unique: 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/sbin/unique - from package: john

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

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