Cron Parser CLI Tool Specification

Cron tab

Overview

This command-line tool parses and evaluates cron expressions. It provides two functionalities:

1. Next Scheduled Runs: Given a cron expression, it outputs the next N scheduled execution times.
2. Validation for a Specific Time: Given a cron expression and a datetime instance, it determines whether the job should run at that time (returns true/false).

The tool supports standard cron syntax, including wildcards (*), step values (/), and ranges (-) across all cron fields.

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Features

1. Next Scheduled Runs

Command:

cron-parser next "<cron_expression>" [--count N]

Description:

• Given a cron expression, it calculates and prints the next N execution times (default is 5 if not specified).
• Supports all five standard cron fields:
  • Minute (0-59)
  • Hour (0-23)
  • Day of Month (1-31)
  • Month (1-12)
  • Day of Week (0-6, where 0 is Sunday)
• The execution times are printed in chronological order.
• If the cron expression is invalid, it returns an error message.

Example:

cron-parser next "*/15 9-17 * * 1-5" --count 3

Output:

2025-02-21 09:00:00
2025-02-21 09:15:00
2025-02-21 09:30:00

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2. Validation for a Specific Time

Command:

cron-parser check "<cron_expression>" "YYYY-MM-DD HH:MM"

Description:

• Given a cron expression and a specific datetime, it evaluates whether the cron job should execute at that moment.
• Returns true if the cron job matches the datetime, otherwise false.
• Supports all cron field syntax variations.
• Expects the datetime in YYYY-MM-DD HH:MM format.
• If the cron expression is invalid, it returns an error message.

Example:

cron-parser check "*/30 9-17 * * 1-5" "2025-02-21 09:30"

Output:

true

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Supported Cron Syntax

Symbol	Description
*	Wildcard, matches all values for a field
,	List separator, allows specifying multiple values
-	Range, allows specifying a range of values
/	Step, allows skipping values at a defined interval

Examples:

• */15 * * * * → Runs every 15 minutes.
• 0 9-17 * * 1-5 → Runs at the start of every hour between 9 AM and 5 PM, Monday to Friday.
• 30 8-18/2 * * * → Runs at 8:30, 10:30, 12:30, etc., up to 18:30 every day.

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Error Handling

• If the cron expression is invalid, the tool should return an appropriate error message.
• If the datetime format is incorrect, the tool should display a usage message and expected format.

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Implementation Considerations

• The tool should be implemented in Python, using the datetime module for date/time calculations.
• Consider using a library like croniter to handle cron parsing and evaluation.
• Ensure the tool runs efficiently, even for complex cron expressions.

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Future Enhancements

• Support for extended cron syntax (e.g., @hourly, @daily).
• Option to output times in different time zones.
• Ability to read cron expressions from a file.

Discussion

can use chain of responsibility but if failure occurs then its not collecting every parser info. If we parse every parser every time them we can collect that 2 parsers are wrong. parsers being minParse, hrParse, domParse, monthParse, dowParse.