readahead: 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:

/sbin/readahead - from package: readahead

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

On Debian possible default full paths are:

/sbin/readahead - from package: readahead-fedora

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

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