gdaldem: 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/gdaldem - from package: gdal

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

On Debian possible default full paths are:

/usr/bin/gdaldem - from package: gdal-bin

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

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