In your Ruby app you may actually need to save a temporary file to the hard drive which you should store in your operating systems /tmp directory. This is because operating systems, usually on reboot, will automatically clear out the /tmp directory for you. So, it is good practice to use that directory instead of some other one.
irb(main):002:0> Dir.tmpdir
=> "/tmp"
It is good to use Dir.tmpdir rather than using /tmp directly because Windows or possibly some other operating system my use something other than /tmp, but Ruby’s Dir.tmpdir will figure this out for you.