Discoveries in Ruby(and Rails): List local/global_variables in the current scope, and the (_) in irb sessions.

Ruby provides a mechanism for tapping into the program at runtime to determine the list of defined local and global variables. Ruby provides two methods to get all declared variables in a local or global scope.

Kernel#local_variables

The local_variables is available globally in your ruby programmes.

x = 5
puts local_variables
# => [:x]

the local_variables method will get the variables in the current local scope. So, for something like

y = 6
def do_something
  x = 5
  puts local_variables
end

do_something
local_variables

The first line printed to the console will be the local variables inside do_something. The second print will be the local variables in the current scope, namely y.

Kernel#global_variables

In Ruby, a global variables is a variable that starts with a dollar sign $. Ruby has a set of built-in globals available for us already.

puts global_variables

You can add to the list of global variables by creating your own global variables, tho you might not want to do that.

$my_global_variable = "Custom global"
global_variables.find { |glob| glob == :$my_global_variable }
 # => :$my_global_variable 

Notice: All variables are stored as a Symbol.

Bonus content, the _ local variable in irb sessions.

When you launch an irb session and log the local_variables you get an _ in the list. The _ provides a way to access the last return value in case it's lost forever.

You can access the return value of the previous line by using _ in your irb sessions.

x = 5
y = _

puts x 
puts y

Running this through irb you get 5 logged two times. That is because _ stores the last return value, which is the assignment of 5 to the variable x.

Resources

• Kernel#local_variables
• Kernel#global_variables