httpd: 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/sbin/httpd - from package: httpd

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

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