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 it has 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

The regular expression that represents a single upper-case letter is [A-Z]. This is a character class that matches any one character from the range A to Z, which are the 26 upper-case letters of the English alphabet. A character class is enclosed in square brackets and can contain a list or a range of characters to match. For example, [abc] matches any one of the letters a, b, or c, and [0-9] matches any one digit from 0 to 9. The other options are not valid regular expressions for a single upper-case letter. The :UPPER: option is not a valid syntax for a character class, and it would be interpreted as a literal string of seven characters. The !a-z option is not a valid syntax for a negated character class, and it would be interpreted as a literal string of four characters. The %C option is not a valid syntax for a character class, and it would be interpreted as a literal string of two characters. The {AZ} option is not a valid syntax for a character class, and it would be interpreted as a literal string of four characters. The curly braces are used for interval expressions, which specify the number of repetitions of a character or a group of characters. For example, a{2,4} matches the letter a repeated two, three, or four times. References:

How to Use Regular Expressions (regexes) on Linux1

How to Use Regular Expressions (RegEx) on Linux2

A beginner’s guide to regular expressions with grep3

 The examples for Bash file globbing that match a file named root-can-do-this.txt are:

root*can?do-this.{txt,odt}: This matches any file that starts with root, followed by any number of characters, followed by can, followed by any single character, followed by do-this, followed by either .txt or .odt extension.

{root,user,admin}-can-??-this.txt: This matches any file that starts with either root, user or admin, followed by a hyphen, followed by can, followed by any two characters, followed by -this, followed by .txt extension.

root***{can,may}-do-this.[tT][xX][tT]: This matches any file that starts with root, followed by any number of characters, followed by either can or may, followed by -do-this, followed by any combination of t, T, x or X for the extension.

The examples that do not match are:

r[oOoO]t-can-do*.txt: This matches any file that starts with r, followed by either o, O, o or O, followed by t-can-do, followed by any number of characters, followed by .txt extension. This does not match because the file name has only one o after r.

rootcando??this.txt: This matches any file that starts with root, followed by any number of characters, followed by can, followed by any number of characters, followed by do, followed by any two characters, followed by this, followed by .txt extension. This does not match because the file name has a hyphen between do and this. References:

LPI Exam 101 Detailed Objectives, Topic 103: GNU and Unix Commands, Weight: 25, Objective 103.3: Perform basic file management

LPI Linux Essentials Study Guide, Chapter 3: Working on the Command Line, Section 3.3: Globbing, Page 58

Pressing the Ctrl-C combination on the keyboard while a command is executing in the foreground sends the SIGINT(2) signal code. The SIGINT(2) signal means interrupt and is usually sent when the user presses Ctrl-C on the keyboard. The signal causes the process to terminate, unless it is caught or ignored by the process. The SIGHUP(1) signal means hang up and is usually sent when the terminal or network connection is disconnected. The SIGQUIT(3) signal means quit and is usually sent when the user presses Ctrl-\ on the keyboard. The SIGKILL(9) signal means kill and is used to force the process to terminate immediately, without any chance to catch the signal or perform any cleanup. The SIGTERM(15) signal means terminate and is the default signal sent by the kill command. References: LPI Exam 101 Detailed Objectives, Topic 103: GNU and Unix Commands, Weight: 25, Objective 103.3: Perform basic file management, Signal List

The dmesg command is used to print or control the kernel ring buffer. The kernel ring buffer is a special kind of buffer that stores the messages generated by the kernel, such as the boot messages, the hardware errors, the driver information, etc. The kernel ring buffer is a fixed-size circular buffer, which means that when it is full, the oldest messages are overwritten by the new ones. The dmesg command can show the messages in the kernel ring buffer, as well as filter, save, or clear them. The syntax of the dmesg command is:

dmesg [options]

The options are optional, and they can modify the behavior of the dmesg command, such as changing the output format, the timestamp, the level, the facility, the color, etc. For more information, see the dmesg man page1 or the Linuxize tutorial2.

One of the common reasons for the error message “device is busy” when trying to unmount a file system is that a user or a process has a file open in the mounted directory. This prevents the kernel from releasing the file system resources and detaching the device. To find out which user or process is holding the file system, one can use the lsof or fuser commands12. For example, lsof /mnt or fuser -m /mnt will list the processes that have open files in /mnt. To force the unmounting of a busy file system, one can use the -l option of the umount command, which will perform a lazy unmount. This means that the file system will be detached as soon as it is not busy anymore3. References: 1: How to solve “device is busy” problem in Linux 2: How to Find Out Which Process Is Using a File in Linux 3: umount(8) - Linux man page

The apt-get command is used to interact with the APT package management system on Debian-based Linux distributions. The apt-get command has several subcommands that perform different operations on packages. One of these subcommands is full-upgrade, which is used to install the newest versions of all currently installed packages, along with their dependencies. The full-upgrade command also removes any packages that are no longer needed or that conflict with the upgraded packages. The full-upgrade command is equivalent to the dist-upgrade command, which is an older name for the same operation. The other options are not valid subcommands of apt-get. The auto-update option does not exist, the install option is used to install specific packages, not to upgrade them, the update option is used to update the list of available packages, not to install them, and the dist-upgrade option is the same as the full-upgrade option. References:

APT-GET Command in Linux {Detailed Tutorial With Examples} - phoenixNAP

Using apt-get Commands in Linux [Ultimate Guide] - It’s FOSS

Ubuntu ‘apt-get’ list of commands (list, update, upgrade, cheatsheet …