How to Use File.readlines in Ruby: Simple Guide

In Ruby, File.readlines reads all lines from a file and returns them as an array of strings. Each element in the array represents one line from the file, including the newline character at the end.

Syntax

The basic syntax of File.readlines is simple:

• File.readlines(filename, chomp: true/false)

Here, filename is the path to the file you want to read.

The optional chomp: keyword argument removes newline characters if set to true.

ruby

lines = File.readlines('example.txt', chomp: true)

Example

This example reads all lines from a file named example.txt and prints each line without the newline character.

ruby

File.write('example.txt', "Hello\nWorld\nRuby\n")
lines = File.readlines('example.txt', chomp: true)
lines.each do |line|
  puts line
end

Output

Hello
World
Ruby

Common Pitfalls

One common mistake is forgetting that File.readlines includes newline characters at the end of each line by default. This can cause unexpected output when printing or processing lines.

Another pitfall is not handling file not found errors, which will raise an exception if the file does not exist.

ruby

begin
  lines = File.readlines('missing.txt')
rescue Errno::ENOENT
  puts 'File not found!'
end

# Wrong way (includes newlines):
lines = File.readlines('example.txt')
puts lines

# Right way (remove newlines):
lines = File.readlines('example.txt', chomp: true)
puts lines

Output

File not found!
Hello
World
Ruby
Hello
World
Ruby

Quick Reference

Feature	Description
File.readlines(filename)	Reads all lines into an array with newlines included
File.readlines(filename, chomp: true)	Reads all lines and removes trailing newlines
Returns	Array of strings, each string is a line from the file
Raises	Errno::ENOENT if file does not exist
Use case	Quickly read entire file line by line for processing

Key Takeaways

File.readlines reads all lines from a file into an array of strings.

Use the chomp: true option to remove newline characters from each line.

Handle file not found errors to avoid program crashes.

Each array element corresponds to one line in the file.

File.readlines is useful for simple line-by-line file reading.