ceph-mon: 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-mon - from package: ceph

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

On Debian possible default full paths are:

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

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

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