Cron Expression Parser

A simple cron expression parser implemented in Python.

Instructions

In order to run the parser, make sure that you are in the root directory of the project and have python 3.8 installed.
The program takes in a single argument encapsulated in a string:
python main.py "<cron string here>"

The argument should contain a cron string of the format: <minute> <hour> <day of month> <month> <day of week> <command>

If run with valid input, the program will print out a table describing the expression on a per attribute basis.

Running the following command */15 0 1,15 * 1-5 /usr/bin/find will produce the following output:

minute: 0 15 30 45  
hour: 0  
day of month: 1 15  
month: 1 2 3 4 5 6 7 8 9 10 11 12  
day of week: 1 2 3 4 5  
command: /usr/bin/find

The characters and values each field can contain are as thus:
minute: , - * / with the values 0-59 inclusive
hour: , - * / with the values 0-23 inclusive
day of month: , - * / ? W with the values 1-31 inclusive
month: , - * / with the values 1-12 inclusive or JAN, FEB, MAR, APR, MAY, JUN, JUL, AUG, SEP, OCT, NOV, DEC
day of week: , - * ? / with the values 1-7 inclusive or SUN, MON, TUE, WED, THU, FRI, SAT

Tests

Unit tests for the parser have been written to ensure it works as expected. These can be run with the following command: python -m unittest

Todo

• Implement remaining cron expressions such as # L C
• Use a date package to validate day of month input, i.e. 31st February would be invalid
• Add more in depth unit tests in order to ensure that exceptions are being raised correctly

For a more information and examples of cron expressions, please visit CRON Expression wiki