File.exist? / directory? / basename / dirname / extname

# Check whether a file exists.
File.exist?("path")

# Check whether the path is a directory.
File.directory?("path")

# Check whether the path is a regular file.
File.file?("path")

# Get the filename from a path.
File.basename("path")
File.basename("path", "extension")  # Filename without the extension

# Get the directory part of a path.
File.dirname("path")

# Get the extension from a path.
File.extname("path")

path = "/home/user/documents/report.txt"

# Decompose the path.
puts File.basename(path)          # report.txt
puts File.basename(path, ".txt")  # report (without extension)
puts File.dirname(path)           # /home/user/documents
puts File.extname(path)           # .txt

# Check that the file exists before processing it.
filename = "sample.txt"
if File.exist?(filename)
  puts "The file exists."
  if File.file?(filename)
    puts "It is a regular file."
  end
else
  puts "The file does not exist."
end

# Use File.join to safely build a path.
directory = "/home/user"
file = "data.csv"
full_path = File.join(directory, "documents", file)
puts full_path  # /home/user/documents/data.csv

# Branch logic based on the file extension.
file_list = ["image.png", "doc.txt", "script.rb", "data.csv"]
file_list.each do |f|
  puts "#{f}: #{File.extname(f)}"
end

The class methods of the File class are specialized for working with file paths. Because they let you retrieve path information without actually opening a file, they are convenient for pre-processing checks.

Use File.join rather than string concatenation to build paths. Windows and Unix-like systems use different path separators, but File.join generates the correct path regardless of the environment. Use File.exist? instead of the older File.exists?, which was removed in Ruby 3.2.