Translated most of the grub2official manual, and then sorted it out myself. Because the content is a bit complicated, the arrangement of chapters may not be very reasonable, so please understand.
Directory of this article:
##1.1 Basic content
1.2 Install grub2
##1.3 grub2 configuration file
1.4 Commands in the command line and menu items
1.5 Several common built-in variables
1.6 grub configuration and installation examples
1.7 Brief description of traditional grub
This article mainly introduces grub2. At the end of the article, traditional grub is briefly described, but the content of grub2 contains a lot of comparisons between grub2 and traditional grub.
If you just want to know the role of boot.img/core.img/diskboot.img/kernel.img in grub2 or the stage1/stage1_5/stage2 file in traditional grub, please directly Jump to related content to read.
1.1 Basic content
1.1.1 The difference between grub2 and grub
Official manual original text:
Only explain a few main ones:
1. The name of the configuration file has changed. In grub, the configuration file is grub.conf or menu.lst (a soft link to grub.conf), and is renamed grub.cfg in grub2.
2.grub2 adds a lot of syntax, which is closer to a scripting language, such as supporting variables, conditional judgments, and loops.
3. In grub2, the device name starts from 1, but in grub it starts from 0.
4.grub2 uses img files and no longer uses stage1, stage1.5 and stage2 in grub.
5. Supports the graphical interface to configure grub, but you need to install the grub-customizer package, which is provided by the epel source.
6. When the operating system environment has been entered, the grub command is no longer provided, that is, the grub interactive interface cannot be entered. It can only be entered when the computer is booted, which is a big shortcoming.
7. There is no useful find command in grub2, which is another major shortcoming.
grub2 supports two ways to boot the operating system:
Direct boot: (direct-load) Directly boot the operating system written in the default configuration file through the default grub2 boot loader
Chain boot (chain-load) Use the default grub2 boot loader to chain boot another boot loader, which will boot the corresponding operating system
Generally only use the first method, only if you want to The second method is used when booting an operating system that grub does not support by default.
1.1.4 File distribution after installation of grub2 program and traditional grub program
After the traditional grub software is installed, some stages will be generated in the /usr/share/grub/RELEASE/ directory document.
After the grub2 software is installed, many module files and img files will be generated in the /usr/lib/grub/i386-pc/ directory, as well as Includes some lst list files.
[root@server7 ~]# ls /usr/lib/grub/i386-pc/*.mod | wc -l257[root@server7 ~]# ls -lh /usr/lib/grub/i386-pc/*.lst
-rw-r--r--. 1 root root 3.7K Nov 24 2015 /usr/lib/grub/i386-pc/command.lst
-rw-r--r--. 1 root root 936 Nov 24 2015 /usr/lib/grub/i386-pc/crypto.lst
-rw-r--r--. 1 root root 214 Nov 24 2015 /usr/lib/grub/i386-pc/fs.lst
-rw-r--r--. 1 root root 5.1K Nov 24 2015 /usr/lib/grub/i386-pc/moddep.lst
-rw-r--r--. 1 root root 111 Nov 24 2015 /usr/lib/grub/i386-pc/partmap.lst
-rw-r--r--. 1 root root 17 Nov 24 2015 /usr/lib/grub/i386-pc/parttool.lst
-rw-r--r--. 1 root root 202 Nov 24 2015 /usr/lib/grub/i386-pc/terminal.lst
-rw-r--r--. 1 root root 33 Nov 24 2015 /usr/lib/grub/i386-pc/video.lst[root@server7 ~]# ls -lh /usr/lib/grub/i386-pc/*.img-rw-r--r--. 1 root root 512 Nov 24 2015 /usr/lib/grub/i386-pc/boot_hybrid.img
-rw-r--r--. 1 root root 512 Nov 24 2015 /usr/lib/grub/i386-pc/boot.img
-rw-r--r--. 1 root root 2.0K Nov 24 2015 /usr/lib/grub/i386-pc/cdboot.img
-rw-r--r--. 1 root root 512 Nov 24 2015 /usr/lib/grub/i386-pc/diskboot.img
-rw-r--r--. 1 root root 28K Nov 24 2015 /usr/lib/grub/i386-pc/kernel.img
-rw-r--r--. 1 root root 1.0K Nov 24 2015 /usr/lib/grub/i386-pc/lnxboot.img
-rw-r--r--. 1 root root 2.9K Nov 24 2015 /usr/lib/grub/i386-pc/lzma_decompress.img
-rw-r--r--. 1 root root 1.0K Nov 24 2015 /usr/lib/grub/i386-pc/pxeboot.img
grub2-install is actually a shell script used to call other tools. The real functions are completed by other tools, so if you are very familiar with grub's internal commands and mechanisms, you don't need grub2-install at all.
The corresponding traditional grub installation command is grub-install, and its usage is the same as grub2-install.
1.2.2 Description of various img and stage files
Official manual original text:
The img file is generated by grub2, and the stage file is generated by traditional grub. Below are descriptions of the various files.
1.2.2.1 img file in grub2
grub2 generates several img files, some are distributed in the /usr/lib/grub/i386-pc directory, and some are distributed in /boot/ In the grub2/i386-pc directory, I believe you will understand the relationship between them after reading the following.
The following figure describes the relationship between various img files. Among them, core.img is dynamically generated, and the path is /boot/grub2/i386-pc/core.img, while other img exists in the /usr/lib/grub/i386-pc directory. Of course, when installing grub, boot.img will be copied to the /boot/grub2/i386-pc directory.
(1)boot.img
Under the BIOS platform, boot.img is the first img file started by grub, and it is written to In the MBR or the boot sector of the partition, because the size of the boot sector is 512 bytes, the size of the img file is also 512 bytes.
The only function of boot.img is to read the first sector belonging to core.img and jump to it, giving control to the img of that sector. Due to size limitations, boot.img cannot understand the structure of the file system, so grub2-install will hard-code the location of core.img into boot.img, so that the location of core.img can be found.
(2)core.img
core.img is dynamically created by the grub2-mkimage program based on diskboot.img, kernel.img and a series of modules. There are enough functional modules embedded in core.img to ensure that grub can access /boot/grub, and relevant modules can be loaded to implement related functions, such as loading the boot menu, loading information about the target operating system, etc. Since grub2 is used extensively The dynamic function module makes the size of core.img small enough.
core.img contains the contents of multiple img files, including diskboot.img/kernel.img, etc.
The installation location of core.img varies with MBR disks and GPT disks, which has been explained above.
(3)diskboot.img
If the boot device is a hard disk, that is, when booting from the hard disk, the content of the first sector in core.img is diskboot.img. The function of diskboo.img is to read the remaining parts of core.img into memory and transfer control to kernel.img. Since the file system is not recognized at this time, all locations of core.img are listed in the block list. Encoded in such a way that diskboot.img can find the remaining content.
Because the img file occupies one sector, the size is 512 bytes.
(4)cdboot.img
If the boot device is an optical drive (cd-rom), that is, when booting from the optical drive, the content of the first sector in core.img is cdboo .img. Its function is the same as diskboot.img.
(5)pexboot.img
If booted from the PXE environment of the network, the content of the first sector in core.img is pxeboot.img.
(6)kernel.img
The kernel.img file contains the basic runtime environment of grub: device framework, file handle, environment variables, command line parser in rescue mode, etc. . It is rarely used directly because they are fully embedded in core.img. Note that kernel.img is grub's kernel and has nothing to do with the operating system's kernel.
If you are careful, you will find that kernel.img itself takes up 28KB of space, but after being embedded into core.img, the core.img file is only 26KB in size. This is because kernel.img in core.img is compressed.
(7)lnxboot.img
The img file is placed at the front of core.img, making grub like the Linux kernel, so that core.img can be used by LILO's "image ="identify. Of course, this is used in conjunction with LILO, but who still uses LILO now?
(8)*.mod
Various functional modules, some of which have been embedded in core.img, or will be automatically loaded by grub, but sometimes it is also necessary to use the insmod command to load them manually.
Set the background image. The background image must be readable by grub. The image file name suffix must be ".png", ".tga", ".jpg", ".jpeg". When needed, grub will press Scale down the size of the image to fit the screen.
(11).GRUB_THEME
Set the theme of grub menu.
(12).GRUB_GFXPAYLOAD_LINUX
When set to "text", Linux will be forced to start in text mode. In some cases, graphics mode may not be supported.
(13).GRUB_DISABLE_OS_PROBER
By default, grub2-mkconfig will try to use the os-prober program (if it is already installed, it should be installed by default) to detect other available operating system kernels , and generate corresponding startup menu items for it. Set to "true" to disable automatic detection.
(14).GRUB_DISABLE_SUBMENU
By default, if grub2-mkconfig finds multiple kernels of the same version or a lower version, it will only generate a top-level menu for the highest version of the kernel. All other lower-version kernel menus are placed into submenus, and setting them to "y" will generate them all as top-level menus.
(15).GRUB_HIDDEN_TIMEOUT (obsolete, but still valid for backward compatibility)
Use "GRUB_TIMEOUT_STYLE={countdown|hidden}" to replace this item
( 16).GRUB_HIDDEN_TIMEOUT_QUIET (obsolete, but still valid for backward compatibility)
Used with GRUB_HIDDEN_TIMEOUT, you can use GRUB_TIMEOUT_STYLE=countdown to replace these two items.
1.3.2 Directly write the grub.cfg file in script
Official manual original text:
Comment symbol: Characters starting with # are considered comments, so grub supports in-line comments
##Connection operators: { } | & $ ; < >
Reserved keywords and symbols: ! [[ ]] { } case do done elif else esac fi for function if in menuentry select then time until while.
Not all keywords are useful, they are only provided in advance for future function expansion.
Quotation marks and escape characters
Special characters need to be escaped. There are three ways to escape: using backslashes, using single quotes, and using double quotes. The backslash escaping method is the same as the shell. All strings within single quotes are literal and have no special meaning, even escape characters within single quotes are considered pure characters. So '\'' cannot retain single quotes. Single quotes need to be transferred using double quotes, so "'" should be written. Double quotes have the same effect as single quotes, but they cannot escape certain special characters, including "$" and "\". The "$" symbol within double quotes retains its original meaning at all times. For "\", only the characters after the backslash are '$', '"', and '\'. In addition, if a line ends with a backslash, it means a line continuation, but the official It is not recommended to use line continuation characters in grub.cfg
Variable expansion
Use the $ symbol to reference variables, you can also use ${var. } refers to var variables. Supports positional variables. For example, $1 refers to the first parameter. also supports special variables, such as $? which represents the exit status code of the previous command. . If positional variables are used, $*, $@ and $# are also supported. All parameters represented by $@ are inseparable from each other. $@ also represents all variables, but each parameter of $@ is Can be split, $# represents the number of parameters
Simple commands
You can use simple commands in grub.cfg. . Use a newline or semicolon between each command to indicate the end of the command. If "!" is used before the command, it indicates the logical inversion. Loop structure: for name in word…; do list; done
Loop structure: while cond; do list; done
Loop structure: until cond; do list; done
Conditional judgment structure: if list; then list; [elif list; then list;] … [else list;] fi
Function structure: function name { command; … }
Menu item command: menuentry title [--class=class …] [--users=users] [ --unrestricted] [--hotkey=key] [--id=id] { command; … }
This is the most important item in grub.cfg, official original text:
search [--file|--label|--fs-uuid] [--set [var]] [--no-floppy] [--hint args] name
Copy after login
Search for devices by file [--file], volume label [--label], and file system UUID [--fs-uuid].
If the "--set" option is used, the first found device will be set to the value of the environment variable "var". The default variable "var" is 'root'.
When searching, you can use the "--no-floppy" option to disable searching for floppy disks, because floppy disks are very slow and have been eliminated.
Sometimes "--hint=XXX" is also specified, which means that devices that meet the prompt conditions are given priority. If multiple hint conditions are specified, the first hint is matched first, and then the second one is matched. And so on.
The first search in the above if statement searches for the device with uuid "367d6a77-033b-4037-bbcb-416705ead095", but multiple hint options are used, indicating that first Matches devices with the /boot partition (hd0, msdos1) under the bios platform, and then specifies several hints. However, because the search uses the uuid search method, these hint options are redundant, because the uuid of the partition on a single disk is only.
For another example, if there are two boot partitions on a boot device (such as when multiple systems coexist), they are (hd0, msdos1) and (hd0, msdos5). If uuid search is not used at this time , instead use label mode to search:
default=0 # 默认启动第一个系统
timeout=5 # 等待超时时间5秒
splashimage=(hd0,0)/grub/splash.xpm.gz # 背景图片
hiddenmenu # 隐藏菜单，若要显式，在启动时按下ESC
title Red Hat Enterprise Linux AS (2.6.18-92.el5) # 定义操作系统的说明信息
root (hd0,0)
kernel /vmlinuz-2.6.18-92.el5 ro root＝/dev/sda2 rhgb quiet
initrd /initrd-2.6.18-92.el5.img
Copy after login
Before describing the configuration method, a key point needs to be explained, whether boot is an independent partition, which affects the configuration of subsequent paths.
Check the /boot/grub/grub.conf file in a normal operating system. You can see a prompt in the NOTICE section, saying whether you have an independent boot partition? If there is, it means that the paths of kernel and initrd start from / instead of /boot, such as /vmlinuz-xxx. If there is no independent boot partition, the boot path needs to be specified in the paths of kernel and initrd. For example, Boot does not have The partition is a directory under the / file system, then /boot/vmlinuz-xxx.
root (hd0,0) defines the root recognized by grub. Generally, the partition where boot is located is defined. grub can only recognize hd, so only hd can be used here. hd0 means that it is on the first disk. The second 0 of hd0,0 means that boot is on the first partition. grub2 The calculation of partitions starts from 1, which is the difference between traditional grub and grub2.
kernel defines the path and startup parameters of the kernel file, which is equivalent to the grub2 linux command or the linux16 command. First, explain the parameters. ro means read-only. Root=/dev/sda[N] or root=UUID="device_uuid_num" specifies the partition where the root file system is located. This is a required parameter. rhgb means to use the graphical interface to output some information during the operating system startup process. Omitting it can speed up the startup speed. Quiet means to output information silently when starting the operating system. Let’s explain the path again. If the boot is an independent partition, the kernel path is defined as /vmlinuz-xxx. If there is no independent partition, specify its absolute path. It is usually a directory under the root file system, so it is usually / boot/vmlinuz-xxx.
initrd defines the path of init ramdisk. The path is defined in the same way as kernel. There are no parameters except the path.
Or use the UUID method as shown below.
If the root= option is not specified, the error "no or empty root... dracut...kernel panic" will be reported. As shown below.
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