ab: 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/ab - from package: httpd-tools

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

On Debian possible default full paths are:

/usr/bin/ab - from package: apache2-utils

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

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