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You may encounter this error while creating a new crontab or updating an existing one that has a syntax error.
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Crontabs have simple, but strict syntax rules. Every crontab file has to:
# 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
# | | | | | .---- username
# | | | | | |
# * * * * * user command to be executed
* * * * * root echo 'Hello world!'
Sometimes you may find that duplicate cronjobs are running at the same time. This may happen when the cronjob takes longer to complete than its execution interval. Here is a simple way to prevent this from happening ever again.
One of the features of Cron is the ability to run Cron jobs as a specific user. Sometimes you may even want to create a special user with limited privileges just to run Cron jobs. There are two methods how to run cronjob as a specific user.
By default, cron checks crontabs for cronjobs every minute. If you want to run a job every n seconds you need to use a simple workaround.
Cron can generate logs, which are very useful in troubleshooting your cron jobs. In this quick tutorial, we will take a look at cron logs – how to find them and how to read them.
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