postgres: 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/postgres - from package: postgresql-server

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

On Debian possible default full paths are:

/usr/lib/postgresql/9.4/bin/postgres - from package: postgresql-9.4

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

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