使用Emacs，我如何缩进/格式化TXT文件中的代码段？

I am writing a document with Emacs. As you know, there are some code segments in the text file I am working with. Typically, when I open this file, emacs will get into text-mode automatically. And it works fine for me to edit the ordinary paragraphs. But for those code segments, how could I indent them into gnu or linux style just like what I could do in c-mode (by c-set-style && do Ctrl-Alt-\ in certain region)?I am writing a document with Emacs. As you know