Which statement correctly describes the shell parameters $@, $$, and $!?

• A. $@ prints all command arguments individually quoted; $$ is the PID of the current shell; $! is the PID of the last background process.
• B. $@ prints the first argument; $$ prints the process ID of the last command; $! prints the PID of the currently running shell.
• C. $@ prints the script name; $$ prints the process ID of the current thread; $! is the exit status of last command.
• D. They are environment variables unrelated to the script's arguments.

Answer: (A)

Understanding shell parameters: $@ represents all positional parameters. When used as "$@", it expands to each argument as a separate, properly quoted word, preserving spaces inside arguments. The current shell’s process ID is given by $$. The PID of the most recently started background job is $!, which is the PID of the last background process you launched (for example, with &). So the statement that describes these three correctly—$@ as all command arguments (each argument as a separate quoted item), $$ as the PID of the current shell, and $! as the PID of the last background process—is the right one. The other descriptions misstate one or more of these parameters (such as mislabeling what $@, $$, or $! expand to).