jruby.rb: 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 Debian possible default full paths are:

/usr/bin/jruby.rb - from package: jruby

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

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