spectrum1d: 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/spectrum1d - 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/spectrum1d - from package: GMT

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

Error in other languages:
spectrum1d: bevel nie gevind nie
spectrum1d: comandă negăsită
spectrum1d: comando não encontrado
spectrum1d: commande introuvable
spectrum1d: command not found
spectrum1d: käsku ei ole
spectrum1d: không tìm thấy lệnh
spectrum1d: komanda nerasta
spectrum1d: Kommando nicht gefunden.
spectrum1d: kommandot finns inte
spectrum1d: komut yok
spectrum1d: nie znaleziono polecenia
spectrum1d: níor aimsíodh an t-ordú
spectrum1d: no se encontró la orden
spectrum1d: no s'ha trobat l'ordre
spectrum1d: opdracht niet gevonden
spectrum1d: parancs nem található
spectrum1d: perintah tidak ditemukan
spectrum1d: príkaz nenájdený
spectrum1d: příkaz nenalezen
spectrum1d: команда не найдена
spectrum1d: командата не е открита
spectrum1d: 命令找不到
spectrum1d: コマンドが見つかりません