How To Check String Nullability In Unix/Linux

In this article we will see how to validate/check whether a variable is NULL or empty. Let see with the help of examples.Lets say we have a variable V_Error_Msg as given below; I will explain all the solution on this variable.

V_Error_Msg="ORA-00942"

Using ==/= Operator

if [ ${V_Error_Msg} = "" ];
then
  echo "V_Error_Msg is Empty/NULL"
else
  echo "V_Error_Msg is Not Empty/NULL"
fi

[OR]

if [ ${V_Error_Msg} == "" ];
then
  echo "V_Error_Msg is Empty/NULL"
else
  echo "V_Error_Msg is Not Empty/NULL"
fi

Output: Both will produce same output in some shells equal(=) to operator do the needful and in some double equal(==) do the needful.
V_Error_Msg is Not Empty/NULL

Using ! Operator

if [ ! "${V_Error_Msg}" ];
then
  echo "V_Error_Msg is Empty/NULL"
else
  echo "V_Error_Msg is Not Empty/NULL"
fi

Output:
V_Error_Msg is Not Empty/NULL
Explanation: If V_Error_Msg holds any value then if return true, means variable is not empty and if V_Error_Msg is NULL then if block return false. Here variable V_Error_Msg is not empty hence it has returned TRUE and ! of TRUE is FALSE hence "V_Error_Msg is Not Empty/NULL" has been printed on terminal.

Using -z Switch

$ [ -z "${V_Error_Msg}" ] && echo "V_Error_Msg is Empty/NULL" || echo "V_Error_Msg is Not Empty/NULL"

Output:
V_Error_Msg is Empty/NULL
Explanation: Syntax reads, if V_Error_Msg is empty then(&&) TRUE else(||) FALSE

$ [ ! -z "${V_Error_Msg}" ] && echo "V_Error_Msg is Not Empty/NULL"  || echo "V_Error_Msg is Empty/NULL"

Output:
V_Error_Msg is Empty/NULL
Explanation: Syntax reads, if V_Error_Msg is Not empty then(&&) TRUE else(||) FALSE

Using -n Switch

$ [ ! -n "${V_Error_Msg}" ] && echo "V_Error_Msg is Empty/NULL" || echo "V_Error_Msg is Not Empty/NULL"

Output:
V_Error_Msg is Empty/NULL
Explanation: Syntax reads, if V_Error_Msg is empty then(&&) TRUE else(||) FALSE

$ [ -n "${V_Error_Msg}" ] && echo "V_Error_Msg is Not Empty/NULL"  || echo "V_Error_Msg is Empty/NULL"

Output:
V_Error_Msg is Empty/NULL

Explanation: Syntax reads, if V_Error_Msg is empty then(&&) TRUE else(||) FALSE
Note: Please observe carefully, there is a space between operator and operand. If we will not provide the space it will through an error
sh: test: argument expected

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