Command Line Flags in Ruby

Here’s the translation of the Go code to Ruby, with explanations in Markdown format suitable for Hugo:

Our example program demonstrates how to use command-line flags in Ruby. Command-line flags are a common way to specify options for command-line programs. For example, in wc -l the -l is a command-line flag.

Ruby provides an optparse library for parsing command-line options. We’ll use this library to implement our example command-line program.

require 'optparse'

# Define default values
options = {
  word: 'foo',
  numb: 42,
  fork: false,
  svar: 'bar'
}

# Parse command-line options
OptionParser.new do |opts|
  opts.banner = "Usage: command_line_flags.rb [options]"

  # Basic flag declarations are available for string, integer, and boolean options.
  # Here we declare a string flag 'word' with a default value 'foo' and a short description.
  opts.on("-w", "--word STRING", "a string") do |w|
    options[:word] = w
  end

  # This declares 'numb' and 'fork' flags, using a similar approach to the 'word' flag.
  opts.on("-n", "--numb INTEGER", Integer, "an int") do |n|
    options[:numb] = n
  end

  opts.on("-f", "--fork", "a bool") do |f|
    options[:fork] = f
  end

  # It's also possible to declare an option that uses an existing var declared elsewhere in the program.
  opts.on("-s", "--svar STRING", "a string var") do |s|
    options[:svar] = s
  end
end.parse!

# Here we'll just dump out the parsed options and any trailing positional arguments.
puts "word: #{options[:word]}"
puts "numb: #{options[:numb]}"
puts "fork: #{options[:fork]}"
puts "svar: #{options[:svar]}"
puts "tail: #{ARGV}"

To experiment with the command-line flags program, save it as command_line_flags.rb and run it with various options:

$ ruby command_line_flags.rb -w opt -n 7 -f -s flag
word: opt
numb: 7
fork: true
svar: flag
tail: []

Note that if you omit flags they automatically take their default values:

$ ruby command_line_flags.rb -w opt
word: opt
numb: 42
fork: false
svar: bar
tail: []

Trailing positional arguments can be provided after any flags:

$ ruby command_line_flags.rb -w opt a1 a2 a3
word: opt
numb: 42
fork: false
svar: bar
tail: ["a1", "a2", "a3"]

Use -h or --help flags to get automatically generated help text for the command-line program:

$ ruby command_line_flags.rb -h
Usage: command_line_flags.rb [options]
    -w, --word STRING                a string
    -n, --numb INTEGER               an int
    -f, --fork                       a bool
    -s, --svar STRING                a string var

If you provide a flag that wasn’t specified to the OptionParser, the program will print an error message:

$ ruby command_line_flags.rb -x
command_line_flags.rb: invalid option: -x

This example demonstrates how to use Ruby’s optparse library to handle command-line flags, providing a flexible way to configure your Ruby programs from the command line.

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Command Line Flags in Ruby