Deleting a specific line in a text file?

Question

How can I delete a single, specific line from a text file? For example the third line, or any other line. I tried this:

line = 2
file = File.open(filename, 'r+')
file.each { last_line = file.pos unless file.eof? }
file.seek(last_line, IO::SEEK_SET)
file.close

Unfortunately, it does nothing. I tried a lot of other solutions, but nothing works.

Answer 1

I think you can't do that safely because of file system limitations.

If you really wanna do a inplace editing, you could try to write it to memory, edit it, and then replace the old file. But beware that there's at least two problems with this approach. First, if your program stops in the middle of rewriting, you will get an incomplete file. Second, if your file is too big, it will eat your memory.

file_lines = ''

IO.readlines(your_file).each do |line|
  file_lines += line unless <put here your condition for removing the line>
end

<extra string manipulation to file_lines if you wanted>

File.open(your_file, 'w') do |file|
  file.puts file_lines
end

Something along those lines should work, but using a temporary file is a much safer and the standard approach

require 'fileutils'

File.open(output_file, "w") do |out_file|
  File.foreach(input_file) do |line|
    out_file.puts line unless <put here your condition for removing the line>
  end
end

FileUtils.mv(output_file, input_file)

Your condition could be anything that showed it was the unwanted line, like, file_lines += line unless line.chomp == "aaab" for example, would remove the line "aaab".