ceph-mds: 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/ceph-mds - from package: ceph

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

On Debian possible default full paths are:

/usr/bin/ceph-mds - from package: ceph-mds

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

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