Lucas Seiki Oshiro

Ruby: a great language for shell scripts!

4 minute read

Intro

Ruby is so associated with its most famous framework, Rails, that many people forget how amazing this language is. I mean, I know a lot of people who says “I don’t like Ruby” and when I ask why, they say something about Rails. Personally, I consider Ruby one of my favorite programming languages, and the last time I touched any Rails code was 7 years ago…

So, if I don’t use Rails anymore, what I do with Ruby? Well, Ruby is a very rich and complete language, perhaps even more than its more famous relative, Python (sadly, I can’t say the same about its ecosystem…). And one of the things that I think that Ruby is better than Python is using it for writing shell scripts.

That is, most of the cases Bash for me is enough, but if the script starts to become complex, I switch to Ruby. Here I show the main features that might be interesting for this case of use.

Goals

  • Show features of Ruby that are useful for writing shell scripts;

  • Compare Ruby to Bash and Python;

Non-goals

  • Replace entirely Bash scripts by Ruby scripts.

Feature 1: calling external commands

The first thing that you expect of language for writing shell scripts is to call external commands. In Ruby, you do that using backticks (`):

`ls`

That’s it! You don’t need system, popen or something like that, or import a library. And if you set that to a variable, you’ll have the output of the command:

my_date=`date`

Note: if you want to use system (e.g. if you want the output to be redirected to stdout instead of a string) or popen (if you want to read or write data from or to a subprocess), those are also available in Ruby!

Feature 2: status code

This is real quick: in Ruby, the variable $? contains the status code of the last executed command. So, it’s really close to Bash:

`true`
puts $? # 0

`false`
puts $? # 1

Feature 3: it’s a typed language

Ruby is not a statically typed language, but it has types. In fact, it is a object-oriented language, and it follow strictly the OOP paradigm (more than Python, in some aspects even more than Java!). Bash, on the other hand, everything is a string, and that leads to several safety issues…

total_lines = `wc -l my_file`.to_i # an int containing the number of lines of a file
half = total_lines.div 2           # integer division
puts `head -n #{half} my_file`     # print half of the file

Feature 4: functional constructions

Ruby implements map, select (filter), reduce, flat_map and other functional operations as methods. So, you can, for example, apply a map over a command output:

puts `ls`.lines.map { |name| name.strip.length } # prints the lengths of the filenames

Feature 5: regex matching

Regex is a type in Ruby, and operations using regex are built-in in the language. Look at this example, where we get the current git branch name calling git branch:

current_branch_regex = /^\* (\S+)/
output_lines = `git branch`.lines
output_lines.each do |line|
  if line =~ current_branch_regex # match the string with the regex
    puts $1                       # prints the match of the first group  
  end
end

Note for Git lovers: I know that I could do that only using git branch --show-current, but that was the first example that came in my mind to demonstrate the use of regex…

Feature 6: easy threads

If want to work with multiple threads, Ruby is perhaps the one of the easiest language to do it. Look:

thread = Thread.new do
  puts "I'm in a thread!"
end

puts "I'm outside a thread!"

thread.join

So, it can be useful for, for example, downloading several files at the same time:

(1..10).map do |i|                       # iterates from i=1 to i=10, inclusive
  Thread.new do
    `wget http://my_site.com/file_#{i}`  # you can use variables inside commands!  
  end
end.each { |thread| thread.join }        # do/end and curly braces have the same purpose!

Feature 7: builtin file and dir operations

In Ruby, all the file operations are methods of the File class and all the directory operations are methods of the Dir class, as it should be. In Python, for example, if you want to read a file you use open, but if you want to delete it you need to use os.remove, and os does a lot of other things that are not related to files.

So, in Ruby:

exists = File.exist? 'My File'           # methods that return booleans end in ?
file_content = File.open('My File').read
File.delete 'My File'                     # parentheses are optional if it's not ambiguous

Conclusion

I hope that after reading this short text you consider using Ruby as a replacement for complex shell scripts. I mean, I don’t expect that you drop Bash entirely, but consider using Ruby when things get complex. Of course, you can do that in Python, Perl, even JS, but, as my personal choice I think that Ruby is the most complete and easier Bash replacement for that!

If you find something wrong, or if you have any suggestion, please let me know here.

Update

This reached #1 on Hacker News! Not only that, but it started some interesting discussions in the comments. Thanks everyone!

#1 on HN!

Update #2

Somehow this reached Matz, the creator of Ruby! And this was tweeted by him! I… can’t believe it!

Updated: