Ruby: Most concise way to use an ENV variable if it exists, otherwise use default value

Question

In Ruby, I am trying to write a line that uses a variable if it has been set, otherwise default to some value:

myvar = # assign it to ENV[\'MY_VAR\'], otherw         


        

Answer 1

myvar = ENV['MY_VAR'].is_set? ? ENV['MY_VAR'] : 'foobar'

This way you keep the .is_set? method.

Answer 2

The most reliable way for a general Hash is to ask if it has the key:

myvar = h.has_key?('MY_VAR') ? h['MY_VAR'] : 'default'

If you don't care about nil or false values (i.e. you want to treat them the same as "not there"), then undur_gongor's approach is good (this should also be fine when h is ENV):

myvar = h['MY_VAR'] || 'foobar'

And if you want to allow nil to be in your Hash but pretend it isn't there (i.e. a nil value is the same as "not there") while allowing a false in your Hash:

myvar = h['MY_VAR'].nil? ? 'foobar' : h['MY_VAR']

In the end it really depends on your precise intent and you should choose the approach that matches your intent. The choice between if/else/end and ? : is, of course, a matter of taste and "concise" doesn't mean "least number of characters" so feel free to use a ternary or if block as desired.

Answer 3

The Demand gem which I wrote allows this to be extremely concise and DRY:

myvar = demand(ENV['MY_VAR'], 'foobar')

This will use ENV['MY_VAR'] only if it is present. That is, it will discard it just if it's nil, empty or a whitespace-only string, giving the default instead.

If a valid value for ENV['MY_VAR'] is falsy (such as false), or an invalid value is truthy (such as ""), then solutions like using || would not work.