rackup: 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/lib/pcsd/vendor/bundle/ruby/bin/rackup - from package: pcs
/usr/lib/pcsd/vendor/bundle/ruby/gems/rack-1.6.4/bin/rackup - from package: pcs
/usr/bin/rackup - from package: rubygem-rack
/usr/lib/ruby/gems/1.8/gems/rack-1.1.0/bin/rackup - from package: rubygem-rack

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

On Debian possible default full paths are:

/usr/bin/rackup - from package: ruby-rack

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

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