Ruby: Append Text to the 2Nd Line of a File

Ruby: Append text to the 2nd line of a file

You write a new file, applying your change at the desired line, then rename result back to the original file name. The following method will copy the file and yield the output file object to a block at the correct line, so that block can output your new lines.

def insert_lines_following_line file, line_no
  tmp_fn = "#{file}.tmp"
  File.open( tmp_fn, 'w' ) do |outf|
    line_ct = 0
    IO.foreach(file) do |line|
      outf.print line
      yield(outf) if line_no == (line_ct += 1)
    end
  end
  File.rename tmp_fn, file
end

insert_lines_following_line( "#{curDir}/Backup_Times.csv", 1 ) do |outf|
  # output new csv lines in this block
  outf.puts ['foo','bar','baz',1,2,3].join(",") # or however you build your csv line
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.

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+')

Delete first two lines and add two lines to file

I'd start with something like this:

NEWLINES = {
  0 => "New Title",
  1 => "\nfff"
}

File.open('test.txt.new', 'w') do |fo|
  File.foreach('test.txt').with_index do |li, ln|
    fo.puts (NEWLINES[ln] || li)
  end
end

Here's the contents of test.txt.new after running:

New Title

fff
aaa
bbb
ccc

The idea is to provide a list of replacement lines in the NEWLINES hash. As each line is read from the original file the line number is checked in the hash, and if the line exists then the corresponding value is used, otherwise the original line is used.

If you want to read the entire file then substitute, it reduces the code a little, but the code will have scalability issues:

NEWLINES = [
  "New Title",
  "",
  "fff"
]

file = File.readlines('test.txt')
File.open('test.txt.new', 'w') do |fo|
  fo.puts NEWLINES
  fo.puts file[(NEWLINES.size - 1) .. -1]
end

It's not very smart but it'll work for simple replacements.

If you really want to do it right, learn how diff works, create a diff file, then let it do the heavy lifting, as it's designed for this sort of task, runs extremely fast, and is used millions of times every day on *nix systems around the world.

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

Add a new line in file?

Use IO#puts.

file.puts @string

How to add new line in a file

Reading and writing a file at the same time can get messy, same thing with other data structures like arrays. You should build a new file as you go along.

Some notes:

• you should use the block form of File.open because it will stop you from forgetting to call f.close
• puts nil is the same as puts without arguments
• single quotes are preferred over double quotes when you don’t need string interpolation
• you should use do ... end instead of { ... } for multi-line blocks
• File.open(...).each can be replaced with File.foreach
• the intermediate result can be stored in a StringIO object which will respond to puts etc.

Example:

require 'stringio'

file = 'test.txt'
output = StringIO.new

File.foreach(file) do |line|
  if line.include? '2014-10'
    output.puts
  else
    output << line
  end
end

output.rewind

File.open(file, 'w') do |f|
  f.write output.read
end

How do I add a line after another line in a file, in Ruby?

Assuming you want to do this with the FileEdit class.

Chef::Util::FileEdit.new('/path/to/file').insert_line_after_match(/three/, 'four')

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