The difference and relation between the inverted quotation mark and the double quotation mark in shell script

Double quotes

Purpose: In order to contain text or function segments. In addition to itself, the function inside the counter-quote, the variable starting with $ and \ Start the inverse of the character outside, the rest are directly output.

The content enclosed in double quotes is treated as a single string. It prevents wildcard expansion, but allows variable expansion. This is different from the way a single argument is handled.

Anti-Quote

The purpose is to keep the function conversion within the double quotation marks. But the single quotation mark has no effect. in the preceding single double quotation mark, enclose the string, but what if the string is a list of command columns? The answer is no execution. To deal with this situation, we have to use anti-single quotes to do it.
fdv= ' Date +%f ' echo "Today $FDV"
The date +%f in the inverted quotation marks is treated as an instruction, and the result of execution is taken into the FDV variable.

The difference and relation between the inverted quotation mark and the double quotation mark in shell script