Cron CheatSheet

Crontab stands for "cron table" because it uses the job scheduler cron to execute tasks, cron itself is named after "chronos"

Crontab is a UNIX command that creates a table or list of commands, each of which is to be executed by the operating system at a specified time that you want.

The schedule is called the crontab, which is also the name of the program used to edit that schedule.

• You more easily have control over when it runs. You control the minute, hour, day, etc, that it will execute
• It's easier to write the code and to manage its operation. It eliminates the looping and timing logic in the task, and you run crontab to change the timing or shut it off.
• It's not occupying memory in your system when it's not running.
• If it fails and exits for some reason, it will start up again when the proper time comes
• Advantages of the infinite loop
• Even if your system is off but still it is run in the background. But, make sure that the server is not to be shut down.
• It doesn't have the overhead of being restarted every time it's needed

• Smallest resolution is 1 minuteIf a task needs to run every 30 seconds, you cant do it with cron.
• Error handling If a job fails, what should happen? Solutions have been built to solve this single problem. Developers love adding more band-aids rather than admitting there is a better way. Deja Vu?
• Logging Crons dont log, unless you tell them too. It is rare to see people log output of their cron jobs.
• Working with cron pulls you out of the application cron is a system-level process. Not an application process. Its challenging to give application developers access to anything at a system level. Developers shouldnt care where their application runs A good example of this is timezones. If a system person changes the timezone of a server, the cron may run at a different time than expected. The less the app developers have to worry about what they run on, the better.

Server admins have been using cron jobs for a long time. But since the target audience of this article is web developers, let's look at a few use cases of cron jobs that are relevant in this area:

• If you have a membership site, where accounts have expiration dates, you can schedule cron jobs to regularly deactivate or delete accounts that are past their expiration dates.
• You can send out daily newsletter e-mails.
• If you have summary tables (or materialized views) in your database, they can be regularly updated with a cron job. For example, you may store every web page hit in a table, but another summary table may contain daily traffic summaries.
• You can expire and erase cached data files in a certain interval.
• You can auto-check your website content for broken links and have a report e-mailed to yourself regularly.
• You can schedule long-running tasks to run from a command-line script, rather than running it from a web script. Like encoding videos, or sending out mass e-mails.
• You can even perform something as simple as fetching your most recent Tweets, to be cached in a text file.

    
        Min  Hour Day  Mon  Weekday
        *    *    *    *    *  command to be executed
        ?    ?    ?    ?    ?
        ?    ?    ?    ?    ??  Day of Week   (0=Sun .. 6=Sat)
        ?    ?    ?    ???????  Month         (1..12)
        ?    ?    ????????????  Day of Month  (1..31)
        ?    ?????????????????  Hour          (0..23)
        ??????????????????????  Minute        (0..59)
    

• Crontab Guru