What command opens Ruby's REPL?
There are several REPLs for Ruby.
The standard library ships with a REPL called IRb (for Interactive Ruby), which installs a program named irb, but since it is just a Ruby library, it can also be invoked from Ruby code and not just from the shell. On Rubinius, IRb can also be invoked by just calling the rbx program without arguments, just like in CPython.
There is also a very nice REPL called Pry, which installs a program named pry, but since it is just a Ruby library, it can also be invoked from Ruby code and not just from the shell.
JRuby comes with an additional REPL called JIRb Swing, which is invoked by calling the jirb_swing program.
The NetBeans Ruby Plugin also comes with a REPL.
Use the irb (Interactive Ruby Shell) command.