It’s finally time to start coding.

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puts "Hello World"

That’s it! Save it as hello_world.rb, exit, and now we can run the file. Note that # is the comment delimiter in Ruby and Shell so I will be using that to give you an idea of the expected result of the line preceding it.

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ruby hello_world.rb
# Hello World

Like shell scripts, you can also run them as executables. Create another file:

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#!/usr/bin/env ruby
puts "Hello World"

And give it executable permission, and run it.

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chmod +x hello_world_exe.rb
./hello_world_exe.rb

Hashbangs will be explained at the end of this post for those curious

Interactive Ruby Shell

irb is a tool that provides an interface to Ruby. It’s a great feature, especially for developers new to Ruby because it allows one to interact with Ruby for instant feedback, and build up to a complex script line by line. Code behaves exactly the same way in irb as it would in the method described above. So open up irb and write Hello World again….puts "Hello World"

You’ll notice that after printing Hello World, it also says => nil; this is the ‘null’ value in Ruby. It’s the value that is returned by the function, and in this case, the value returned by puts.

If you want to import files that you’ve written, there are two ways to do so. A file that is imported in irb is executed just as it would if it had been imported through normal code, and it is the same as if it had been copy and pasted into irb. The following demonstrates the two major difference between the two options so try this out in irb. Whereas require is your standard ‘import this library’, load is more like ‘run this bit of code’.

The first is subtle, but important. The argument to load is a path to the file. A file you load can be anywhere on the system, it accepts both relative and absolute paths. The argument to require is just the name of the file, and it will look for that file only in Ruby’s PATH. Since 1.9.2 however, your working directory is no longer included in it.

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puts $:
# /root/.rbenv/versions/2.0.0-p247/lib/ruby/site_ruby/2.0.0
# /root/.rbenv/versions/2.0.0-p247/lib/ruby/site_ruby/2.0.0/x86_64-linux
# /root/.rbenv/versions/2.0.0-p247/lib/ruby/site_ruby
# /root/.rbenv/versions/2.0.0-p247/lib/ruby/vendor_ruby/2.0.0
# /root/.rbenv/versions/2.0.0-p247/lib/ruby/vendor_ruby/2.0.0/x86_64-linux
# /root/.rbenv/versions/2.0.0-p247/lib/ruby/vendor_ruby
# /root/.rbenv/versions/2.0.0-p247/lib/ruby/2.0.0
# /root/.rbenv/versions/2.0.0-p247/lib/ruby/2.0.0/x86_64-linux

To fix this, we start irb with irb -I .

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puts $:
# /root/ruby-intro
# /root/.rbenv/versions/2.0.0-p247/lib/ruby/site_ruby/2.0.0
# /root/.rbenv/versions/2.0.0-p247/lib/ruby/site_ruby/2.0.0/x86_64-linux
# /root/.rbenv/versions/2.0.0-p247/lib/ruby/site_ruby
# /root/.rbenv/versions/2.0.0-p247/lib/ruby/vendor_ruby/2.0.0
# /root/.rbenv/versions/2.0.0-p247/lib/ruby/vendor_ruby/2.0.0/x86_64-linux
# /root/.rbenv/versions/2.0.0-p247/lib/ruby/vendor_ruby
# /root/.rbenv/versions/2.0.0-p247/lib/ruby/2.0.0
# /root/.rbenv/versions/2.0.0-p247/lib/ruby/2.0.0/x86_64-linux

Let’s try again.

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load 'hello_world.rb'
# Hello World
# => true
load 'hello_world.rb'
# Hello World
# => true

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require 'hello_world'
# Hello World
# => true
require 'hello_world'
# => false

The other difference is that require will not import a file more than once.

Hashbangs

You might have come across this in COMP2041. This is the line at the top of scripts that tells your shell what to execute it with – your shell will simply interpret it as a shell script without it.

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chmod +x hello_world.rb
./hello_world.rb
# ./hello_world.rb: line 1: puts: command not found

/usr/bin/env

The characters following the exclamation mark is the path of the executable that you want to run the script with.

Since you now have several different versions of Ruby installed, it’s best to make the script run with the version that you intend to. rbenv controls the Ruby version in your environment, so that’s the best one to use! By /usr/bin/env ruby, the script will be run with the same ruby as typing ruby in the command line. To demonstrate, create the 2 files below, and then enter the following commands (make note of the versions):

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#!/usr/bin/ruby
puts RUBY_VERSION

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#!/usr/bin/env ruby
puts RUBY_VERSION

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ruby -v
# ruby 2.0.0p247 (2013-06-27 revision 41674) [x86_64-darwin12.4.0]
/usr/bin/ruby -v
# ruby 1.8.7 (2012-02-08 patchlevel 358) [universal-darwin12.0]
chmod +x version_env.rb
./version_env.rb
# 2.0.0
chmod +x version_system.rb
./version_system.rb
# 1.8.7

1. Wikipedia on Hashbangs
2. Wikipedia on irb
3. Alan Skorkin on Require vs Load