Make a customized page header

Question

I have the following template to writing my thesis.

\documentclass[12pt,a4paper,twoside]{report}
\usepackage{pdfpages}
\usepackage{geometry}\geometry{top=3cm,bottom=3cm,left=3.7cm,right=2.5cm}
\usepackage{fancyhdr}
\pagestyle{fancy}
\fancyhead{}
\fancyhead[CO,CE]{This is the title of the thesis}
\renewcommand{\headrulewidth}{0.4pt}
\setlength{\headheight}{15pt}

\setcounter{secnumdepth}{4}

\usepackage{titlesec}
\titleformat{\chapter}[display]{\normalfont\Huge\bfseries}{\chaptertitlename \ \thechapter}{15pt}{\Huge}
\titlespacing*{\chapter}{0pt}{-3\baselineskip}{20pt}[3.7cm]

\hyphenpenalty=10000
\tolerance=2500
\raggedbottom

\usepackage{units}
\usepackage{nomentbl}
\usepackage{enumitem,calc}

\usepackage{setspace}\linespread{1.6}

\title{This is the title of the thesis}

Obviously, it gives all the page header as ''This is the title of the thesis'', which is centered.

1. I want the page headers on odd page numbers to be the current section title and right alignment, which is on the second level, such as 1.1, 1.2,... and those on even page numbers to be the current chapter title and left alignment, which is on the first level, such as 1, 2,...

2. Some titles of the chapter or section are too long to be shown in one line in the header. In this case, I want to show it as a shorter title.

How do I achieve the customized page header by modifying the template? Thank you a lot!

Answer 1

Replace \fancyhead[CO,CE]{This is the title of the thesis} by

\fancyhead[LE]{\leftmark}
\fancyhead[RO]{\rightmark}

Example (without the not related code):

\documentclass[12pt,a4paper,twoside]{report}

\usepackage{fancyhdr}
\pagestyle{fancy}
\fancyhead{}
%\fancyhead[CO,CE]{This is the title of the thesis}% <- removed
\fancyhead[LE]{\leftmark}% <- added
\fancyhead[RO]{\rightmark}% <- added
\renewcommand{\headrulewidth}{0.4pt}
\setlength{\headheight}{15pt}

\usepackage{lipsum}% only for dummy text
\begin{document}
\chapter{A chapter}
\section{A section}
\lipsum[1-20]
\chapter[Short chapter title]{Long chapter title in the text body}
\section[Short section title]{Another section with a long title}
\lipsum[1-20]
\end{document}

Result:

Note that on chapter pages pagestyle plain is used by default. Therefore the page header is empty on chapter pages.