ifup: 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/ifup - from package: initscripts
/usr/share/rear/skel/default/bin/ifup - from package: rear

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

On Debian possible default full paths are:

/sbin/ifup - from package: ifupdown
/sbin/ifup - from package: netscript-2.4

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

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