Cron & crontab.

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Cron is a daemon which is run every minute and has the job of running other programs at set times. These times are set by crontab files (cron tables). The system has a crontab and also users can run their own.
Use crontab -e to edit the crontab for the current user.
I suppose cron takes its name from Cronos, the ancient Greek god of time.
 

Crontab format

Crontab examples

*****/path/to/command
12345 6 

1Minute of hour0-59
2Hour of day0-23
3Day of month1-31
4Month of year1-12
5Day of week0-6 (0=Sunday)
6Command to run
#comments start with #.
#you cannot mix comments and commands on the same line.
#force command output to be mailed to specific user
MAILTO="admin@myserver.com"
#or send all command output to the bitbucket
MAILTO=""
/5 * * * * /usr/bin/mrtg /data/mrtg/172.25.0.1.cfg --logging /var/log/mrtg.log
2 11,15 * * * /data/scripts/portprobe-0.5.pl

*/5 means run every 5 minutes
2 11,15 means run 11:02 and 15:02 every day

/
Meaning of *
1Do job every minute, 60 times per hour
2Do job every hour, 24 times per day
3Ignore this field and use field 5
4Do job every month, 12 times per year
5Ignore this field and use field 3
Asterisk in both field 3 & 5 means job is run every day
# * * * * * command to be executed
# - - - - -
# | | | | |
# | | | | +—– day of week (0 - 6) (Sunday=0)
# | | | +——- month (1 - 12)
# | | +——— day of month (1 - 31)
# | +———– hour (0 - 23)
# +————- min (0 - 59)
 

null


 


Send all output to /dev/null:- /path/to/myfile/file.pl > /dev/null 2>&1