Cron:

Cron is a job scheduler in linux. It is used to run jobs periodically at fixed times, dates, or intervals. It performs specified operations at a predefined time or occurrence of an event without the intervention of the user. Cron is used to automate repeated tasks. It starts automatically from /etc/init.d on entering the multi-user run levels. Cron is a daemon (A background process executing non-interactive jobs).

Cron Jobs:

Tasks scheduled in cron are called cron jobs. Users can determine what kind of tasks they want to automate and when it shold be executed.

Cron Files:

Cron file is a simple text file that contains commands to run periodically at a specific time. The default system cron table or crontab configuration file is /etc/crontab .

Use Cases for Cron Job Scheduling:

• Automated Backups:

  Regularly schedule backups of databases, files, or entire systems.

• System Maintenance:

  Periodically run maintenance tasks such as clearing temporary files, updating software packages, or optimizing databases.

• Log Rotation:

  Rotate log files at specified intervals to manage disk space efficiently.

• Scheduled Reports:

  Generate and distribute reports automatically at scheduled times.

• Data Synchronization:

  Synchronize data between different systems or databases at regular intervals.

• Security Scans:

  Run security scans or vulnerability assessments on systems periodically.

• Monitoring and Alerts:

  Schedule scripts to monitor system health and send alerts if certain conditions are met.

• File or Data Transfer:

  Automate file transfers between servers or systems on a regular basis.

• Periodic Cleanup:

  Clean up temporary files, old backups, or other unnecessary data at scheduled times.

• Database Maintenance:

  Schedule tasks to optimize, backup, or perform other maintenance on databases.

Cron Job Scheduling Limitations:

• Can't be distributed to multiple computers in a network.

• Won't be able to repeat a job every 59 seconds or less.

• Won't run again until the next scheduled time.

Crontab Syntax:

Crontab syntax consist of five fields and a command.

• Minute (value between 0-59)

• Hour (value between 0-23)

• Day of Month (value between 1-31)

• The month of the year (value between 1-12 or Jan-Dec)

• Day of the week (value between 0-6 or Sun-Sat)

Command

Operators of Crontab:

• Asterisk (*):

  This operator signifies all possible values in a field. e.g. write an * in a minute field to make the cronjob run every minute.

• Comma (,):

  This operator is used for listing multiple values. e.g. writing 1,5 in the day-of-week field will schedule the job to run every Monday and Friday.

• Hyphen (-):

  This operator is used to determine range of the values. e.g. Write 6-9 in the month-of-the-year field to schedule the job from June to September.

• Separator (/):

  This operator is used to divide a value. e.g. If we want to run a script every twelve hours, write */12 in the hour field.

Cron Job Special Strings:

• @hourly :

  This string will run once in an hour. This command is written as

  @hourly [command] . Its equivalent command is 0 * * * * [command] .

• @daily or @midnight:

  These strings will run the task everyday at midnight. This command is written as @daily [command] or @midnight [command]. Its equivalent command is \

  0 0 * * * [command] .

• @weekly:

  This string will run the task once a week at midnight on Sunday. This command is written as @weekly [command] . Its equivalent command is

  0 0 * * 0 [command] .

• @monthly:

  This string will run the task once on the first day of every month. This command is written as @monthly [command] . Its equivalent command is

  0 0 1 * * [command] .

• @yearly:

  This string will run the task once a year at midnight on January 1st. This command is written as @yearly [command] . Its equivalent command is

  0 0 1 1 * [command] .

• @reboot:

  This string will run the task once during the system startup. This command is written as @reboot /path/to/script/script.sh .

Writing Cron Jobs:

• Every Minute: * * * * * [command]

• Every 15 Minutes: */15 * * * * [command]

• On the 30^th Minute of Every Hour: 30 * * * * [command]

• At the beginning of every hour: 0 * * * * [command]

• Everyday at midnight: 0 0 * * * [command]

• At 2am everyday: 0 2 * * * [command]

• Every 1st of the month: 0 0 1 * * [command]

• Every 15th of the month: 0 0 15 * * [command]

• On December 1st Midnight: 0 0 1 12 * [command]

• Saturdays at midnight: 0 0 * * 6 [command]

• Every weekday at 4am: 0 4 * * 1-5 [command]

• At 4am on Tuesdays and Thursdays: 0 4 * * 2,4 [command]

• Every Other Day at 37 Minutes Past the Hour: 37 1-23/2 * * * [command]

• Every 20 Minutes - Multiple Commands: */20 * * * * [command1]; [command2]

• On Saturdays and Sundays at 12 PM: 0 12 * * 6,0 [command]

• Monday to Friday - Every Hour 9 AM to 5 PM: 0 9-17 * * 1-5 [command]

Using Crontab:

• Edit Crontab File:

  To edit the crontab file crontab -e is used.

  

  The file will be opened in the nano text editor.

• List Existing Jobs:

  To list the existing jobs crontab -l is used.

• Delete All Jobs:

  To delete all the jobs crontab -r is used.