Build your own Crontab-tool

Implementation of the https://codingchallenges.fyi/challenges/challenge-cron/ using Scala

Implementation Details

The parser itself is using a Monadic Parser, based on Graham Hutton and Erik Meijer paper https://people.cs.nott.ac.uk/pszgmh/monparsing.pdf although a simplified version at this point, I am not using a StateMonad which could be nice to have to be more detailed when reporting parser errors.

Features

Providing the crontab expression (ie: */15 0 1,15 * 1-5 /usr/bin/find)

• Validates the crontab
• Details an explanation on shape of a table
• Displays a human-readable explanation
• Displays the next 5 instances of the cron

How To Run

Requirements

This is a Scala project, built using sbt, so ideally we will require the sbt to be installed. It can easily be installed using sdkman or homebrew, more details in https://www.scala-sbt.org/1.x/docs/Installing-sbt-on-Mac.html

#SDKMAN!
sdk install java $(sdk list java | grep -o "\b8\.[0-9]*\.[0-9]*\-tem" | head -1)
sdk install sbt

# Homebrew
brew install sbt

Running The Tests

Once SBT is installed, to run the tests suite, simply use sbt from the root directory of the project.

sbt test

The command will build and run the tests

Running the App with SBT

An option to run the application can be using directly sbt, for that we will need to scape the string argument.

sbt "run \"*/15 0 1,15 * 1-5 /usr/bin/find\""

cronTabTool */15 0 1,15 * 1-5 /usr/bin/find

minutes       0 15 30 45
hours         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

Every 15 minutes, at 0 hours, Monday through Friday

Friday, 1 November 2024, 00:00:00 Greenwich Mean Time
Friday, 1 November 2024, 00:15:00 Greenwich Mean Time
Friday, 1 November 2024, 00:30:00 Greenwich Mean Time
Friday, 1 November 2024, 00:45:00 Greenwich Mean Time
Friday, 15 November 2024, 00:00:00 Greenwich Mean Time