unicorn_rails: 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/unicorn_rails - from package: unicorn

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

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