Shell programming ~ Encountered a problem ~ Who can explain it?

Shell programming ~ Encountered a problem ~ Who can explain-Linux general technology-Linux technology and application information? The following is a detailed description. The problems I encountered during programming are simplified as follows:
1) generate a file:
$ Echo-n "abcd"> file
2) write a script (bash environment) to read the file:
$ Cat f1
#! /Bin/bash
While read
Do
Echo $
Done <file

No data is found ~~~
However, if the while statement is not used, you can read the following:
$ Cat f2
#! /Bin/bash
Read a <file
Echo $
I tried it for a long time and found it related to '-n' in my echo statement. After this option is added, I cannot read data using the while statement. Why ?? Looking forward to expert answers! Thank you!