opengrok: 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/opengrok - from package: opengrok

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

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