Single quote, double quotation mark in shell

# Abc=haha

# echo $ABC

haha

# echo "$abc"

haha

# echo ' $abc '

$abc

# echo ' $abc '

haha

# echo ' $abc ' $ABC '

Haha$abc

From the above example, it is not difficult to see that the double quotation marks are not shielded from the variable and some special symbols escaped, and all the contents of the single quotation marks will be intact output, and the single quotation marks in single quotes to the variable, the variable can be normal display, a bit like the negative negative in mathematics is positive.

Single quote, double quotation mark in shell