part of any embedded Linux system. After the kernel has initialized itself, it must mount a root file system and execute a set of developer-defined initialization routines. In this chapter, we examine the details of post-kernel system initialization.
We begin by looking at the root file system and its layout. Next we develop and study a minimal system configuration. Later in this chapter, we add functionality to the minimal system configuration to produce useful example embedded system configurations. We complete the coverage of system initialization by introducing the
6.1. Root File System
In Chapter 5, 'Kernel Initialization,' we examined the Linux kernel's behavior during the initialization process. We made several references to mounting a
The root file system refers to the file system mounted at the base of the file system hierarchy, designated simply as /. As you will discover in Chapter 9, 'File Systems,' even a small embedded Linux system typically mounts several file systems on different locations in the file system hierarchy. The proc file system, introduced in Chapter 9, is an example. It is a special-purpose file system mounted at /proc under the root file system. The root file system is simply the first file system mounted at the base of the file system hierarchy.
As you will shortly see, the root file system has special requirements for a Linux system. Linux expects the root file system to contain programs and utilities to boot a system, initialize services such as networking and a system console, load device drivers, and mount additional file systems.
6.1.1. FHS: File System Hierarchy Standard
Several kernel developers authored a standard governing the organization and layout of a UNIX file system. The File System Hierarchy Standard (FHS) establishes a minimum baseline of compatibility between Linux distributions and application programs. You'll find a reference to this standard in Section 6.7.1 'Suggestions for Additional Reading' at the end of this chapter. You are encouraged to review the FHS standard for a better background on the layout and rationale of UNIX file system organization.
Many Linux distributions have directory layouts closely matching that described in the FHS standard. The standard exists to provide one element of a common base between different UNIX and Linux distributions. The FHS standard allows your application software (and developers) to predict where certain system elements, including files and directories, can be found on the file system.
6.1.2. File System Layout
Where space is a concern, many embedded systems developers create a very small root file system on a bootable device (such as Flash memory) and later mount a larger file system from another device, perhaps a hard disk or network NFS server. In fact, it is not uncommon to mount a larger root file system right on top of the original small one. You'll see an example of that when we examine the
A simple Linux root file system might contain the following top-level directory entries:
.
|
|--bin
|--dev
|--etc
|--lib
|--sbin
|--usr
|--var
|--tmp
Table 6-1 details the most common contents of each of these root directory entries.
Table 6-1. Top-Level Directories
| Directory | Contents |
|---|---|
| bin | Binary executables, usable by all users on the system[47] |
| dev | Device nodes (see Chapter 8, 'Device Driver Basics') |
| etc | Local system-configuration files |
| lib | System libraries, such as the standard C library and many others |
| sbin | Binary executables usually reserved for superuser accounts on the system |
| usr | A secondary file system hierarchy for application programs, usually read- only |
| var |
