Introduction

CRON is a time-based job scheduler in Unix-like operating systems. It allows users to schedule scripts or commands to run automatically at specific intervals. Understanding CRON syntax is crucial for automating tasks effectively.

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Basic CRON Syntax

A CRON expression consists of five fields followed by the command to execute:

* * * * * command-to-execute
- - - - -
| | | | |
| | | | +---- Day of the week (0 - 7) (Sunday = 0 or 7)
| | | +------ Month (1 - 12)
| | +-------- Day of the month (1 - 31)
| +---------- Hour (0 - 23)
+------------ Minute (0 - 59)

Each field represents a specific time unit:

• Minute: Values from 0 to 59.

• Hour: Values from 0 to 23.

• Day of the Month: Values from 1 to 31.

• Month: Values from 1 to 12.

• Day of the Week: Values from 0 to 7 (where both 0 and 7 represent Sunday).

Special characters used in CRON expressions:

• * (asterisk): Any value.

• , (comma): List of values.

• - (dash): Range of values.

• / (slash): Step values.

Examples:

• Run a script every day at midnight:

0 0 * * * /path/to/script.sh

• Run a script every Monday at 8 AM:

0 8 * * 1 /path/to/script.sh

• Run a script every 30 minutes:

*/30 * * * * /path/to/script.sh

• Run a script on the 1st of every month at 3 PM:

0 15 1 * * /path/to/script.sh

• Run a script at 10 AM on weekdays:

0 10 * * 1-5 /path/to/script.sh

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Special Strings in CRON

Instead of using the standard five-field syntax, CRON allows special strings for common scheduling tasks:

• @reboot – Run once at startup.

• @hourly – Run every hour.

• @daily – Run once a day (midnight).

• @weekly – Run once a week (Sunday midnight).

• @monthly – Run once a month (1st day, midnight).

• @yearly – Run once a year (January 1st, midnight).

Example:

@daily /path/to/script.sh