In this chapter we have looked at two examples in detail: Linux and Android, which built on top on Linux. Linux has been around now for a bit over 30 years and has grown from a hobby project by one person who wanted a production version of MINIX to a large and powerful system that powers most of the Internet. It is also the most successful open-source project in history.

We started with a brief overview of the user interface and the shell, with some examples of what you can do on the command line. Then we took a brief look at some of the standard UNIX programs that are available in Linux. Next we saw how Linux is structured in layers.

After that we moved onto the core of the Linux material, how it works inside. This included processes and threads, memory management, input/output, the file system, and, of course, security. For each of these we showed some of the system calls available and how they are implemented. Then we moved on to Android, which is layered on top of Linux. Linux itself is mostly used on desktops, notebooks, and servers but Android is aimed at mobile devices such as smartphones and tablets. This changes its goals and requirements considerably. For example, how long it takes to start a program is of only minor interest on notebooks, but it is crucial on mobile devices. Notebook users really don’t mind if Word takes 3 seconds to start, but smartphone users would go crazy if hitting the app to make a phone call took 3 seconds to boot. This simple difference in goals has vast implications for the respective designs.

Another enormous difference between Linux and Android is that while Linux tries to avoid wasting energy, Android goes to great lengths to prevent draining the battery to fast. A smartphone whose battery lasted only 4 hours would not be a big hit.

After going over tge design goals and history of Android, we took a look at the system architecture. Then we studied the innards in detail, includng ART, binder IPC, how apps work, intents, and the Android process model. Next come the everimportant security, which has evolved over the years in Android as it has become more important. Finally we looked at background execution, which is quite different from how it is done on desktops and notebooks.