This output will be displayed when the user fred executes the following command:

echo 'fred $USER'

The echo command is a built-in Linux feature that prints out arguments as the standard output. The syntax of the echo command is:

echo [option] [string]

The option can modify the behavior of the echo command, such as enabling the interpretation of escape characters or omitting the newline after the output. The string is the text that is displayed as the output.

The single quotation marks (’ ') are used to enclose the string argument and prevent any expansion or substitution of the characters inside the quotation marks. This means that any variables, commands, or special characters inside the single quotation marks are treated as literal characters, not as expressions.

The $USER variable is a shell variable that holds the username of the current user. However, since it is enclosed in single quotation marks, it is not expanded to its value, but printed as it is.

Therefore, the command echo ‘fred $USER’ will print the string fred $USER as the output, without any changes. The output will be the same for any user who executes the command, not just fred.

The other outputs are incorrect for the following reasons:

A, fred fred: This output would be displayed if the $USER variable was expanded to its value, which is fred for the user fred. However, since the $USER variable is enclosed in single quotation marks, it is not expanded, but printed as it is.

B, fred /home/fred/: This output would be displayed if the $USER variable was expanded to its value, which is fred for the user fred, and then concatenated with the string /home/ to form a path. However, since the $USER variable is enclosed in single quotation marks, it is not expanded, but printed as it is. Also, there is no concatenation operator in the echo command, so the string /home/ would not be added to the output.

C, ‘fred $USER’: This output would be displayed if the single quotation marks were also printed as part of the output. However, the single quotation marks are not part of the string argument, but only used to enclose it and prevent any expansion or substitution. They are not displayed as the output.

E, ‘fred fred’: This output would be displayed if the single quotation marks were also printed as part of the output, and the $USER variable was expanded to its value, which is fred for the user fred. However, neither of these conditions are true. The single quotation marks are not part of the string argument, but only used to enclose it and prevent any expansion or substitution. They are not displayed as the output. The $USER variable is enclosed in single quotation marks, so it is not expanded, but printed as it is.

References:

How to use Echo Command in Linux (With Examples) - phoenixNAP

How to Use the Echo Command on Linux - How-To Geek

echo command in Linux with Examples - GeeksforGeeks