ri: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/bin/ri - from package: ruby-ri

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/ri - from package: ruby
/usr/lib/jruby/bin/ri - from package: jruby
/usr/lib/ruby/gems/2.1.0/gems/rdoc-4.1.0/bin/ri - from package: libruby2.1
/usr/lib/ruby-standalone/bin/ri - from package: ruby-standalone

You may install the required package with command apt-get install {package}

Error in other languages:
ri: bevel nie gevind nie
ri: comandă negăsită
ri: comando não encontrado
ri: commande introuvable
ri: command not found
ri: käsku ei ole
ri: không tìm thấy lệnh
ri: komanda nerasta
ri: Kommando nicht gefunden.
ri: kommandot finns inte
ri: komut yok
ri: nie znaleziono polecenia
ri: níor aimsíodh an t-ordú
ri: no se encontró la orden
ri: no s'ha trobat l'ordre
ri: opdracht niet gevonden
ri: parancs nem található
ri: perintah tidak ditemukan
ri: príkaz nenájdený
ri: příkaz nenalezen
ri: команда не найдена
ri: командата не е открита
ri: 命令找不到
ri: コマンドが見つかりません