Summary of how shell assigns values to multiple variables

The requirements are as follows:

There is an AAAA file with the following content:

 Aaaaaaaaaaaaaaaa
Bbbbbbbbbbbbbbbbbb
Mmmccccccccccccccc
Dddddddddddddd
Mmmmmm
Ffffffffffffff

I want to extract the row numbers of "mm". The methods are summarized as follows:

Method 1:

 #! /Bin/sh

For J in 'grep-n "mm" AAAA | cut-d ":"-f1'
Do
Echo $ J
Done

Method 2:

 #! /Bin/sh

A = 'grep-n "mm" AAAA | cut-d ":"-f1'
For J in $ {A [@]}
Do
Echo $ J
Done

Method 3:

Note:

The first <is targeted, and the second <() is replaced by a process.