What Is "$," in Ruby?

In Ruby, "$," is a global variable that's used to specify the default separator to be used for joining elements in the output produced by the Array#join and Kernel#print methods.

In the following example, you can see how the output differs for Array#join with and without setting a value for $,:

# without setting `$,`
puts [1, 2, 3].join #=> "123"

# setting `$,`
$, = ', '
puts [1, 2, 3].join #=> "1, 2, 3"

The value you set $, to is only used when you don't specify a separator as an argument to Array#join. This means that when you explicitly specify a separator via the argument to Array#join, it supersedes the value $, is set to:

# without setting `$,`
puts [1, 2, 3].join(',') #=> "1,2,3"

# setting `$,`
$, = ', '
puts [1, 2, 3].join(',') #=> "1,2,3"

Consider, for example, the following as well, where you can see how the output of Kernel#print is affected when a value for $, is set:

# without setting `$,`
print 1, 2, 3 #=> 123

# setting `$,`
$, = ', '
print 1, 2, 3 #=> "1, 2, 3"

The value of $, defaults to nil, which means that, in that case, the Array#join and Kernel#print methods won't use a separator between elements.

Please note that using global variables is not considered a good practice as they can lead to confusing and hard-to-maintain code. It is generally recommended to use explicit arguments or options rather than relying on global variables.