In bash, what is the difference between a null string and an empty string?

Question

I wonder if there is a difference in how bash represents variables with null values

var=

and with empty strings

var=""

What care should I take when manipulating variables like these?

Answer 1

Consider the following:

var1=

var2=""

The variable var1 has no value, has its null value (null). A null value is exactly a "NOTHING"! Different from the variable value var2 that has an empty string.

Imagine that we will be using some object language (just to be more didactic). Using the Python example:

$ python
Python 2.7.12 (default, Jun 29 2016, 14:05:02) 
[GCC 4.2.1 Compatible Apple LLVM 7.3.0 (clang-703.0.31)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> var1 = None
>>> var2 = ""
>>> type(var1)
<type 'NoneType'>
>>> type(var2)
<type 'str'>

ps: None in python is the same thing as null in other languages.

The situation of var1 demonstrates that the variable does not initialize a value. Therefore, a "nothing" cannot receive any feedback if you try... you will receive the famous error NullPointerException because the "nothing" cannot receive attribute(s): it does not receive values and points to anything.

>>> len(var1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: object of type 'NoneType' has no len()

To var2 is a variable of the type string empty. Something "empty" has its value of zero because this way the variable is initialized. That is, even if it is a "blank character" it has its reserved place in memory. So, when used the len() (length) will be returned 0:

>>> len(var2)
0

Answer 2

Please consider the shell script in sh:

#! /bin/sh 

aNullString=
anEmptyString=""

if [ -z ${aNullString} ] ; then
    echo The string aNullString is empty 
else
    echo The string aNullString is not empty 
fi

if [ -z ${anEmptyString} ] ; then
    echo The string anEmptyString is empty 
else
    echo The string anEmptyString is not empty 
fi

if [ ${aNullString} = ${anEmptyString}] ; then
    echo The strings aNullString and anEmptyString are equal
else
    echo The strings aNullString and anEmptyString differ 
fi

He suggests that a= and b='' are the same character string as its result is:

The string aNullString is empty
The string anEmptyString is empty
The strings aNullString and anEmptyString are equal