Prepend a Single Line to File with Ruby

Prepend a single line to file with Ruby

This is a pretty common task:

original_file = './original_file'
new_file = original_file + '.new'

Set up the test:

File.open(original_file, 'w') do |fo|
  %w[something else].each { |w| fo.puts w }
end

This is the actual code:

File.open(new_file, 'w') do |fo|
  fo.puts 'hello'
  File.foreach(original_file) do |li|
    fo.puts li
  end
end

Rename the old file to something safe:

File.rename(original_file, original_file + '.old')
File.rename(new_file, original_file)

Show that it works:

puts `cat #{original_file}`
puts '---'
puts `cat #{original_file}.old`

Which outputs:

hello
something
else
---
something
else

You don't want to try to load the file completely into memory. That'll work until you get a file that is bigger than your RAM allocation, and the machine goes to a crawl, or worse, crashes.

Instead, read it line by line. Reading individual lines is still extremely fast, and is scalable. You'll have to have enough room on your drive to store the original and the temporary file.

can you create / write / append a string to a file in a single line in Ruby

Ruby has had IO::write since 1.9.3. Your edit shows you're passing the wrong args. The first arg is a filename, the second the string to write, the third is an optional offset, and the fourth is a hash that can contain options to pass to the open. Since you want to append, you'll need to pass the offset as the current size of the file to use this method:

File.write('some-file.txt', 'here is some text', File.size('some-file.txt'), mode: 'a')

Hoisting from the discussion thread:
This method has concurrency issues for append because the calculation of the offset is inherently racy. This code will first find the size is X, open the file, seek to X and write. If another process or thread writes to the end between the File.size and the seek/write inside File::write, we will no longer be appending and will be overwriting data.

If one opens the file using the 'a' mode and does not seek, one is guaranteed to write to the end from the POSIX semantics defined for fopen(3) with O_APPEND; so I recommend this instead:

File.open('some-file.txt', 'a') { |f| f.write('here is some text') }

Append line every n lines to file - ruby

Here are a couple options.

Option 1:

File.open('output.txt', 'w') do |outfile|
    File.foreach('input.txt').each_with_index do |line, i|
        outfile.puts(line)
        outfile.puts '--- 1000 ---' if (i + 1) % 1000 == 0 && i != 0
    end
end

This inserts the line '--- 1000 ---' after every 1000 lines from the original file. It has some drawbacks though. Mainly it has to check each index and check that we are not at line zero with every line! But it works. And it works on large files without hogging memory.

Option 2:

File.open('output.txt', 'w') do |outfile|
    File.foreach('input.txt').each_slice(1000) do |lines|
        outfile.puts(lines)
        outfile.puts '--- 1000 ---'
    end
end

This code does almost exactly the same thing using Enumerable's each_slice method. It yields an array of every 1000 lines, writes them out using puts (which accepts Arrays), then writes our marker line after it. It then repeats for the next 1000 lines. The difference is if the file isn't a multiple of 1000 lines the last call to this block will yield an array smaller than 1000 lines and our code will still append our line of text after it.

We can fix this by testing the array's length and only writing out our line if the array is exactly 1000 lines. Which will be true for every batch of 1000 lines except the last one (given a file that is not a multiple of 1000 lines).

Option 2a:

File.open('output.txt', 'w') do |outfile|
    File.foreach('input.txt').each_slice(1000) do |lines|
        outfile.puts(lines)
        outfile.puts '--- 1000 ---' unless lines.size < 1000
    end
end

This extra check is only needed if appending that line to the end of the file is a problem for you. Otherwise you can leave it out for a small performance boost.

Speaking of performance, here is how each option performed on a 335.5 MB file containing 1,000,000 paragraphs of Lorem Ipsum. Each benchmark is total time to process the entire file 100 times.

Option 1:

103.859825  44.646519 148.506344 (152.286349)
[Finished in 152.6s]

Option 2:

 96.249542  43.780160 140.029702 (145.210728)
[Finished in 145.7s]

Option 2a:

 98.041073  45.788944 143.830017 (149.769698)
[Finished in 150.2s]

As you can see, option 2 is the fastest. Keep in mind, options 2/2a will in theory use more memory since it loads 1000 lines at a time, but even then it's capped at a very small level so handling enormous files shouldn't be a problem. However they are all so close I would recommend going with whatever option reads the best or makes the most sense.

Hope this helped.

How to append a text to file succinctly

Yes. It's poorly documented, but you can use:

File.write('foo.txt', 'some text', mode: 'a+')

How to append a new line write to a file with ruby?

try this

def write_to_file(line, my_file)
  File.open(my_file, 'a') do |file|
     p '-----loop number:' + line.to_s
     file.puts "#{line}"
  end
end

[1,2,3,4].each do |line|
  write_to_file(line, my_file)
end

How to insert a line in the middle of a text file?

Think of file as a piece of paper. Once you've written something on it, the only safe way to add content is to append. You can't write text in the middle of the paper without overwriting something.

Generally, this kind of insertion is done via a temporary file. It goes like this:

1. Open source file source for reading
2. Create a new file target for writing
3. Read from source up to the point of insertion and write that to target
4. Write your new content to target
5. Copy the rest of source to target
6. Close the files
7. Delete/rename source
8. Rename target to what source was named.

Add a new line in file?

Use IO#puts.

file.puts @string

---

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