Книга: Embedded Linux Primer: A Practical, Real-World Approach

11.3.1. BusyBox Init

11.3.1. BusyBox Init

Notice the symlink in Listing 11-5 called init. In Chapter 6 "System Initialization," you learned about the init program and its role in system initialization. Recall that the kernel attempts to execute a program called /sbin/init as the last step in kernel initialization. There is no reason why BusyBox can't emulate the init functionality, and that's exactly how the system illustrated by Listing 11-5 is configured. BusyBox handles the init functionality.

BusyBox handles system initialization differently from standard System V init. A Linux system using the System V (SysV) initialization as described in Chapter 6 requires an inittab file accessible in the /etc directory. BusyBox also reads an inittab file, but the syntax of the inittab file is different. In general, you should not need to use an inittab if you are using BusyBox. I agree with the BusyBox man page: If you need run levels, use System V initialization.[78]

Let's see what this looks like on an embedded system. We have created a small root file system based on BusyBox. We configured BusyBox for static linking, eliminating the need for any shared libraries. Listing 11-6 contains a tree listing of this root file system. We built this small file system using the steps outlined in Chapter 9, "File Systems," Section 9.10, "Building a Simple File System." We do not detail the procedure again here. The files in our simple file system are those shown in Listing 11-6.

Listing 11-6. Minimal BusyBox Root File System

$ tree
|-- bin
|   |-- busybox
|   |-- cat -> busybox
|   |-- dmesg -> busybox
|   |-- echo -> busybox
|   |-- hostname -> busybox
|   |-- ls -> busybox
|   |-- ps -> busybox
|   |-- pwd -> busybox
|   '-- sh -> busybox
|-- dev
|   '-- console
|-- etc
'-- proc
4 directories, 10 files

This BusyBox-based root file system occupies little more than the size needed for busybox itself. In this configuration, using static linking and supporting nearly 100 utilities, the BusyBox executable came in at less than 1MB:

# ls -l /bin/busybox
-rwxr-xr-x 1 root root 824724 Dec 3 2005 /bin/busybox

Now let's see how this system behaves. Listing 11-7 captures the console output on power-up on this BusyBox-based embedded system.

Listing 11-7. BusyBox Default Startup

Looking up port of RPC 100003/2 on
Looking up port of RPC 100005/1 on
VFS: Mounted root (nfs filesystem).
Freeing init memory: 96K
Bummer, could not run '/etc/init.d/rcS': No such file or directory
Please press Enter to activate this console.
BusyBox v1.01 (2005.12.03-19:09+0000) Built-in shell (ash)
Enter 'help' for a list of built-in commands.
-sh: can't access tty; job control turned off
/ #

The example of Listing 11-7 was run on an embedded board configured for NFS root mount. We export a directory on our workstation that contains the simple file system image detailed in Listing 11-6. As oneof the final steps in the boot process, the Linux kernel on our target board mounts a root file system via NFS. When the kernel attempts to execute /sbin/init, it fails because there is no /sbin/init on our file system. However, as we have seen, the kernel also attempts to execute /bin/sh. In our BusyBox-configured target, this succeeds, and busybox is launched via the symlink /bin/sh on our root file system.

The first thing BusyBox displays is the complaint that it can't find /etc/init.d/rcS. This is the default initialization script that BusyBox searches for. Instead of using inittab, this is the preferred method to initialize an embedded system based on BusyBox.

When it has completed initialization, BusyBox displays a prompt asking the user to press Enter to activate a console. When it detects the Enter key, it executes an ash shell session waiting for user input. The final message about job control is a result of the fact that we are creating the system console on a serial terminal. The Linux kernel contains code to disable job control if it detects the console on a serial terminal.

This example produced a working system, with nearly 100 Linux utilities available, including core utilities, file utilities, network support, and a reasonably capable shell. You can see that this simple package provides a powerful platform upon which to build your own system applications. Of course, it should be noted that without any support for libc and other system libraries, you would face a formidable task implementing your applications. You would have to provide support for all the usual system calls and other library functions that a typical C program relies on. Alternatively, you could statically link your applications against the libraries it depends on, but if you have more than a couple applications using this method, your applications will likely exceed the combined size of linking dynamically and having the shared libraries on your target.

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