hp_median_filter: 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/hp_median_filter - from package: healpix

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

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