grep: 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:

/bin/grep - from package: grep

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

On Debian possible default full paths are:

/bin/grep - from package: grep
/usr/lib/plan9/bin/grep - from package: 9base

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

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