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Crontab Sections
Each of the sections is separated by a space, with the final section having one or more spaces in it. No spaces are allowed within Sections 1-5, only between them. Sections 1-5 are used to indicate when and how often you want the task to be executed. This is how a cron job is laid out:
minute (0-59), hour (0-23, 0 = midnight), day (1-31), month (1-12), weekday (0-6,
0 = Sunday), command
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https://help.ubuntu.com/community/CronHowto