Best practices with STDIN in Ruby?

Question

I want to deal with the command line input in Ruby:

> cat input.txt | myprog.rb
> myprog.rb < input.txt
> myprog.rb arg1 arg2 arg3 ...

What is the best way to do it? In particular I want to deal with blank STDIN, and I hope for an elegant solution.

#!/usr/bin/env ruby

STDIN.read.split("\n").each do |a|
   puts a
end

ARGV.each do |b|
    puts b
end

Answer 1

I am not quite sure what you need, but I would use something like this:

#!/usr/bin/env ruby

until ARGV.empty? do
  puts "From arguments: #{ARGV.shift}"
end

while a = gets
  puts "From stdin: #{a}"
end

Note that because ARGV array is empty before first gets, Ruby won't try to interpret argument as text file from which to read (behaviour inherited from Perl).

If stdin is empty or there is no arguments, nothing is printed.

Few test cases:

$ cat input.txt | ./myprog.rb
From stdin: line 1
From stdin: line 2

$ ./myprog.rb arg1 arg2 arg3
From arguments: arg1
From arguments: arg2
From arguments: arg3
hi!
From stdin: hi!

Answer 2

Something like this perhaps?

#/usr/bin/env ruby

if $stdin.tty?
  ARGV.each do |file|
    puts "do something with this file: #{file}"
  end
else
  $stdin.each_line do |line|
    puts "do something with this line: #{line}"
  end
end

Example:

> cat input.txt | ./myprog.rb
do something with this line: this
do something with this line: is
do something with this line: a
do something with this line: test
> ./myprog.rb < input.txt 
do something with this line: this
do something with this line: is
do something with this line: a
do something with this line: test
> ./myprog.rb arg1 arg2 arg3
do something with this file: arg1
do something with this file: arg2
do something with this file: arg3

Answer 3

This was just things I found in my collection of obscure ruby.

So, in Ruby, a simple no-bells implementation of the unix command cat would be:

#!/usr/bin/env ruby
puts ARGF.read

ARGF is your friend when it comes to input; it is a virtual file that gets all input from named files or all from STDIN.

ARGF.each_with_index do |line, idx|
    print ARGF.filename, ":", idx, ";", line
end

# print all the lines in every file passed via command line that contains login
ARGF.each do |line|
    puts line if line =~ /login/
end

Thank goodness we didn’t get the diamond operator in Ruby, but we did get ARGF as a replacement. Though obscure, it actually turns out to be useful. Consider this program, which prepends copyright headers in-place (thanks to another perlism, -i) to every file mentioned on the command-line

#!/usr/bin/env ruby -i

Header = DATA.read

ARGF.each_line do |e|
  puts Header if ARGF.pos - e.length == 0
  puts e
end

__END__
#--
# Copyright (C) 2007 Fancypants, Inc.
#++

Credits to: http://www.oreillynet.com/ruby/blog/2007/04/trivial%5Fscripting%5Fwith%5Fruby.html#comment-565558 and http://blog.nicksieger.com/articles/2007/10/06/obscure-and-ugly-perlisms-in-ruby

Answer 4

Using some of the stuff that was listed on this page, I put together a post with some code snippets. This should answer the question sufficiently: http://eric.lubow.org/2010/ruby/multiple-input-locations-from-bash-into-ruby/