nokogiri: 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/nokogiri - from package: rubygem-nokogiri
/usr/lib/ruby/gems/1.8/gems/nokogiri-1.4.3.1/bin/nokogiri - from package: rubygem-nokogiri

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

On Debian possible default full paths are:

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

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

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