hdfunpac: 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/hdfunpac - from package: hdf

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

On Debian possible default full paths are:

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

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

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