Ruby - how to write console input to file

Question

I want to write a simple program, that writes console input to file with name i entered.

file_name = ARGV[0]
of = File.open(file_name, 'w')

while a = gets.chomp
  puts a
  of.puts a
end

# ruby write_script.rb file_name.txt

returns: main: undefined method chomp for nil:NilClass (NoMethodError)

update:

file_name = ARGV[0]
File.open(file_name, 'w') do |file|
  while (a = gets)
    print a
    file.write a
  end
end

After executing this code - program terminates at start and empty file creates.

What is the right way to do it?

R.daneel.olivaw · Accepted Answer · 2012-02-05 07:37:31Z

2

I think, the problem lies in the fact, that puts does some stuff behind the scenes. Specifically, if there is a file name as a command line argument, gets reads the next line from that file (see these docs Ruby docs, Kernel:gets).

So, if a one line script:

puts gets

...gets called with CLI arguments like this ruby onelinescript.rb some_file.txt, it will print the first line of some_file.txt.

The problem in your case, I think, is that you open the file from the CLI argument list first for writing and then implicitly try to open it for reading with gets, which doesn't work. To fix that, you should explicitly state the IO object you call gets on:

file_name = ARGV[0]
of = File.open(file_name, 'w')

while a = STDIN.gets.chomp
  puts a
  of.puts a
end

P. S. In case I am wrong, please don't hesitate to point that out.

edited Feb 5, 2012 at 7:37

R.daneel.olivaw

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answered Feb 4, 2012 at 21:22

jst

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1 Comment

Vladimir Tsukanov Over a year ago

Thank you, it seems to be true

Kassym Dorsel · Accepted Answer · 2012-02-04 22:02:23Z

1

The following code works for me :

begin
  File.open(ARGV.shift, 'w') do |f|
    while a = gets.chomp
      p a
      break if a == 'exit'
      f.puts a
    end
  end
rescue SignalException
  puts
  puts "#Completed#"
  exit
end

It should be noted that I'm using ARG.shift instead of ARG[0] because when using the latter gets will access whatever is in ARG before moving on the STDIN. ARG.shift removes the original argument.

edited Feb 4, 2012 at 22:02

answered Feb 4, 2012 at 21:27

Kassym Dorsel

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3 Comments

Vladimir Tsukanov Over a year ago

Beatiful solution and just i had problem with ctr-c. Thank you)

Kassym Dorsel Over a year ago

That's normal since by interrupting you are throwing an error. If you want a clean finish even when using ctr-c you need to catch that error . I've updated the above code to catch it.

Phrogz Over a year ago

To finish cleanly on OS X or *nix, ctrl-d.

Daniel Pittman · Accepted Answer · 2012-02-04 20:42:49Z

0

You misspelled chomp as chop in your code.

answered Feb 4, 2012 at 20:42

Daniel Pittman

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Comments

farnoy · Accepted Answer · 2012-02-04 20:43:45Z

0

The method here is chomp, which strips a string (taken from stdin by gets()) from the trailing newline character \n.

answered Feb 4, 2012 at 20:43

farnoy

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Comments

David Grayson · Accepted Answer · 2012-02-05 00:18:18Z

0

Code:

file_name = ARGV[0]
of = File.open(file_name, 'w')

while a = STDIN.gets
  print a
  of.write a
end
of.close

Your problem is that gets returns nil when you get to the end of stdin.

edited Feb 5, 2012 at 0:18

answered Feb 4, 2012 at 20:59

David Grayson

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5 Comments

David Grayson Over a year ago

Sorry , my iPhone totally makes it impossible to format code properly, I'm not sure why.

Vladimir Tsukanov Over a year ago

Thanks for answer, but i tried this by myself, after executing that code with ruby write_script.rb file_name.txt the program terminates and empty file file_name.txt creates

Phrogz Over a year ago

Note: you must close the file at the end of the program! Best to use the block form of File.open (see my answer).

David Grayson Over a year ago

Oops! I added a call to of.close. That will probably fix the problem.

David Grayson Over a year ago

I also changed my answer to use STDIN.gets. Thanks jst!

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