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Cause of the error
You may encounter this error while creating a new crontab or updating an existing one that has a syntax error.
Solution
Crontabs have simple, but strict syntax rules. Every crontab file has to:
- Start whit a correct cron schedule (can be an environmental variable) or comment on every line
- Contain a username for each cron job
- Contain valid and executable shell expression
- End with a newline
# 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!'
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This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.
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How to prevent duplicate cron jobs from running?
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.
Questions -
How to run a cronjob as a specific user?
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How to run cron jobs every 5, 10, or 30 seconds?
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.
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How to view and read cron logs on Ubuntu, Debian and CentOS?
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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