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Ruby OS Interaction

Ruby isn't just for shuffling numbers and text around — it can also reach out and poke the actual operating system: run commands, create files, check what machine it's running on, all that. Here's what that looks like, broken down simply.

Running commands like you would in a terminal

There are a few different ways to tell Ruby "hey, run this shell command for me":

output = `ls -la`        # backticks: run it, hand me back whatever it printed
puts output

Think of backticks like texting a friend "what's in your fridge?" and they text back the whole list.

success = system("mkdir test_dir")   # just tells you true/false: did it work?

This one doesn't care what the command printed — just whether it succeeded or not.

files = %x{find . -name "*.rb"}      # same idea as backticks, just different-looking syntax

require 'open3'
stdout, stderr, status = Open3.capture3("ls /nonexistent")

Open3 is the "grown-up" option — it separates the normal output, the error messages, and the exit code (a number that tells you if something failed) into three neat little boxes instead of mashing it all together.

Making and messing with files/folders

Dir.mkdir("new_folder") unless Dir.exist?("new_folder")
Dir.chdir("new_folder")
puts Dir.pwd   # "where am I right now?"

This says: "make a folder, but only if it doesn't already exist, then step inside it."

Dir.glob("*.txt").each { |file| puts file }

Translation: "find every file ending in .txt and print its name, one by one."

File.open("sample.txt", "w") { |f| f.write("Hello OS!") }
content = File.read("sample.txt") if File.exist?("sample.txt")

Open a file in "write" mode, put text in it, close it automatically. Then read it back — but only if it actually exists (so you don't crash trying to read something that isn't there).

File.chmod(0755, "sample.txt")   # sets who's allowed to read/write/run this file
puts File.size("sample.txt")     # how big is it, in bytes
puts File.mtime("sample.txt")    # when was it last changed

Environment variables and process stuff

An environment variable is just a little labeled setting your whole computer can see — like a sticky note the OS keeps handy.

puts ENV['PATH']          # read one
ENV['MY_VAR'] = "custom_value"   # create/change one

Every running program has an ID number (like a name tag) so the OS can keep track of it:

puts Process.pid    # "what's MY ID number?"
puts Process.ppid   # "what's the ID of whoever started me?"

Forking is a neat (and slightly weird) trick where a program basically clones itself into a second, independent copy:

if RUBY_PLATFORM !~ /mswin|mingw/   # skip this on Windows, it doesn't support forking
  pid = fork do
    puts "Child process: #{Process.pid}"
  end
  Process.wait(pid) if pid   # wait for the clone to finish before moving on
end

Imagine hitting "duplicate" on yourself, having your clone go do a task, and you just standing there waiting for them to finish.

Figuring out what computer you're on

require 'rbconfig'
puts RbConfig::CONFIG['host_os']    # e.g. "linux", "darwin" (that's mac), "mingw" (windows)
puts RbConfig::CONFIG['host_cpu']
puts RUBY_VERSION

You can even make your script behave differently depending on the OS it's running on:

case RbConfig::CONFIG['host_os']
when /mswin|mingw|cygwin/
  system("dir")        # Windows uses "dir" instead of "ls"
when /darwin|mac os/
  system("ls -la")     # Mac
when /linux/
  system("ps aux")     # Linux
end

It's like a script that speaks three different dialects and automatically picks the right one depending on where it wakes up.

Temporary, throwaway files

Sometimes you just need a scratch file that disappears when you're done — no cleanup headaches:

require 'tempfile'

temp = Tempfile.new('ruby_temp')
temp.write("Temporary data")
temp.close
puts temp.path   # where did Ruby put it?
temp.unlink      # delete it, all cleaned up

Like writing a note on a napkin, using it, then tossing the napkin — instead of a permanent notebook.

Why any of this matters

All of this is what makes Ruby genuinely good at system administration, automation, and DevOps work — scripts that create folders, run shell commands, check what machine they're on, and clean up after themselves, all without you touching the terminal by hand.

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