pg_standby: 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/pg_standby - from package: postgresql-contrib

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

On Debian possible default full paths are:

/usr/lib/postgresql/9.4/bin/pg_standby - from package: postgresql-contrib-9.4

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

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