Working with Files

Learning Goals

  • Recognize the significance of files in modern computing systems
  • Be able to create a new file from the command line, our editor, and ruby
  • Be able to read data from a file from the command line, our editor, and ruby
  • Be able to write data to a file from the command line, our editor, and ruby
  • Be able to read command-line arguments from within a ruby program using ARGV


  • 5 - Warmup
  • 15 - Understanding Terms
  • 20 - Exercising Raw Files
  • 5 - Progress Checks & Questions


  • Unix Command
  • File I/O (Input/Output)
  • Argument Vector (ARGV)


Spend 5 minutes answering the following questions:

  1. What Unix commands (command line commands) do you use for creating/reading/writing files?
  2. Name 3 kinds of files you’ve worked with in the past.
  3. Why would you want to read files, especially text files, in a Ruby application?
  4. What other kinds of files might you want to read from a program?

Reading & Writing to Files

Interacting with Files from the Command Line

Creating an Empty File

  • Using touch from the command line
    Creates a file in your present working directory(pwd).
touch <filename>

Reading an Existing File

  • Using cat in your terminal
    Prints out the contents of your file.
cat <filename>

Writing Data to a File

  • Using echo to add content from the command line
echo "test" > <filename>
  • Using cat to add content from the command line
cat > <filename>

Interacting with Files in a Ruby Program

From Pry or IRB

Creating an Empty File

  • Using Ruby and
    Creates a Ruby File object that is “writable.”<filename>, "w")

Reading an Existing File

  • Using Ruby
    Creates a Ruby File object that is “readable.”
file ='<filename>', "r") 

Writing Data to a File

  • Using Ruby and File.write() in Pry, IRB, or your Ruby file.
new_file ='<filename>', "w")
new_file.write("all the text you want")

Command-line Arguments and ARGV

Working with files represents 1 common way our ruby programs will interact with the system environment.

Another common interaction involves reading “Command Line Arguments”

So far we’ve basically used our command line as a way to tell our computer to run various ruby files. I enter this command, this file is run. However, it might be more useful to think of it as a communication tool in which the developer can communicate with their computer, their ruby program, etc.

Reading Arguments from a Ruby Program

  • ARGV - Argument Vector is basically a special array
  • Arguments from the command line are provided as strings in the ARGV array
  • Arguments are separated by spaces in the command line
  • ARGV is a “constant” and is globally accessible from anywhere in a ruby program

From the command line:

ruby colors.rb red.txt blue.txt

In your file:

ARGV == ["red.txt", "blue.txt"]
ARGV[0] == "red.txt"
ARGV[1] == "blue.txt"


Work with a partner next to you to do the following:

Use cat to create a quiet_quotes.txt from the command line and paste the following into it:

Bob Ross Quotes
“We don't make mistakes, just happy little accidents.”
“Talent is a pursued interest. Anything that you're willing to practice, you can do.”
“There's nothing wrong with having a tree as a friend.”
“I guess I’m a little weird. I like to talk to trees and animals. That’s okay though; I have more fun than most people.”
“All you need to paint is a few tools, a little instruction, and a vision in your mind.”
“The secret to doing anything is believing that you can do it. Anything that you believe you can do strong enough, you can do. Anything. As long as you believe.”
“I started painting as a hobby when I was little. I didn't know I had any talent. I believe talent is just a pursued interest. Anybody can do what I do.”
“Mix up a little more shadow color here, then we can put us a little shadow right in there. See how you can move things around? You have unlimited power on this canvas -- can literally, literally move mountains”
“Let's build a happy little cloud. Let's build some happy little trees.”

Using ARGV, write a Ruby script that will take in the above file and write out an all caps version to loud_quotes.txt.

The program must be executed from the command line like so:

ruby <filename.rb> quiet_quotes.txt loud_quotes.txt

Additional Resources

Reading/Writing to Files with Ruby Video More on cat

Lesson Search Results

Showing top 10 results