dirdiff: 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/dirdiff - from package: dirdiff

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

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