What are all the common ways to read a file in Ruby?

Be wary of "slurping" files. That's when you read the entire file into memory at once.

The problem is that it doesn't scale well. You could be developing code with a reasonably sized file, then put it into production and suddenly find you're trying to read files measuring in gigabytes, and your host is freezing up as it tries to read and allocate memory.

Line-by-line I/O is very fast, and almost always as effective as slurping. It's surprisingly fast actually.

I like to use:

IO.foreach("testfile") { |x| print "GOT ", x }

or

File.foreach('testfile') { |x| print "GOT", x }

File inherits from IO, and foreach is in IO, so you can use either.

I have some benchmarks showing the impact of trying to read big files via read vs. line-by-line I/O at "https://stackoverflow.com/q/25189262/128421".

The easiest way if the file isn't too long is:

puts File.read(file_name)

Indeed, IO.read or File.read automatically close the file, so there is no need to use File.open with a block.

You can read the file all at once:

content = File.readlines 'file.txt'
content.each_with_index{|line, i| puts "#{i+1}: #{line}"}

When the file is large, or may be large, it is usually better to process it line-by-line:

File.foreach( 'file.txt' ) do |line|
  puts line
end

Sometimes you want access to the file handle though or control the reads yourself:

File.open( 'file.txt' ) do |f|
  loop do
    break if not line = f.gets
    puts "#{f.lineno}: #{line}"
  end
end

In case of binary files, you may specify a nil-separator and a block size, like so:

File.open('file.bin', 'rb') do |f|
  loop do
    break if not buf = f.gets(nil, 80)
    puts buf.unpack('H*')
  end
end

Finally you can do it without a block, for example when processing multiple files simultaneously. In that case the file must be explicitly closed (improved as per comment of @antinome):

begin
  f = File.open 'file.txt'
  while line = f.gets
    puts line
  end
ensure
  f.close
end

References: File API and the IO API.

File.open("my/file/path", "r") do |f|
  f.each_line do |line|
    puts line
  end
end
# File is closed automatically at end of block

It is also possible to explicitly close file after as above (pass a block to open closes it for you):

f = File.open("my/file/path", "r")
f.each_line do |line|
  puts line
end
f.close