How to Schedule Crontab
Crontab has a robust scheduling system. There are 5 fields and you put a number in each to set the appropriate counter when your command or script triggers. The following examples will help you.
Scheduling in Crontab
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# 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) # | | | | | # * * * * * command to be executed |
- fields are separated by spaces or tabs
- commas can be used to specify a list
- slashes can be used to step through every (ranges)
- asteriks signify “all” for that field
Be careful of changing timezone. Your crontab could be affected each time you change them.
Crontab Schedule Examples
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# hourly and don't mail it, send info to syslog @hourly /path/to/script.sh >/dev/null 2>&1 # Create output log and watch for "ERROR" also create syslog script.sh 2>&1 | tee -a output.log | grep -C 100 ERROR # if as root, noon every day update 00 12 * * * apt-get update # immediately after reboot (several issues though, recommended not to use) @reboot /path/to/script.sh # on reboot, mail uptime a @reboot echo `uptime` | mail -s "`uname -n` rebooted" admin@site.com # 60 seconds after reboot @reboot sleep 60 && /path/to/script.sh # every minute */1 * * * * /path/to/script.sh # every hour on the 1st minute (1:01, 2:01, etc) 01 * * * * /path/to/script.sh # every monday at midnight 0 0 * * MON /path/to/script.sh # every midnight 0 0 * * * /path/to/script.sh # every day at 9:15am 15 9 * * * /path/to/script.sh # every other hour 0 */2 * * * /path/to/script.sh |
You can type in and get human readable cron schedules by using this website: https://crontab.guru