h5diff: 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/lib64/mpich/bin/h5diff - from package: hdf5-mpich
/usr/lib/mpich/bin/h5diff - from package: hdf5-mpich
/usr/lib64/openmpi/bin/h5diff - from package: hdf5-openmpi
/usr/lib/openmpi/bin/h5diff - from package: hdf5-openmpi
/usr/bin/h5diff - from package: hdf5

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

On Debian possible default full paths are:

/usr/bin/h5diff - from package: hdf5-tools

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

Error in other languages:
h5diff: bevel nie gevind nie
h5diff: comandă negăsită
h5diff: comando não encontrado
h5diff: commande introuvable
h5diff: command not found
h5diff: käsku ei ole
h5diff: không tìm thấy lệnh
h5diff: komanda nerasta
h5diff: Kommando nicht gefunden.
h5diff: kommandot finns inte
h5diff: komut yok
h5diff: nie znaleziono polecenia
h5diff: níor aimsíodh an t-ordú
h5diff: no se encontró la orden
h5diff: no s'ha trobat l'ordre
h5diff: opdracht niet gevonden
h5diff: parancs nem található
h5diff: perintah tidak ditemukan
h5diff: príkaz nenájdený
h5diff: příkaz nenalezen
h5diff: команда не найдена
h5diff: командата не е открита
h5diff: 命令找不到
h5diff: コマンドが見つかりません