Linux Shell programming Variable reference

Shell A total of three reference symbols, double quotes, single quotes and anti-quotes ' (in the English state of the TAB key, 1 keys to the left of the key).

Double quotation mark "": References all characters except dollar sign $, anti-quote ' and backslash \.

Single quote ': refers to all characters.

Anti-quote ': The shell interprets the contents of the anti-quotation mark as a system command.

Backslash \: Escape symbol, masking the special meaning of the next character.

A variable with no double quotation marks is normally the same, for example:

num=10
Echo $num
echo "$num"
The two will print the same content.
Str= "X Y Z"
Echo $str
echo "$str"
The top two will print out different content. $STR print out: x y z, and "$str" prints x y z, and the double quotation marks will print out the original content, without adding multiple spaces to a single space.

Linux Shell programming Variable reference