Back quotes, single quotes, double quotes in the Linux Shell

The back quotation marks the top of the TAB key of the (') keyboard, and the 1 key to the left.
Plays the role of command substitution in Linux. Command substitution refers to the ability of the shell to interpolate the standard output of a command anywhere on a command line.
as follows, the shell executes the date command in the anti-quotation marks and inserts the result into the content displayed by the echo command.
echo Today is ' date '
Today is April 19, 2016 Tuesday 15:53:59 CST
　　
Single and double quotation marks are used when the user assigns a string with a space to the variable's delimiter.
Str= "Today is Tuesday"
Echo $str
Today is Tuesday

If there is no single or double quotation marks, the shell interprets the string after the space as a command.

Str=today is Tuesday

./date.sh: Line 2:is: Command not found

The difference between single and double quotation marks. Single quotes tell the shell to ignore all special characters, while double quotes ignore most, but do not include $, \, '.

Such as:

echo "' Date '"

Output: April 19, 2016 Tuesday 16:09:17 CST

Echo ' Date '

Output: ' Date '

Back quotes, single quotes, double quotes in the Linux Shell