Ruby File Handling

A file is a named location used for storing data. For example, main.rb is a file that is always used to store Ruby code.

In Ruby, you can read from, write to, and manipulate files using the built-in File class. This process is known as File Handling.

Opening a File in Ruby

We need to open a file first to perform any operations on it. For this, we use the File.open method. Let's look at an example:

Suppose we have a file named file1.txt.

We can open this file using the File.open method:

my_file = File.open("file1.txt", "r")

Here,

• file1.txt - The name of the file we want to open.
• r - Open the file for reading.

This code creates a File object named my_file. Now, we can use this object to work with files.

my_file = File.open("file1.txt")

# Open a file in default mode (read)
# Equivalent to open("test.txt", "r")
my_file = File.open("test.txt")

# Open a file in write mode 
my_file = File.open("test.txt", "w")  

# Open a file in read and write mode
my_file = File.open("test.txt", "r+")

file_path = "/home/user/documents/file1.txt"
my_file = File.open(file_path)

Reading Files in Ruby

Ruby offers several ways to read files. Let's look at two common methods to read it:

1. Using File objects.

Here, we first create a File object like we previously learned. Then, we use the each_line method with a do...end block to print the file contents line by line. For example,

# Create a File object to open a file
my_file = File.open("file1.txt")

# Read the file line by line
my_file.each_line do |line|
  puts line
end

# Close the file
my_file.close

Here, line is a placeholder variable in the loop. In each iteration, the line variable takes a single line from the file and moves to the next line in the following iteration.

Important! Once you create a File object such as my_file, you must close it after you've completed all file operations. This frees your system resources and also ensures data is safely written to the disk.

2. Using a block (auto-closes the file).

You can also use a block where you open the file when defining the loop itself. For example,

File.open("file1.txt", "r") { |file| puts file.read }

Here, file is a placeholder variable that represents the File object we just opened. We then use the read method with the placeholder to read the file.

Important! This code loads the entire contents of the file into memory as a single string. Thus, while this code is easy to read, it's not ideal for large files.

Instead, you can read the file line by line using each_line:

File.open("file1.txt", "r") do |file|
  file.each_line { |line| puts line }
end

In each iteration of this code, the line placeholder stores a single line of the file and moves to the next line in the following iteration.

Note: Using a block is safer and more idiomatic in Ruby, as the file is closed automatically when the block finishes.

Other File Reading Methods

Apart from the read method (which reads the whole file), you can also use the following methods as per your needs:

• readline - Reads one line of the file.
• readlines - Reads all lines into an array, where each line is an element in the array.

Let's use these methods to read the following file:

1. Using the readline method.

my_file = File.open("file1.txt", "r")

puts my_file.readline    # Output: This is a test file.
puts my_file.readline    # Output: Hello from test file.

Here, readline reads a single line of the file and moves the file pointer to the next line. So, when you use readline again, it prints the next line.

2. Using the readlines method.

my_file = File.open("file1.txt", "r")

puts my_file.readlines

# Output:
# This is a test file.
# Hello from test file.

As you can see, readlines reads the entire file.

Difference between read and readlines.

The read method returns the file contents as a single string. For example, read returns the contents of file1.txt as:

# String returned after using 'read' for file1.txt
"This is a test file.\nHello from test file."

On the other hand, the readlines method returns an array, where each line is an element of the array. For example, readlines returns the contents of file1.txt as:

# Array returned after using 'readlines' for file1.txt
["This is a test file.", "Hello from test file."]

Writing to a File

To write to a file, we open the file in "w" (writing) mode and use the write method inside a do loop. Let's try this with our file1.txt file:

File.open("file1.txt", "w") do |file|
  file.write("My text file!")
end

This code creates (or overwrites) file1.txt and writes the string "My text file!" to the file.

Important! "w" will overwrite the file if it already exists.

Since file1.txt already exists, all previous content will be replaced with the new text content inside the write method. So, our file now looks like this:

Appending to a File

We open a file in "a" (append) mode to add text to the end of an existing file. Let's see how we can do this using our original file1.txt file:

Then, we can add the sentence "My text file!" using append mode like this:

File.open("file1.txt", "a") do |file|
  file.puts("My text file!")
end

Here, file.puts adds new content to the file (along with a new line at the end) without erasing the original content. So, our file now looks like this:

You can see that it attached the new text right at the end of the original file content.

To fix this, add a newline character "\n" at the beginning of the appended text:

File.open("file1.txt", "a") do |file|
  file.puts("\nMy text file!")
end

As you can see, writing "\nMy text file!" shifted our appended text to the next line:

Checking if a File Exists

We can use the exist? method to check if a file is present:

if File.exist?("file1.txt")
  puts "File found!"
else
  puts "File not found."
end

# Try to open the file inside the begin statement
begin
  File.open("file1.txt") do |file|
    puts "File found!"
  end

# If file doesn't exist, print a message
rescue Errno::ENOENT
  puts "File not found."

# Code to execute if there are any other errors
rescue StandardError => e
  puts "An error occurred: #{e.message}"
end

Renaming and Deleting a File

Rename a File

We can rename a file using the rename method:

File.rename("file1.txt", "renamed_file.txt")

Here, we renamed file1.txt to renamed_file.txt.

Delete a File

We can delete a file using the delete method:

File.delete("file1.txt")

Here, we used the delete method to remove file1.txt.

Things to Remember in Ruby File Handling

• Always close the File object using the close method (unless you're using blocks, which closes the file automatically after the operation).
• Use blocks with File.open for safer, cleaner code.
• Handle exceptions (e.g., file not found) using begin...rescue for robust applications.
• Choose the right mode to avoid accidentally deleting or overwriting data (e.g., use append mode "a" instead of write mode "w" to add new content to an existing file).