mdsum: 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/mdsum - from package: ucommon-bin

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

On Debian possible default full paths are:

/usr/bin/mdsum - from package: ucommon-utils

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

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