with_csv.rb

require "csv"

filename, column_name, replacement, output = *ARGV[0..3]
csv = begin
        CSV.read(filename, headers: true)
      rescue
        puts "input file missing"
        exit 1
      end

unless csv.headers.include?(column_name)
  puts "column name doesn’t exist in the input file"
  exit 1
end

CSV.open(output, "wb") do |out|
  out << csv.headers
  csv.each do |row|
    row[column_name] = replacement
    out << row
  end
end

without_csv.rb

filename, column_name, replacement, output = *ARGV[0..3]
csv = begin
        File.read(filename).split("\n")
      rescue
        puts "input file missing"
        exit 1
      end
headers = csv.shift
column_index = headers.split(?,).index(column_name)

if column_index.nil?
  puts "column name doesn’t exist in the input file"
  exit 1
end

out = csv.map { |row|
  cols = row.split(?,)
  cols[column_index] = replacement
  cols.join(?,)
}
out.unshift(headers)

File.write(output, out.join("\n"))