The difference between a single quotation mark and a double quote delimiter

Double quote resolution variable, single quote non-parsed variable, delimiter parse variable
Direct output single quotation marks in double quotes, double quotes directly in single quotation marks,
Single quote output single quotes need to be escaped, double quote output double quotes need to be escaped. (Escape character "\")
The double quote output variable name needs to be escaped (' \ ');

Cases:

Double quotes

$a = "123";
$b = "456";
$c = $a. $b;
echo "\ $c";d ie;
The output is the variable name $c

Echo $c;

The output result is $c=123456;

$a = ' jjjjjj ';
$b = "AAA $a";
Echo $b;

The output result is $b= "AAAJJJJJJ";

Delimiter: The delimiter text behaves just like a double-quoted string, with no double quotes

This means that you do not need to escape quotes in the delimiter text, but you can still use the escaped code listed above. Variables are expanded, but when you express complex variables in the delimiter text, you should be aware of the same as the string. (Note: There must be no space in front of the end of ABC)

Echo <<<ABC
111 $a
Abc

The difference between a single quotation mark and a double quote delimiter