Small questions about shell Programming

Ask a small shell programming question-Linux general technology-Linux programming and kernel information. The following is a detailed description. 6
#! /Bin/bash
# Demo for if/then/elif/else/fi
If [$ #-eq 2] // $ # What does it mean that the value is smaller than 2 and cp is as follows?
Then
Cp $1 $2 // $0 $1 $2
Elif [$ #-eq 1]
Then
Cat $1
Else
Echo $0 file [dir/ls]
Fi

7
// Sum. sh
#! /Bin/bash
Please input an integer a for read-p
I = 0
Sum = 0
While ($ I <= $ a) # How to write with ??
Do
Sum = $ [sum + $ I]
(I ++ ))
Done
The sum of the values within echo $ {a} is $ {sum}

8
How to use shell programming to print out all prime numbers within 100