printf "Hello %s, welcome to %s.\n" "$USER"

printf "Hello %s, welcome to %s.\n" "$USER" "$HOSTNAME"

ShellCheck has noticed that you're using a printf format string with more %s variables than arguments to fill them.

In the problematic example case, the last %s will just become an empty string every time.

Either remove the unused variables from the format string, or add enough arguments to fill them.

When using the Ksh/Bash %T timestamp extension, such as printf 'The time is %(%H:%M)T\n', an argument of -1 and no argument are both taken to mean the current time. In these cases, consider specifying -1 explicitly.