）
第3列表示日期1～31
第4列表示月份1～12
第5列標識號星期0～6（0表示星期天）
第6列要運行的命令
crontab文件的一些例子：
每晚的21:30重啟apache。
30 21 * * * /usr/local/etc/rc.d/lighttpd restart
每月1、10、22日
45 4 1,10,22 * * /usr/local/etc/rc.d/lighttpd restart
每天早上6點10分
10 6 * * * date
每兩個小時
0 */2 * * * date
晚上11點到早上8點之間每兩個小時，早上8點
0 23-7/2，8 * * * date
每個月的4號和每個禮拜的禮拜一到禮拜三的早上11點
0 11 4 * mon-wed date
1月份日早上4點
0 4 1 jan * date
crontab最小維度是分鐘，想實現秒級別的方式有兩種
- crontab -e 中實現
* * * * * /bin/date >>/tmp/date.txt
* * * * * sleep 10; /bin/date >>/tmp/date.txt
* * * * * sleep 20; /bin/date >>/tmp/date.txt
* * * * * sleep 30; /bin/date >>/tmp/date.txt
* * * * * sleep 40; /bin/date >>/tmp/date.txt
* * * * * sleep 50; /bin/date >>/tmp/date.txt
2.在shell腳本中實現
vim test.sh
#!/bin/bash
for((i=1;i<=5;i++));
do
/bin/date >>/tmp/date.txt
sleep 10
done &
crontab -e
* * * * * test.sh
在執行crontab的是經常碰到找不到命令的情況，這時候是path的問題制定path即可
例如
crontab -e
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
* * * * * test.sh
