Basic Operators & Arrays

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

The program above would be used to add two numbers

The following points need to be considered while adding:

There must be spaces between operators and expressions. (2+2 is not correct; it should be written as 2 + 2)
The complete expression should be enclosed between backticks (` `).

Basic Operators

Operator	Description	Example
+ (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 operand	a = $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.

To evaluate floating point we must use other utilities such as bc or awk.

Relational Operators

Numeric Operator	Description	Example
-eq	Checks if the value of two integer numeric operands are equal or not; if yes, then the condition becomes true.	[[ $a -eq $b ]] is not true.
-ne	Checks 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.
-gt	Checks 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.
-lt	Checks 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.
-ge	Checks 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.
-le	Checks 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.

Boolean Operators

Operator	Description	Example
!	This is logical negation. This inverts a true condition into false and vice versa.	[[ ! false ]] is true.
\|\|	This is logical OR. If one of the operands is true, then the condition becomes true.	[[ $a -lt 20 \|\| $b -gt 100 ]] is true.
&&	This is logical AND. If both the operands are true, then the condition becomes true otherwise false.	[[ $a -lt 20 && $b -gt 100 ]] is false.

String Operators

String 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.

File Test Operators

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

Shell Arrays

You can access all the items in an array in one of the following ways:

Get the indexes of each element in an array by: