PDF 101-500 Study Guide

The umask value is a four-digit octal number that determines the default permissions for new files and directories created by a user. The umask value specifies the permissions that are not granted to the user, group, and others. The permissions are represented by three bits for each category, where 1 means execute, 2 means write, and 4 means read. The sum of these values indicates the combination of permissions. For example, 7 means read, write, and execute; 6 means read and write; 5 means read and execute; 4 means read only; 3 means write and execute; 2 means write only; 1 means execute only; and 0 means no permission.

The umask value is subtracted from the maximum permissions for files and directories, which are 666 and 777, respectively. The maximum permissions are then converted to binary and bitwise ANDed with the bitwise complement of the umask value. The result is the default permissions for the new files and directories. For example, if the umask value is 0027, the default permissions for a new file are:

666 - 027 = 639 639 in octal = 110 011 001 in binary 027 in octal = 000 010 111 in binary Bitwise complement of 027 = 111 101 000 in binary 110 011 001 AND 111 101 000 = 110 001 000 in binary 110 001 000 in binary = 608 in octal 608 in octal = rw- — —

The default permissions for a new directory are:

777 - 027 = 750 750 in octal = 111 101 000 in binary 027 in octal = 000 010 111 in binary Bitwise complement of 027 = 111 101 000 in binary 111 101 000 AND 111 101 000 = 111 101 000 in binary 111 101 000 in binary = 750 in octal 750 in octal = rwx r-x —

Therefore, the umask value of 0027 ensures that new files can be read and written by their owning user, and are not accessible at all for everyone else. New directories can be read, written and listed by their owning user, read and listed by their owning group, and are not accessible at all for everyone else.

References:

• Umask command in Linux with examples - GeeksforGeeks
• What Is umask in Linux, and How Do You Use It? - How-To Geek
• What is UMASK and how to set UMASK in Linux/Unix? - Kernel Talks

The correct answer is A, tr -d ‘\r’ < userlist.txt > newlist.txt. This command will use the tr utility to delete the carriage return characters (\r) from the userlist.txt file and redirect the output to the newlist.txt file. The tr utility is used to translate, delete, or squeeze characters from the standard input and write the result to the standard output. The syntax of the tr command is:

tr [options] set1 [set2]

The options can modify the behavior of the tr command, such as complementing, squeezing, or truncating the sets of characters. The set1 and set2 are strings of characters that specify the translation to be performed. The characters in set1 are replaced by the corresponding characters in set2. If set2 is omitted, the characters in set1 are deleted.

The -d option tells tr to delete the characters in set1 from the output. The \r character is a special escape sequence that represents the carriage return character, which is used in Windows systems to mark the end of a line, along with the line feed character (\n). The < and > symbols are redirection operators that redirect the input and output of a command to a file or device. The < symbol redirects the standard input of a command from a file, while the > symbol redirects the standard output of a command to a file, overwriting any existing content.

Therefore, the command tr -d ‘\r’ < userlist.txt > newlist.txt will read each character from the userlist.txt file, delete any carriage return characters, and write the output to the newlist.txt file. This will effectively change all CR-LF line breaks (\r\n) in the userlist.txt file to Linux standard LF line breaks (\n) and store the result in newlist.txt.

The other commands are incorrect for the following reasons:

• B, tr -c ‘\n\r’ ‘’ userlist.txt: This command will not work as expected, because it has several errors. First, the -c option tells tr to complement the set of characters in set1, which means that the operations will apply to the characters that are not in the set1. However, the set1 in this command is ‘\n\r’, which includes both the line feed and the carriage return characters, so the complement will be all the other characters except these two. Second, the set2 in this command is an empty string (‘’), which means that tr will delete the characters that match the set1 (or its complement, in this case). Third, the redirection operators are reversed, which means that tr will read the input from the newlist.txt file and write the output to the userlist.txt file, instead of the other way around. Therefore, the command tr -c ‘\n\r’ ‘’ userlist.txt will delete all the characters except the line feed and the carriage return characters from the newlist.txt file and overwrite the userlist.txt file with the output, which will be a series of blank lines.
• C, tr ‘\r\n’ ‘’ newlist.txt: This command will not work as expected, because it has several errors. First, the set1 in this command is ‘\r\n’, which means that tr will match the carriage return and the line feed characters separately, not as a pair. Second, the set2 in this command is an empty string (‘’), which means that tr will delete the characters that match the set1. Third, the redirection operators are missing, which means that tr will not read the input from the userlist.txt file or write the output to the newlist.txt file, but instead it will expect the input from the standard input and write the output to the standard output. Therefore, the command tr ‘\r\n’ ‘’ newlist.txt will delete all the carriage return and the line feed characters from the standard input and write the output to the standard output, and treat the userlist.txt and newlist.txt as additional arguments that will cause an error.
• D, tr ‘\r’ ‘\n’ userlist.txt newlist.txt: This command will not work as expected, because it has several errors. First, the set1 and set2 in this command are ‘\r’ and ‘\n’, which means that tr will replace the carriage return characters with the line feed characters, not delete them. Second, the redirection operators are missing, which means that tr will not read the input from the userlist.txt file or write the output to the newlist.txt file, but instead it will expect the input from the standard input and write the output to the standard output. Third, the userlist.txt and newlist.txt are treated as additional arguments that will cause an error. Therefore, the command tr ‘\r’ ‘\n’ userlist.txt newlist.txt will replace all the carriage return characters with the line feed characters from the standard input and write the output to the standard output, and report an error for the extra arguments.
• E, tr -s ‘/M/J/’ userlist.txt newlist.txt: This command will not work as expected, because it has several errors. First, the -s option tells tr to squeeze the repeated characters in set1 with a single occurrence, not delete them. Second, the set1 in this command is ‘/M/J/’, which is not a valid escape sequence for the carriage return and the line feed characters. The correct escape sequences are ‘\r’ and ‘\n’, respectively. Third, the redirection operators are missing, which means that tr will not read the input from the userlist.txt file or write the output to the newlist.txt file, but instead it will expect the input from the standard input and write the output to the standard output. Fourth, the userlist.txt and newlist.txt are treated as additional arguments that will cause an error. Therefore, the command tr -s ‘/M/J/’ userlist.txt newlist.txt will squeeze the repeated occurrences of the characters /, ^, M, and J from the standard input and write the output to the standard output, and report an error for the extra arguments.

References:

• Tr Command in Linux with Examples | Linuxize
• tr command in Unix/Linux with examples - GeeksforGeeks
• How to Use the Linux tr Command - How-To Geek
• [tr(1) - Linux manual page - man7.org]
• [Linux tr command help and examples - Computer Hope].

The boot sequence of a PC using a BIOS is the process of loading and executing the operating system from the power-on state. The BIOS (Basic Input/Output System) is a firmware program that is stored in a ROM chip on the motherboard and provides the interface between the hardware and the software. The BIOS performs the following tasks during the boot sequence12:

• It runs the power-on self-test (POST) to check the integrity and functionality of the hardware components and peripherals.
• It reads the BIOS settings from the CMOS memory and applies them to the system configuration. Some parts of the boot process, such as the boot device order, can be configured from the BIOS by pressing a specific key (usually F2, F10, or Del) during the POST.
• It locates and loads the boot loader program from the first sector of the boot device, which is usually the hard disk. The boot loader program can be GRUB, LILO, or Windows Boot Manager, depending on the operating system installed. The boot loader is responsible for loading the kernel and the initial ramdisk (initrd) into memory and passing control to them.
• It transfers the execution to the boot loader and exits the boot sequence.

The other options are false or irrelevant. Linux does require the assistance of the BIOS to boot a computer, at least until the kernel takes over the hardware control. The BIOS boot process does not start only if secondary storage is functional, it can also boot from other devices such as USB drives, CD-ROMs, or network. The BIOS is not started by loading hardware drivers from secondary storage, it is stored in a ROM chip and runs before any secondary storage is accessed. References:

• Linux Essentials - Linux Professional Institute Certification Programs1
• Exam 101 Objectives - Linux Professional Institute2
• The Linux Booting Process - 6 Steps Described in Detail - freeCodeCamp.org3
• Guide to the Boot Process of a Linux System - Baeldung4

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