[ruby] How to write to a JSON file in the correct format

I am creating a hash in Ruby and want to write it to a JSON file, in the correct format.

Here is my code:

tempHash = {
    "key_a" => "val_a",
    "key_b" => "val_b"
}
fJson = File.open("public/temp.json","w")
fJson.write(tempHash)
fJson.close

And here is the contents of the resulting file:

key_aval_akey_bval_b

I'm using Sinatra (don't know what version) and Ruby v 1.8.7.

How can I write this to the file in the correct JSON format?

This question is for ruby 1.8 but it still comes on top when googling.

in ruby >= 1.9 you can use

File.write("public/temp.json",tempHash.to_json)

other than what mentioned in other answers, in ruby 1.8 you can also use one liner form

File.open("public/temp.json","w"){ |f| f.write tempHash.to_json }

With formatting

require 'json'
tempHash = {
    "key_a" => "val_a",
    "key_b" => "val_b"
}
File.open("public/temp.json","w") do |f|
  f.write(JSON.pretty_generate(tempHash))
end

Output

{
    "key_a":"val_a",
    "key_b":"val_b"
}

To make this work on Ubuntu Linux:

1. I installed the Ubuntu package ruby-json:

   apt-get install ruby-json
   

2. I wrote the script in ${HOME}/rubybin/jsonDEMO

3. $HOME/.bashrc included:

   ${HOME}/rubybin:${PATH}
   

(On this occasion I also typed the above on the bash command line.)

Then it worked when I entered on the command line:

jsonDemo