LPI 101-500 - LPIC-1 Exam 101, Part 1 of 2, version 5.0

The wall command is a command-line utility that displays messages to all logged-in users on the terminal12. The wall command takes the following basic syntax:

$ wall OPTION { file | message }

The OPTION can be one of the following:

-n or --nobanner: Suppress the banner (the header line with the hostname, date, and time) from the output. This option can only be used by the root user.

-v or --version: Display version information and exit.

-h or --help: Display help message and exit.

The file or message argument is the source of the message to be displayed. If a file is specified, the wall command will read the message from the file. If a message is specified, the wall command will read the message from the standard input. The message can be terminated by pressing Ctrl+D.

The other commands in the options are not valid or do not have the same functionality as the wall command:

bcast: There is no such command in Linux.

mesg: This command is used to control write access to the terminal. It does not send messages to other users.

print: This command is used to print files or data to a printer. It does not send messages to other users.

yell: There is no such command in Linux.

The first program that is usually started, at boot time, by the Linux kernel when using SysV init is /sbin/init. This program is responsible for reading the /etc/inittab file and executing the appropriate scripts and programs for each runlevel. The other options are not valid programs that are started by the kernel. /lib/init.so is a shared library that is used by some init programs, but not by SysV init. /etc/rc.d/rcinit is a script that is run by init, not by the kernel. /proc/sys/kernel/init is a kernel parameter that can be used to specify a different init program, but the default value is /sbin/init. /boot/init is not a standard location for an init program, and it is unlikely that the kernel would find it there. References:

1: SysVinit - ArchWiki

2: Linux: How to write a System V init script to start, stop, and restart my own application or service - nixCraft

3: sysvinit - Gentoo wiki

The bootloader is a program that is executed by the BIOS after it completes its tasks of initializing the hardware and performing the POST (Power-On Self Test). The bootloader is responsible for loading the kernel and other necessary files into memory and passing control to the kernel. The bootloader can be either installed in the Master Boot Record (MBR) of the disk or in a separate partition. Some examples of bootloaders are GRUB, LILO, and SYSLINUX. References: LPI Linux Essentials - 1.101.1, LPI Linux Administrator - 102.1

The USB device filesystem can be found under /proc/bus/usb/1. This is a virtual filesystem that provides information about the USB devices and buses connected to the system12. It contains files and directories that correspond to the USB host controllers, hubs, and devices12. For example, the following output shows the contents of /proc/bus/usb/ on a system with one USB host controller and two USB devices:

The directories 001 and 002 represent the USB buses, and each contain files that represent the USB devices on that bus. The file devices contains a summary of all the USB devices and their configurations. The file drivers contains a list of the USB drivers and the devices they are bound to12.

The /proc/bus/usb/ filesystem is deprecated and should not be used anymore3. It has been replaced by the /sys/bus/usb/ filesystem, which is a sysfs mount that provides more detailed and structured information about the USB devices and buses3 .

The usual way to pass this change to the kernel from the boot loader is to pass init=/bin/sh on the kernel parameter line12. The init kernel parameter is used to specify the program that is run as the first process after the kernel is loaded3. By default, this program is /sbin/init, which is responsible for starting and managing other processes and services4. However, by passing init=/bin/sh, the kernel will run /bin/sh instead, which is a shell program that allows the user to execute commands interactively or from a script5. This way, the user can bypass the usual initialization process and run /bin/sh as the root user, which can be useful for troubleshooting or recovery purposes12.

The other options in the question are not correct because:

A. Start in runlevel 1: This option would not bypass the /sbin/init program, but rather instruct it to start the system in single-user mode, which is a mode that allows only the root user to log in, and disables all network services and graphical interfaces. To start in runlevel 1, the user would need to pass single or 1 on the kernel parameter line, not init=/bin/sh.

C. Pass /bin/sh on the kernel parameter line: This option would not work, because the kernel would not recognize /bin/sh as a valid parameter and would ignore it. The kernel only accepts parameters that have a specific format, such as name=value or name.flag3. To specify the init program, the user would need to use the init= prefix, as in init=/bin/sh3.

D. Pass start=/bin/sh on the kernel parameter line: This option would also not work, because the kernel does not have a start parameter. The user would need to use the init parameter, as in init=/bin/sh3.

 Run levels are predefined modes of operation in the SysV init system that determine which processes and services are started or stopped. The default run level is the one that the system enters after booting. It is usually specified in the /etc/inittab file with a line like id:5:initdefault:. The run levels 0 and 6 should never be declared as the default run level because they are used to halt and reboot the system, respectively. If they are set as the default, the system will enter an endless loop of shutting down and restarting. The other run levels (1-5) have different meanings depending on the distribution, but they usually correspond to single-user mode, multi-user mode, network mode, graphical mode, etc. References: LPI Linux Essentials - 1.101.2, LPI Linux Administrator - 101.3

The GRUB boot process consists of three stages1:

Stage 1: This stage is located in the Master Boot Record (MBR) of the first hard disk or the boot sector of a partition. Its main function is to load either Stage 1.5 or Stage 22.

Stage 1.5: This stage is located in the first 30 KB of hard disk immediately following the MBR or in the boot sector of a partition. It contains the code to access the file system that contains the GRUB configuration file. Its main function is to load Stage 22.

Stage 2: This stage is located in an ordinary file system, usually in the /boot/grub directory. It contains the code to display the GRUB menu and to load the kernel and initrd images. It can also load additional modules to support other file systems or features2.

The message “Hard Disk Error” is displayed on the screen during Stage 1 of the GRUB boot process. This indicates that the next Stage (either Stage 1.5 or Stage 2) cannot be read from the hard disk because GRUB was unable to determine the size and geometry of the disk3. This could occur because the BIOS translated geometry has been changed by the user or the disk is moved to another machine or controller after installation, or GRUB was not installed using itself (if it was, the Stage 2 version of this error would have been seen during that process and it would not have completed the install)3.

The other options in the question are not correct because:

A. The kernel was unable to execute /bin/init: This error would occur in Stage 2, after the kernel and initrd images are loaded, not in Stage 14.

C. One or more of the filesystems on the hard disk has errors and a filesystem check should be run: This error would also occur in Stage 2, when GRUB tries to access the file system that contains the GRUB configuration file, not in Stage 15.

D. The BIOS was unable to read the necessary data from the Master Boot Record to begin the boot process: This error would prevent GRUB from starting at all, not in Stage 16.

The shutdown command is used to bring the system down in a safe and controlled way. It can take various options and arguments, such as the time of shutdown, the message to broadcast to users, the halt or reboot mode, etc. The option -r instructs the shutdown command to reboot the system after shutting down. The argument now means to shut down immediately. Therefore, shutdown -r now will reboot the system without delay. The telinit command is used to change the run level of the system. It takes a single argument that specifies the new run level. The run level 6 is reserved for rebooting the system. Therefore, telinit 6 will also reboot the system. The other options are either incorrect or irrelevant. shutdown -r “rebooting” will also reboot the system, but with a delay of one minute and a message to the users. telinit 0 will halt the system, not reboot it. shutdown -k now “rebooting” will only send a warning message to the users, but not actually shut down or reboot the system. References: LPI Linux Essentials - 1.101.2, LPI Linux Administrator - 101.3