grdmask: 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/grdmask - 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/grdmask - from package: GMT

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

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