Use busybox for a mini Linux

Use busybox as a mini Linux-general Linux technology-Linux programming and kernel information. The following is a detailed description. By cutebunny

1. Basic Knowledge
An operating system can be simply abstracted as a boot program + kernel + file system.

Vmlinuz is an image file of the Linux kernel. It can be loaded by the boot program to start the Linux system.

The full name of initrd is boot loader initialized RAM disk, which is the root file system image file used during system startup. This file system contains several driver modules, it is used to load the actual root file system, such as an IDE or scsi drive module. After the kernel is loaded into this module, it is used to drive the hard disk and mount the hard disk to a subdirectory of ramdisk, then, use the cmdt_root command to convert the hard disk file system into the root file system and run the init process. At this time, the init ram disk will be umount, and it will end up.

From this we can see that we can use vmlinuz + initrd. img to create a file system with the same long resident memory for mini Linux.

Now the topic is started:

2. Compile the kernel
Cd/usr/src/... ---- enter the kernel source code directory. If not, go to the official website and click
Make menuconfig ----- add RAM disk support and initial RAM disk to configure compilation options.
(Initrd) support (in block device ). In addition, it is best to compile the ext3 file system into the kernel instead of modules. Save the configuration as. config (default)
Make bzImage ---- compile

In many kernel compilation references
Make modules
Make modules_install
However, we didn't add the modules support here, so we don't need it. The generated bzImage file is in usr/src/linux/arch /... /boot, which is omitted according to your machine architecture, will prompt during the make process, for example, my is x86_64. note that this file is very important. It is actually our final vmlinuz.

3. busybox
Busybox is a software that integrates more than one hundred common linux commands and tools. It even integrates an http server and a telnet server, all these functions only have a size of about 1 MB. the linux commands we usually use are like force-splitting electronic components, while busybox is like an integrated circuit, which compresses the integration of common tools and commands into an executable file, functions remain unchanged, but the size is many times smaller. In Embedded linux applications, busybox has a wide range of applications. In addition, busybox appears in most installation programs of linux releases, when linux is installed, ctrl + alt + F2 can get a console, and all commands in this console are links to busybox. in our mini Linux, what we need is the busybox command and tool.

Download http://busybox.net/downloads/
Many later versions have compilation errors during use, so a original version busybox-1.00 is used
# Cp busybox-1.00.tar.gz/tmp/bunny
# Cd/tmp/bunny
# Tar xvfz busybox-1.00.tar.gz
# Cd busybox-1.00
# Make menuconfig ---- compile the configuration

The following are functional options that need to be compiled into busybox. Other options can be customized as needed.
General Configuration options
Show verbose applet usage messages
Runtime SUID/SGID configuration via/etc/busybox. conf
Build Options
Build BusyBox as a static binary (no shared libs)

This option must be selected so that busybox can be compiled into a static link executable file, and the runtime is independent of other function libraries. otherwise, other library files must be required to run and cannot work properly on a single Linux kernel.

Installation Options
Don't use/usr

This option must also be selected. Otherwise, after make install, busybox will be installed in the/usr of the original system, which will overwrite the original command of the system. after this option is selected, make install will generate a directory named _ install under the busybox Directory, which contains busybox and links to it.

Other options are some basic linux Command Options. You can compile the commands you need. Generally, you can use the default command.
After configuration, exit and save. config.
Make
Make install
After compilation, generate the subdirectory _ install in the busybox directory. The content is as follows:
Bin
Linuxrc-> bin/busybox
Sbin
The executable file busybox is in the bin directory, and all others point to its symbolic link.

4. Create your own root fs
1) directory structure
Mkdir/tmp/myOS/rootfs
Cd/tmp/myOS/rootfs
Mkdir etc usr var tmp proc home root dev
Among them, etc, proc and dev must be created, bin and sbin do not need to be created, because busybox already has.
Others can be symbolic.
Copy busybox
# Cp-R/tmp/bunny/busybox-1.00/_ install/*/tmp/myOS/rootfs/

2) Device Files
The method is as follows:
# Cp-R/dev/console/tmp/myOS/rootfs/dev/
# Cp-R/dev/null/tmp/myOS/rootfs/dev/
# Cp-R/dev/zero/tmp/myOS/rootfs/dev/
All you think you need is cp.
Some references indicate that fd0, hda, ram, ram1, tty1, loop1, fb0, fb, tty, and so on are essential, but some files do not

3) create a configuration file under the etc directory
I copied the built-in example of busybox directly.
Cp-R tmp/bunny/busybox-1.00/examples/bootfloppy/etc/*/tmp/myOS/rootfs/etc

4) Create an initrd. img Image File
Cd/tmp/myOS/
Dd if =/dev/zero of =/tmp/disk bs = 1 M count = 32 ------ initializing 32 M memory space
Mkfs. ext3-m0/tmp/disk ------ format as ext3
Mkdir/mnt/ram
Mount-o loop/tmp/disk/mnt/ram ------ mount to/mnt/ram
Cp-R rootfs/*/mnt/ram ------ write rootfs into memory
Umount/mnt/ram
Dd if =/tmp/disk of =/tmp/myOS/initrd. img ------ retrieve the content in the memory as an image
OK, this initrd. img is our rootfs.
Some documents use ram0, that is, change/tmp/disk in the above process to/dev/ram0. Although there is no problem in the production process, it has a size limit.

5. Integration and startup
1) kernel
Cp/usr/src/linux/arch/x86_64/bzImage/boot/vmlinuz
2) rootfs is generally named as initrd. img.
Cp/tmp/myOS/initrd. img/boot
3) With the above two files, you can use the network dhcp, tftp server to start, and add the startup script to the tftp server as follows:
DEFAULT linux
PROMPT 0
LABEL linux
KERNEL vmlinuz
Append initrd = initrd. img devfs = nomount ramdisk_size = 52000

Problem: theoretically, this small Linux should also be able to boot from grub.