arch: 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/arch - from package: coreutils
/usr/lib/mailman/bin/arch - from package: mailman

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

On Debian possible default full paths are:

/usr/lib/mailman/bin/arch - from package: mailman
/usr/bin/arch - from package: coreutils

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

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