Finding $HOME

It's common to want to find the current user's home directory, for instance to read or write a config file.

Since Ruby version 1.9.2, this has become particularly easy. You can just use the Dir.home method.

config_file = File.join(Dir.home, ".rubytapas") 
# => "/home/avdi/.rubytapas"

So you might think is going to be an extra short episode! But there's a small complication.

Dir.home depends on the $HOME environment variable being set. We can demonstrate this fact by removing the $HOME variable and trying to use Dir.home:

ENV.delete('HOME')
config_file = File.join(Dir.home, ".rubytapas") 
# ~> -:2:in `home': couldn't find HOME environment -- expanding `~' (ArgumentError)
# ~>    from -:2:in `<main>'

Actually, that's not completely true. Dir.home doesn't need the $HOME environment variable to be set if you pass it an explicit username.

ENV.delete('HOME')
config_file = File.join(Dir.home("avdi"), ".rubytapas")
# => "/home/avdi/.rubytapas"

So now the question becomes: how do we get a hold of the current user's login? We might look at the $USER variable, but in a situation where the $HOME variable isn't set, I wouldn't rely on the $USER variable being available either.

Of course, the operating system knows who the current user is. Can't we ask it? As a matter of fact we can. To do so, we use the etc standard library. etc provides a module called (surprise) Etc, which exposes a method called #getlogin which returns the login of the current user.

require 'etc'

user = Etc.getlogin             # => "avdi"
config_file = File.join(Dir.home(user), ".rubytapas") 
# => "/home/avdi/.rubytapas"

By combining Dir.home and Etc, we can reliably find the current user's home directory without depending on environment variables. This technique should work on any UNIX-like operating system, including Linux and Mac OS X. It does not work on Windows.

OK, that's all for today. Happy hacking!