bash array syntax and loops, please explain [duplicate]

Question

I get confused on array syntax in bash. I typically use quotes to enclose strings to loop through, but tutorials typically use parentheses. For kicks I made 4 simple loops. The first two work as I expected by echoing each item in the array, the second two gave unexpected output.

Why does this happen?

LOOP #1

$ List="item1 item2 item3"
$ for f in $List; do echo ${f}; done
item1
item2
item3

LOOP #2

$ List=(item1 item2 item3)
$ for f in ${List[@]}; do echo ${f}; done
item1
item2
item3

LOOP #3

$ List="item1 item2 item3"
$ for f in ${List[@]}; do echo ${f}; done
item1
item2
item3
item2
item3

LOOP #4

$ List=(item1 item2 item3)
$ for f in $List; do echo ${f}; done
item1

Answer 1

The first two work as I expected by echoing each item in the array, the second two gave unexpected output.

Your fourth loop is equivalent to for f in ${List[0]}; do echo ${f}; done. This is documented in man bash:

Referencing an array variable without a subscript is equivalent to referencing the array with a subscript of 0.

Third loop works but is not printing the original array. Try:

List=(item1 item2 item3)     # Declares array List
printf '%s\n' "${List[@]}"
item1
item2
item3

List="item1 item2 item3"     # Overwrites first element of array... 
                             # ... equivalent to List[0]="item1 item2 item3"
printf '%s\n' "${List[@]}"
item1 item2 item3
item2
item3

Answer 2

First 3 snippets should work, but the problem with 3rd is that List="item1 item2 item3" will replace item1 with "item1 item2 item3" and that's why you are getting the output

item1
item2
item3
item2
item3 

Change the name of the variable and it will work

List2="item1 item2 item3"   

In last snippet $List is used without index and this is same as referring to the content of the first element of the array List.