SHELL

$@ bash

file:test.sh
#! /bin/sh
echo '$#' $#
echo '$@' $@
echo '$?' $?

*If you run the above script as*

> ./test.sh 1 2 3

You get output:
$#  3
$@  1 2 3
$?  0

*You passed 3 parameters to your script.*

$# = number of arguments. Answer is 3
$@ = what parameters were passed. Answer is 1 2 3
$? = was last command successful. Answer is 0 which means 'yes'

${1:?} bash

bar=${1:?First argument missing} 
# It exits with code 1 the non-interactive shell if argument 1 
# does not exist and prints the message to sterr.