Files, JSON, CSV and Time

Everyday scripting tasks

Ruby is excellent for scripts that process files and data. File.read and File.write handle small files; File.foreach(path) streams line by line for large ones; and the block form File.open(path, "w") { |f| ... } closes the file automatically, even on errors. Dir.glob("data/*.csv") lists files and FileUtils copies and moves them. Pathname offers an object-oriented path API. The json library parses with JSON.parse(text, symbolize_names: true) and generates with to_json or JSON.pretty_generate. The csv library (a bundled gem since Ruby 3.4, so add it to your Gemfile) reads CSV with headers (CSV.foreach(path, headers: true)) and writes it safely with quoting. For dates and times, use Time (with time zones via Time.now.utc, getlocal("+05:30") or the tzinfo gem), Date from the date library, and ISO 8601 strings for interchange (Time.now.utc.iso8601). Use Process.clock_gettime(Process::CLOCK_MONOTONIC) for measuring durations, not wall-clock time.

Scripting with files

Ruby scripts read files, transform data and write results in a few readable lines.

Summarising a CSV and writing JSON

Streaming rows, grouping and writing a report.

require "csv"
require "json"
require "time"

totals = Hash.new(0)
counts = Hash.new(0)

CSV.foreach("data/orders.csv", headers: true) do |row|     # streams one row at a time
  city = row["city"].strip.capitalize
  totals[city] += Integer(row["total_paise"])
  counts[city] += 1
end

report = totals.map do |city, paise|
  { city:, orders: counts[city], revenue_rupees: (paise / 100.0).round(2) }
end.sort_by { -_1[:revenue_rupees] }

File.write("output/revenue.json", JSON.pretty_generate({
  generated_at: Time.now.utc.iso8601,
  cities: report
}))

Dir.glob("data/*.csv").each { |path| puts "#{File.basename(path)}: #{File.size(path)} bytes" }

Integer() fails loudly, to_i does not

"12abc".to_i silently returns 12, and "abc".to_i returns 0. Integer("abc") raises an error. Use Integer() and Float() when parsing data that must be valid.

त्वरित जाँच: Why use the block form File.open(path, "w") { |f| ... }?

• It is faster
• It encrypts the file
• The file is closed automatically when the block ends, even if an error occurs
• It is the only way to write files