Unix / Linux - Shell Basic Operators

There are various operators supported by each shell. We will discuss in detail about Bourne shell (default shell) in this chapter.

We will now discuss the following operators −

• Arithmetic Operators
• Relational Operators
• Boolean Operators
• String Operators
• File Test Operators

Bourne shell didn't originally have any mechanism to perform simple arithmetic operations but it uses external programs, either awk or expr.

The following example shows how to add two numbers −

#!/bin/sh

val=`expr 2 + 2`
echo "Total value : $val"

The above script will generate the following result −

Total value : 4

The following points need to be considered while adding −

• There must be spaces between operators and expressions. For example, 2+2 is not correct; it should be written as 2 + 2.
• The complete expression should be enclosed between ‘ ‘, called the backtick.

Arithmetic Operators

The following arithmetic operators are supported by Bourne Shell.

Assume variable a holds 10 and variable b holds 20 then −

It is very important to understand that all the conditional expressions should be inside square braces with spaces around them, for example [ $a == $b ] is correct whereas, [$a==$b] is incorrect.

All the arithmetical calculations are done using long integers.

Relational Operators

Bourne Shell supports the following relational operators that are specific to numeric values. These operators do not work for string values unless their value is numeric.

For example, following operators will work to check a relation between 10 and 20 as well as in between "10" and "20" but not in between "ten" and "twenty".

Assume variable a holds 10 and variable b holds 20 then −

It is very important to understand that all the conditional expressions should be placed inside square braces with spaces around them. For example, [ $a <= $b ] is correct whereas, [$a <= $b] is incorrect.

Boolean Operators

The following Boolean operators are supported by the Bourne Shell.

Assume variable a holds 10 and variable b holds 20 then −

String Operators

The following string operators are supported by Bourne Shell.

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

File Test Operators

We have a few operators that can be used to test various properties associated with a Unix file.

Assume a variable file holds an existing file name "test" the size of which is 100 bytes and has read, write and execute permission on −