markdown: 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/markdown - from package: python-markdown

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

On Debian possible default full paths are:

/usr/bin/markdown - from package: discount
/usr/bin/markdown - from package: libtext-markdown-perl
/usr/bin/markdown - from package: markdown

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

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