How do I compile Readline support into Ruby

ruby readline irb

Install readline to /usr/local
Recompile ruby from scratch and use the --with-readline-dir=/usr/local switch

or if you have downloaded the ruby sources earlier and built it by hand,

Go to the ext/readline folder of your ruby source tree
Type ruby extconf.rb and then run the make && make install procedure for ruby.

ruby readline irb

There's also a pure ruby readline.