unit_diff: 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/unit_diff - from package: ruby-zentest

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

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