Geany is a free and open codex application for GNU/Linux that allows you to create and edit codices easily in a simple environment.
It's capable of recognizing C, Java, PHP, HTML, Python, Perl or Pascal files. This gives you various advantages, like syntax highlighting or being able to use the auto-completion function. It also includes the necessary tools to compile and execute the files.
Furthermore, Geany can amplify its functions thanks to the use of plugins.