shotgun: 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/shotgun - from package: rubygem-shotgun
/usr/lib/ruby/gems/1.8/gems/shotgun-0.8/bin/shotgun - from package: rubygem-shotgun

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

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