x_update: 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/x_update - 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/x_update - from package: GMT

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

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