grdlandmask: 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/grdlandmask - 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/grdlandmask - from package: GMT

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

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