Talking about: Common symbol summary of Linux cron program task _linux Shell

[Root@wx-a ~]# 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) deletes all crontab tasks
-I (Prompt before deleting user ' s crontab)
-S (selinux context)

By default, any user who is not included in the/etc/cron.deny can perform "crontab–e" to edit his or her routine commands.

The 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)	It means to be accepted at all times,
, (comma)	Represents the meaning of the split period, 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, and 4 represents 2:00 and 4:00
-(minus sign)	Represents a period of time, for example, 10 minutes per hour between 7 and 10. Job: 7-10 * * * command The second column becomes 7-10 representing 7 8 9 10
/n (Slash)	That n represents the number, that 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 Schedule

[Root@wx-a ~]# Cat/etc/crontab
Shell=/bin/bash
Path=/sbin:/bin:/usr/sbin:/usr/bin
Mailto=root
home=/
# for details, man 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

The CRONTAB-E is designed for users ' cron.

Basically, Cron's minimum detection limit for this service is "minutes," so cron reads the contents of the/etc/crontab and/var/spool/cron data once per minute.

Standard output (Stout): Code 1 using > or >>

Standard error Output (STDERR): Code is 2 using 2> or 2>>

Black Hole/dev/null

Error Output 2>&1

Cron lets scheduled tasks not output on screen using >/dev/null 2>&1

[Root@wx-a ~]# Crontab-l
*/1 * * * echo "Hello" >>/tmp/test.txt
[Root@wx-a ~]# Cat/tmp/test.txt
Hello
[Root@wx-a ~]# Cat/tmp/test.txt
Hello
Hello

Let the output go inside the black hole.

[Root@wx-a ~]# Crontab-l
*/1 * * * * echo "Hello"
[Root@wx-a ~]# Crontab-l
*/1 * * * * echo "Hello" >/dev/null 2>&1 output to black hole

Other help is available to view man cron or man crontab

PS:OCSNG Linux Client Installation

Aptitude-y install gcc automake autoconf libtool make
aptitude-y install Dmidecode Libxml-simple-perl-libcompress -zlib-perl libnet-ip-perl libwww-perl libdigest-md5-perl libnet-ssleay-perl
aptitude-y Install Libcrypt-ssleay-perl libnet-snmp-perl Libproc-pid-file-perl Libproc-daemon-perl net-tools Libsys-syslog-perl Pciutils smartmontools read-edid nmap
Tar–xvzf ocsinventory-agent-2.0.x.tar.gz
CD ocsinventory-agent-2.0.x
Env perl_autoinstall=1 PERL makefile.pl
make
make install