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Monday, 12 May 2014

Crontab Interview Questions and Answers


Crontab Interview Questions and Answers

A cron job is a Linux command for scheduling script on your server to complete repetitive tasks automatically. Scripts executed as a cron job are typically used to modify files, databases and manage caching.

Cron is controlled by "crontabs". There is the master file in /etc/crontab. Each user's cronjob is stored in /var/spool/cron/username directory.


Benefits of Cron
  • Cron already built, and works, very reliable
  • You more easily have control over when it runs. You control the minute, hour, day, month and weekday  etc.
  • You can run many cron with difference of 15 seconds
  • You can manage cronjob from admin panel
  • Need not run manually, Schedule once will execute manually.




Install crontab
crontab -a filename

Edit the crontab
$ crontab -e

Display crontab
crontab -l

Display the last edit the crontab file
crontab -v

Remove crontab
crontab -r

Following are the syntax for cron
minute(s) hour(s) day(s) month(s) weekday(s) command(s) "Argument1" "Argument2"
1 * 3 4 5 /path/to/command arg1 arg2

If you don't have parameter put star(*)


Description of Cron fields





Field       Description
minute(0-59) The exact minute that the command sequence executes
hour (0-23) The hour of the day that the command sequence executes
day(1-31)   The day of the month that the command sequence executes
month(1-12) The month of the year that the command sequence executes
weekday(0-6)         The day of the week that the command sequence executes (Sunday = 0, Monday = 1, Tuesday = 2, and so forth)
command(s) File Path    Absolute path CronFile
Argument               Argument (IF you want to pass an argument)

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