LPI Updated 101-500 Exam Questions and Answers by kaya

The content of the kernel ring buffer is reset when the ring buffer is explicitly reset using the command dmesg --clear or when the system is shut down or rebooted. The kernel ring buffer is a portion of the physical memory that holds the kernel’s log messages. It has a fixed size, which means once the buffer is full, the older logs records are overwritten. The dmesg command-line utility is used to print and control the kernel ring buffer in Linux and other Unix-like operating systems1. The dmesg --clear option clears the ring buffer’s contents, which can be useful for debugging purposes or to free up some memory2. The systemshutdown or reboot also clears the ring buffer’s contents, as the physical memory is reset and the kernel is reloaded3.

The other options are false or irrelevant. The ring buffer is not reset when it is read using dmesg without any additional parameters, as this option only displays the current contents of the buffer without modifying them2. The ring buffer is not reset when a configurable amount of time, 15 minutes by default, has passed, as there is no such time limit for the buffer’s persistence4. The ring buffer is not reset when the kernel loads a previously unloaded kernel module, as this action only affects the kernel’s functionality and not its log messages5. References:

• Dmesg Command in Linux | Linuxize1
• dmesg(1) - Linux manual page2
• Linux Dmesg Command Help and Examples - Computer Hope3
• Lockless Ring Buffer Design — The Linux Kernel documentation4
• Linux Kernel Module Management 101 - Linux.com5

This command creates a hard link to a.txt with the name c.txt. The new file c.txt will have the same inode number as a.txt (Inode 525385).

A hard link is a directory entry that points to the same data blocks as another file. A hard link is indistinguishable from the original file, and it shares the same inode number, permissions, ownership, and timestamps. A hard link can only be created within the same file system, and it cannot link to directories or special files. A hard link increases the link count of the file, which is the number of directory entries that refer to the file. The link count can be seen by using the ls -l command. The file is only deleted when the link count reaches zero, which means that all the hard links to the file are removed.

The ln command is used to create hard links or symbolic links. The syntax of the ln command is:

ln [options] source target

The source is the name of the existing file, and the target is the name of the new link. By default, the ln command creates a hard link, unless the -s option is used to create a symbolic link. A symbolic link is a special file that contains the path to another file or directory. A symbolic link is different from a hard link, as ithas its own inode number, permissions, ownership, and timestamps. A symbolic link can link to any file or directory, even across file systems, and it does not affect the link count of the file. A symbolic link can be identified by the ls -l command, as it shows the link name followed by an arrow and the target name.

For example, to create a hard link to a.txt with the name c.txt, use the following command:

ln a.txt c.txt

This command will create a new file c.txt that has the same inode number as a.txt (Inode 525385). The output of the ls -i command will show something like:

525385 a.txt 525385 c.txt

To create a symbolic link to a.txt with the name b.txt, use the following command:

ln -s a.txt b.txt

This command will create a new file b.txt that has a different inode number than a.txt (Inode 526053), and contains the path to a.txt. The output of the ls -i command will show something like:

525385 a.txt 526053 b.txt -> a.txt

References:

• Linux Hard and Soft Links - Linuxize
• ln(1) - Linux manual page
• How to Create Hard and Symbolic Links in Linux - How-To Geek

Systemd is a system and service manager for Linux systems, and it can manage the mounting and unmounting of file systems. Systemd can automatically create and start mount units for file systems that are listed in /etc/fstab or are known to the system. Mount units are unit files that encode information about a file system mount point controlled and supervised by systemd. Mount units must be named after the mount point directories they control, and they have the suffix .mount. For example, the mount point /home must be configured in a unit file home.mount.

Systemd can also handle file systems that are neither listed in /etc/fstab nor known to the system, but are mounted manually by the user. In this case, systemd automatically generates a transient mount unit and monitors the mount point without changing it. A transient mount unit is a unit that is created dynamically and temporarily, and is not backed by a unit file on disk. A transient mount unit has the same name and properties as a regular mount unit, but it is not persistent across reboots. Systemd does not interfere with the manual mount, and does not unmount it unless explicitly requested by the user. The user can use the mount command or the systemd-mount command to create a manual mount, and the umount command or the systemd-umount command to remove it. The user can also use the systemctl command to inspect and control the transient mount unit. For example, to show the status of the transient mount unit for the mount point /mnt, use the following command:

systemctl status mnt.mount

References:

• systemd.mount - freedesktop.org
• systemd-mount - freedesktop.org
• How to name systemd mount unit properly? - Server Fault
• Working with Systemd Mount Units - Pluralsight

This command will display the output of the foo command on the screen and also write it to a file called /tmp/foodata. The syntax of the command is:

foo | tee [options] [file]

The foo command is any command that produces some output. The | symbol is a pipe operator that redirects the standard output of one command to the standard input of another command. The tee command reads from the standard input and writes to both the standard output and one or more files. The options can modify the behavior of the tee command, such as appending to the file instead of overwriting it, or ignoring interrupt signals. The file is the name of the file to which the output is written. If no file is given, the tee command will only write to the standard output.

Therefore, the command foo | tee /tmp/foodata will run the foo command, pipe its output to the tee command, which will display the output on the screen and write it to the file /tmp/foodata.

The other commands are incorrect for the following reasons:

• A, foo | less /tmp/foodata: This command will not write the output of the foo command to a file, but it will display the output of the foo command on the screen in a pager. The less command is a program that allows the user to view and scroll through a file or the output of a command. The syntax of the command is:

foo | less [options] [file]

The foo command is any command that produces some output. The | symbol is a pipe operator that redirects the standard output of one command to the standard input of another command. The less command reads from the standard input or a file and displays it on the screen in a pager. The options can modify the behavior of the less command, such as setting the number of lines per screen, or searching for a pattern. The file is the name of the file to be viewed. If no file is given, the less command will read from the standard input.

Therefore, the command foo | less /tmp/foodata will run the foo command, pipe its output to the less command, which will display the output on the screen in a pager. However, the /tmp/foodata argument will be ignored by the less command, because it will read from the standard input instead of the file. The command will not write anything to the file /tmp/foodata.

• B, foo | cp /tmp/foodata: This command will not work as expected, because it has several errors. First, the cp command is not a valid command to write the output of a command to a file. The cp command is used to copy files or directories from one location to another. The syntax of the command is:

cp [options] source destination

The options can modify the behavior of the cp command, such as preserving the attributes of the files, or creating backups of the existing files. The source is the name of the file or directory to be copied. The destination is the name of the file or directory where the source is copied to.

Second, the pipe operator is not a valid way to redirect the output of a command to the cp command. The pipe operator redirects the standard output of one command to the standard input of another command. However, the cp command does not read from the standard input, but from the source argument. Therefore, the command foo | cp /tmp/foodata will run the foo command, pipe its output to the cp command, which will ignore the standard input and report an error for missing the destination argument. The command will not write anything to the file /tmp/foodata.

• C, foo > /tmp/foodata: This command will not display the output of the foo command on the screen, but it will write it to a file called /tmp/foodata. The > symbol is a redirection operator that redirects the standard output of a command to a file or device, overwriting any existing content. The syntax of the command is:

foo > file

The foo command is any command that produces some output. The > symbol redirects the standard output of the foo command to the file. The file is the name of the file to which the output is written.

Therefore, the command foo > /tmp/foodata will run the foo command, redirect its output to the file /tmp/foodata, and overwrite any previous content. The command will not display anything on the screen.

• E, foo > stdout >> /tmp/foodata: This command will not work as expected, because it has several errors. First, the stdout argument is not a valid file name or device name. The stdout is an abbreviation for the standard output, which is a stream that a program uses to write its output. However, the stdout is not a file or device that can be used as a destination for the redirection operator. Second, the >> symbol is a redirection operator that redirects the standard output of a command to a file or device, appending to any existing content. The syntax of the command is:

foo >> file

The foo command is any command that produces some output. The >> symbol redirects the standard output of the foo command to the file. The file is the name of the file to which the output is appended.

Therefore, the command foo > stdout >> /tmp/foodata will run the foo command, redirect its output to the stdout argument, which will cause an error, and then redirect its output again to the file /tmp/foodata, which will append the output to the file. The command will not display anything on the screen.

References:

• Linux Tee Command with Examples | Linuxize
• tee command in Linux with examples - GeeksforGeeks
• Linux tee command explained for beginners (6 examples) - HowtoForge
• Command Options and Examples of Tee Command in Linux - UbuntuPIT
• Linux tee Command Explained for Beginners (6 Examples) - Linux Handbook.