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'] || 'foobar'

N.B. This is slightly incorrect (if the hash can contain the value nil) but since ENV contains just strings it is probably good enough.

Answer 2

Although it's not relevant in the specific example you gave since you're really asking about hash keys, not variables, Ruby does give a way to check variable definition. Use the defined? keyword (it's not a method, but a keyword since it needs special handling by the interpreter), like so:

a = 1
defined? a
  #=> "local-variable"

@a = 2
defined? @a
  #=> "instance-variable"

@@a = 3
defined? @@a
  #=> "class-variable"

defined? blahblahblah
  #=> nil

Hence you could do:

var = defined?(var) ? var : "default value here"

As far as I know, that's the only way other than an ugly begin/rescue/end block to define a variable in the way that you ask without risking a NameError. As I said, this doesn't apply to hashes since:

hash = {?a => 2, ?b => 3}
defined? hash[?c]
  #=> "method"

i.e. you're checking that the method [] is defined rather than the key/value pair you're using it to access.

Answer 3

hash.fetch(key) { default_value }

Will return the value if it exists, and return default_value if the key doesn't exist.

Answer 4

myvar = ENV.fetch('MY_VAR') { 'foobar' }

'foobar' being the default if ENV['MY_VAR'] is unset.

Answer 5

Another possible alternative, which will work even if ENV['MY_VAR'] turnsout to be a false value

myvar  = ENV['MY_VAR'].presence || 'foobar'

Answer 6

This works best for me:

ENV.fetch('VAR_NAME',"5445")