rdoc: 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/rdoc - from package: ruby-rdoc

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

On Debian possible default full paths are:

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

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

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