croon is a CLI written in rust and provides provides parsing and formatting of standard cron format with five time fields plus a command:

* * * * * command
- - - - -
| | | | |
| | | | +----- day of week    (0 - 6)  (Sunday=0)
| | | +------- month          (1 - 12)
| | +--------- day of month   (1 - 31)
| +----------- hour           (0 - 23)
+------------- minute         (0 - 59)

• Install rustup

cargo run "*/10 0 1,15 * 1-5 /usr/bin/find"

cargo test

croon consists of two main parts:

• CLI entry point
• Library to parse cron expressions

It has two external dependencies for development convenience: nom, linked_hash_set and lazy_static.

croon expects only a single input argument and it parses the argument as in the format of minute hour day_of_month month day_of_week command. Everything except command MUST be a valid cron expression otherwise parsing will fail. command is a "free-text" area. Users can write valid/invalid commands and it is not the responsibility of croon to check whether the command could run.

Parser type in parser.rs is just a thin wrapper around cron_table parser combinator. The type is not really necessary to have but it provides a logical distinction between a parser and cron table.

CronTab in cron_table.rs is instantiated by Parser when the parsing is successful. from_cron_expression_list function is a constructor-like function that converts all parsed CronExpressions to lists of u32 values.

When the input argument is parsed successfully, CLI produces an output according the requirement of a table with the field taking the first 14 columns and the times as a space-separated list.

minute        0 10 20 30 40 50
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"

If there is any failure during the parsing, croon will write to stderr and exits with code 1.

main.rs is the entry point of cron. It is just a thin wrapper around the library and does a few basic checks for error handling and formats crontab output as requested.

Library is where the main functionality of croon is implemented. It uses nom to combine multiple parsers and parses cron expressions according to the standard cron format. There are only tests in the parser which could be considered as functional tests.

Error type is created to enable FromStr trait on CronTab type. It does not do a good job about giving details of the error ocurred during parsing or validation.

The purpose of using linked_hash_set dependency is solely to have HashSet that preserves the order of insertion.

As a software engineer, I didn't have any chance to use parsers or parser combinators during my career. However, since I started playing with rust, I found myself implementing parsers for the most of the projects that I delivered. I find rust really comfortable to build parsers because of the ergonomics it provides with enums (union type functionality but in the form of algebraic data type), powerful pattern matching and absence of null.

Licensed under MIT license (LICENSE or http://opensource.org/licenses/MIT)