Prepend a single line to file with Ruby

Question

I\'d like to add a single line to the top a of file with Ruby like this:

# initial file contents
something
else

# file contents after prepending \"hello\" o         


        

Answer 1

Pure, but works. Minimize file-related operations.

`#!/bin/ruby
inv_file = File.open("./1.txt","r+") { |f|
    #Read file string by-string
    until f.eof?
        #Searching for some
        if f.readline[/condition here/]
            #remember position
            offset = f.pos
            #Find it? - stop reading
            break
        end
    end
    #Remember what contains rest of file
    tail = f.readlines
    #Again to offset
    f.pos = offset
    #insert what necessary
    f.puts "INSERTED"
    #reconstruct tail
    f.write tail.join
    }`