blockmedian: 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/blockmedian - from package: GMT

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

On Debian possible default full paths are:

/usr/lib/gmt/bin/blockmedian - from package: GMT

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

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