[Email protected] ~]# crontab--help
Crontab:invalid option--'-'
Crontab:usage error:unrecognized option
Usage:crontab [-u user] File
crontab [-u user] [-e |-l |-r]
(default operation is replace, per 1003.2)
-E (Edit user ' s crontab) editing crontab work content
-L (list user ' s crontab) view crontab work content
-R (Delete user ' s crontab) Delete all crontab tasks
-I (Prompt before deleting user ' s crontab)
-S (selinux context)
By default, any user can perform "crontab–e" to edit their own routine commands as long as they are not included in the/etc/cron.deny.
Meaning of the representative |
Minutes |
Hours |
Date |
Month |
Week |
Number Range |
0-59 |
0-23 |
1-31 |
1-12 |
0-7 |
When the number of weeks is 0-7, it means "Sunday".
Auxiliary characters
Special characters |
Representative meaning |
* (asterisk) |
Mean to accept at any moment, |
, (comma) |
Represents the meaning of the split window, if you want to perform 2:00 and 4:00, 0 2,4 * * command The time parameter has five columns, the second column is 2, 4 means 2:00 and 4:00 |
-(minus) |
Represents a period of time, such as 10 minutes per hour between 7 and 10 points Work: 7-10 * * command The second column becomes 7-10 for 7 8 9 10 |
/n (Slash) |
That n represents the number, which is the meaning of every n unit interval, for example, every 5 minutes, Then: */5 * * * * command Can also be written as 0-59/5 meaning the same |
System Task Scheduler
[Email protected] ~]# Cat/etc/crontab
Shell=/bin/bash
Path=/sbin:/bin:/usr/sbin:/usr/bin
Mailto=root
home=/
# for details see Mans 4 Crontabs
# Example of Job definition:
#.----------------Minute (0-59)
# |.-------------Hour (0-23)
# | |.----------DAY of Month (1-31)
# | | |.-------month (1-12) OR jan,feb,mar,apr ...
# | | | |.----DAY of Week (0-6) (sunday=0 or 7) or Sun,mon,tue,wed,thu,fri,sat
# | | | | |
# * * * * * * user-name command to be executed
CRONTAB-E is designed for users ' cron.
Basically cron the minimum detection limit for this service is "minutes", so cron reads the contents of/etc/crontab and/var/spool/cron data once every minute.
Standard output (Stout): Code 1 using > or >>
Standard error Output (STDERR): Code 2 using 2> or 2>>
Black Hole/dev/null
Error Output 2>&1
Cron makes scheduled tasks out of the screen output with >/dev/null 2>&1
[Email protected] ~]# crontab-l
*/1 * * * * echo "Hello" >>/tmp/test.txt
[Email protected] ~]# Cat/tmp/test.txt
Hello
[Email protected] ~]# Cat/tmp/test.txt
Hello
Hello
Let the output go inside the black hole.
[Email protected] ~]# crontab-l
*/1 * * * * echo "Hello"
[Email protected] ~]# crontab-l
*/1 * * * * echo "Hello" >/dev/null 2>&1 output to black hole
Other help can be seen in man cron or man crontab
This article is from the "Little Rookie" blog, please be sure to keep this source http://xiaocainiaox.blog.51cto.com/4484443/1701975
Summary of common symbols for Linux cron scheduling tasks