I usually work on more traditional command line applications and daemons so my approach might be a little different to a web developer. I always start by gaugi

I usually work on more traditional command line applications and daemons so my approach might be a little different to a web developer. I always start by gauging how much source code there is and how it's structured. The *nix utility "tree" and the source code line counter "cloc" are usually the first 2 things I run on a codebase. This tells me what languages the applications uses, how much of each, how well commented it is and where those files are. The next thing I usually do is find the entry point of the program. In my case this is usually an executable that calls into the core of the library and sets up the initial application state and starts the core loop and routine that does the guts of the work. Once I have found said core routine I try to get a grasp for how the state machine of the program looks like. If it's a complicated program this step takes quite a while but is very important for gaining an intuitive understanding of how to either add new features or fix bugs. I like to use my pen and paper to help me explore this part as I often have to back track over source files and re-evaluate what portions mean. Once I have what I think is the state machine worked out I like to understand how the program takes input or is configured. In the case of a daemon that often means understanding how configuration files are loaded and how the configuration is represented in memory. Important to cover here is how default values are handled etc. I actually prioritise this over exploring the core loops ancillary functions (the bits that do the "real" work) as I find it hard to progress to that stage without understanding how the initial state is setup. Which brings us to said "real" work. Hanging off of the core loop will be all the functions/modules are called to do the various parts of the programs function. By this time you should already know what these do even if you don't know how they work. Because you already have a good high level understanding at this point you can pick and choose which modules you need to cover and when to cover them.