Unix / Linux - Shell String Operators Example

The following string operators are supported by Bourne Shell.

Assume variable a holds "abc" and variable b holds "efg" then −

Operator	Description	Example
=	Checks if the value of two operands are equal or not; if yes, then the condition becomes true.	[ $a = $b ] is not true.
!=	Checks if the value of two operands are equal or not; if values are not equal then the condition becomes true.	[ $a != $b ] is true.
-z	Checks if the given string operand size is zero; if it is zero length, then it returns true.	[ -z $a ] is not true.
-n	Checks if the given string operand size is non-zero; if it is nonzero length, then it returns true.	[ -n $a ] is not false.
str	Checks if str is not the empty string; if it is empty, then it returns false.	[ $a ] is not false.

Example

Here is an example which uses all the string operators −

Live Demo

#!/bin/sh

a="abc"
b="efg"

if [ $a = $b ]
then
   echo "$a = $b : a is equal to b"
else
   echo "$a = $b: a is not equal to b"
fi

if [ $a != $b ]
then
   echo "$a != $b : a is not equal to b"
else
   echo "$a != $b: a is equal to b"
fi

if [ -z $a ]
then
   echo "-z $a : string length is zero"
else
   echo "-z $a : string length is not zero"
fi

if [ -n $a ]
then
   echo "-n $a : string length is not zero"
else
   echo "-n $a : string length is zero"
fi

if [ $a ]
then
   echo "$a : string is not empty"
else
   echo "$a : string is empty"
fi

The above script will generate the following result −

abc = efg: a is not equal to b
abc != efg : a is not equal to b
-z abc : string length is not zero
-n abc : string length is not zero
abc : string is not empty

The following points need to be considered while using the operator −

There must be spaces between the operators and the expressions. For example, 2+2 is not correct. It should be written as 2 + 2.
if...then...else...fi statement is a decision-making statement which has been explained in the next chapter.

unix-basic-operators.htm