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.According to the operation performed on the operators it has been classified into the below types.

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 −

OperatorDescriptionExample
+ (Addition)Adds values on either side of the operator`expr $a + $b` will give 30
– (Subtraction)Subtracts right hand operand from left hand operand`expr $a – $b` will give -10
* (Multiplication)Multiplies values on either side of the operator`expr $a \* $b` will give 200
/ (Division)Divides left hand operand by right hand operand`expr $b / $a` will give 2
% (Modulus)Divides left hand operand by right hand operand and returns remainder`expr $b % $a` will give 0
= (Assignment)Assigns right operand in left operanda = $b would assign value of b into a
== (Equality)Compares two numbers, if both are same then returns true.[ $a == $b ] would return false.
!= (Not Equality)Compares two numbers, if both are different then returns true.[ $a != $b ] would return true.

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 −

OperatorDescriptionExample
-eqChecks if the value of two operands are equal or not; if yes, then the condition becomes true.[ $a -eq $b ] is not true.
-neChecks if the value of two operands are equal or not; if values are not equal, then the condition becomes true.[ $a -ne $b ] is true.
-gtChecks if the value of left operand is greater than the value of right operand; if yes, then the condition becomes true.[ $a -gt $b ] is not true.
-ltChecks if the value of left operand is less than the value of right operand; if yes, then the condition becomes true.[ $a -lt $b ] is true.
-geChecks if the value of left operand is greater than or equal to the value of right operand; if yes, then the condition becomes true.[ $a -ge $b ] is not true.
-leChecks if the value of left operand is less than or equal to the value of right operand; if yes, then the condition becomes true.[ $a -le $b ] is true.

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 −

OperatorDescriptionExample
!This is logical negation. This inverts a true condition into false and vice versa.[ ! false ] is true.
-oThis is logical OR. If one of the operands is true, then the condition becomes true.[ $a -lt 20 -o $b -gt 100 ] is true.
-aThis is logical AND. If both the operands are true, then the condition becomes true otherwise false.[ $a -lt 20 -a $b -gt 100 ] is false.

String Operators

The following string operators are supported by Bourne Shell.

Assume variable a holds “abc” and variable b holds “efg” then −

OperatorDescriptionExample
=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.
-zChecks if the given string operand size is zero; if it is zero length, then it returns true.[ -z $a ] is not true.
-nChecks if the given string operand size is non-zero; if it is nonzero length, then it returns true.[ -n $a ] is not false.
strChecks if str is not the empty string; if it is empty, then it returns false.[ $a ] is not false.

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 readwrite and execute permission on −

OperatorDescriptionExample
-b fileChecks if file is a block special file; if yes, then the condition becomes true.[ -b $file ] is false.
-c fileChecks if file is a character special file; if yes, then the condition becomes true.[ -c $file ] is false.
-d fileChecks if file is a directory; if yes, then the condition becomes true.[ -d $file ] is not true.
-f fileChecks if file is an ordinary file as opposed to a directory or special file; if yes, then the condition becomes true.[ -f $file ] is true.
-g fileChecks if file has its set group ID (SGID) bit set; if yes, then the condition becomes true.[ -g $file ] is false.
-k fileChecks if file has its sticky bit set; if yes, then the condition becomes true.[ -k $file ] is false.
-p fileChecks if file is a named pipe; if yes, then the condition becomes true.[ -p $file ] is false.
-t fileChecks if file descriptor is open and associated with a terminal; if yes, then the condition becomes true.[ -t $file ] is false.
-u fileChecks if file has its Set User ID (SUID) bit set; if yes, then the condition becomes true.[ -u $file ] is false.
-r fileChecks if file is readable; if yes, then the condition becomes true.[ -r $file ] is true.
-w fileChecks if file is writable; if yes, then the condition becomes true.[ -w $file ] is true.
-x fileChecks if file is executable; if yes, then the condition becomes true.[ -x $file ] is true.
-s fileChecks if file has size greater than 0; if yes, then condition becomes true.[ -s $file ] is true.
-e fileChecks if file exists; is true even if file is a directory but exists.[ -e $file ] is true.