jirb: 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/jirb - from package: jruby
/usr/lib/jruby/bin/jirb - from package: jruby

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

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