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

You can try this:

File.copy_stream(myfile,tempfile)
f = File.open(myfile,'w')
f.write("Hello\n#{File.open(tempfile,'r').read}")
f.close
File.delete(tempfile)

Answer 2

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.

Answer 3

No mechanism exists to do what you want to do easily.

Instead, you will need to open the file, delete the file, open a new file by the old name for writing, write your content, and then write the new file from the old file's contents. That's pretty convoluted sounding but is straightforward in code:

$ cat prepend.rb 
#!/usr/bin/ruby

File.open("passwd", "r") do |orig|
    File.unlink("passwd")
    File.open("passwd", "w") do |new|
        new.write "test string"
        new.write(orig.read())
    end
end

Note that the mechanism I've used doesn't bother checking errors -- you should probably handle errors on each File.open() request and the File.unlink() request -- and it assumes that the entire contents of the file will fit in memory. A short example:

$ rm passwd
$ cp /etc/passwd .
$ ./prepend.rb 
$ head passwd
test stringroot:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/bin/sh
bin:x:2:2:bin:/bin:/bin/sh
sys:x:3:3:sys:/dev:/bin/sh
sync:x:4:65534:sync:/bin:/bin/sync
games:x:5:60:games:/usr/games:/bin/sh
man:x:6:12:man:/var/cache/man:/bin/sh
lp:x:7:7:lp:/var/spool/lpd:/bin/sh
mail:x:8:8:mail:/var/mail:/bin/sh
news:x:9:9:news:/var/spool/news:/bin/sh
$ 

If you want to handle files that might not fit entirely in memory, you should code a loop sort of like this (untested -- better consider it pseudo-code):

while (data=orig.read(4096)) {
    new.write(data)
}

As an alternative, you could write to a temporary file, and if the writing process succeeds, then delete the file and rename the temporary file in place. Whichever approach makes the most sense to you.

Answer 4

As some have said, probably don’t use this for larger files, but it is a simple start.

rd = IO.read 'myfile'
IO.write 'myfile', "hello\n" + rd

Answer 5

fwiw this seems to work:

#!usr/bin/ruby

f = File.open("myfile", "r+")
lines = f.readlines
f.close

lines = ["something\n"] + lines

output = File.new("myfile", "w")
lines.each { |line| output.write line }
output.close

Answer 6

From the command line, you can do:

ruby -i -pe '$_= "prepended text\n"+$_ if $. == 1' myfile

or more efficiently

ruby -i -pe 'BEGIN { gets; print "prepended text\n" + $_ }; ' myfile

Sadly, it turns out, the -i (in-place) options isn't truly in-place, though (and nor is seds in-place option for that matter) -- my file will have a different inode after the op.

That made me sad, because effectively, I can't filter through (prepend to) a huge file if I don't have enough diskspace for two copies of it.

It's not really difficult to do it in-place, though (filtering in general, not just for the first line). All you need to do is:

1) make sure, you have separate read and write pointers (or separate File objects for reading and writing) 2) make sure you have buffered the unread parts that you are going to rewrite 3) truncate to file at the end if your filtering operation should end up with a shorter file

I wrote wrapper class for that at https://github.org/pjump/i_rewriter .

With it, you can do

Rewriter.new("myfile") do |line, index|
  index == 0 ? "prepended text\n" + line : line
end

or with the executable:

$  i_rewriter 'index == 0 ? "prepended text\n" + line : line' myfile

Use carefully (it can corrupt your file if interrupted).