What output will be displayed when the user fred executes echo 'fred $USER'?

When the command echo 'fred $USER' is executed by the user fred, the output will be fred $USER. This occurs because single quotes in shell scripting (like Bash) prevent variable expansion. In this case, $USER is treated as a literal string rather than being expanded to the value of the variable that holds the username of the current user.

The shell processes the command and recognizes the single quotes, interpreting everything within them as a raw string. Therefore, it does not substitute $USER with the actual username. As a result, the output consists of the exact characters that were input, including the dollar sign and the variable name.

This behavior contrasts with double quotes, where variable expansion would occur, resulting in fred fred if the command were executed as echo "fred $USER" while logged in as user fred.