hdiff: 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/hdiff - from package: hdf

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

On Debian possible default full paths are:

/usr/bin/hdiff - from package: hdf4-tools

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

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