Use of quotation marks in Shell

In shell scripts, quotation marks are usually used, but what are the differences between quotation marks?

Single quotes
Purpose: To prevent text conversion. Except for the single quotation marks, all words in the single quotation marks are output as is.

Double quotation marks
Purpose: To include text or function segments, except for functions in the backquotes, variables starting with $, and characters in the backticks are directly output.

Unquoted
Function Conversion, but it does not work in single quotes.
For example, if a = 'date' is used as the command to execute date and then the result is paid to.

Therefore, when writing, you should pay attention to the use of quotation marks, otherwise the error will be in these details.

Have fun!