Shell Basic Operators

The Shell, like other programming languages, supports a variety of operators, including:

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

Native Bash does not support simple math operations, but can be implemented with other commands , such as awk and expr,expr, which are most commonly used.

Expr is an expression evaluation tool that uses it to perform evaluation operations on expressions.

For example, two numbers are added ( note the use of anti-quotes ' instead of single quotes '):

#!/bin/bashval=' Expr 2 + 2 '"sum of two numbers: $val"   

Two points Note:

• There is a space between the expression and the operator.

Arithmetic operators

Suppose the variable A is 10 and the variable B is 20:

operator	Description	Example
+	Addition	The ' expr $a + $b ' result is 30.
-	Subtraction	The ' expr $a-$b ' result is 10.
*	Multiplication	The ' expr $a \* $b ' result is 200.
/	Division	The ' expr $b/$a ' result is 2.
%	Take surplus	The ' expr $b% $a ' result is 0.
=	Assign value	A= $b assigns the value of variable B to a.
==	Equal. Used to compare two numbers, the same returns true.	[$a = = $b] returns FALSE.
!=	Not equal. Used to compare two numbers, and returns true if they are different.	[$a! = $b] Returns TRUE.

Note: conditional expressions are placed between square brackets and have spaces, for example: [$a = = $b] is wrong and must be written as [$a = = $b].

Relational operators

Relational operators only support numbers, and strings are not supported unless the value of the string is a number.

Suppose the variable A is 10 and the variable B is 20:

operator	Description	Example
-eq	Detects whether two numbers are equal and returns true for equality.	[$a-eq $b] returns false.
-ne	Detects whether two numbers are equal and returns true if they are not equal.	[$a-ne $b] returns TRUE.
-gt	Detects if the number on the left is greater than the right and, if so, returns True.	[$a-gt $b] returns false.
-lt	Detects if the number on the left is less than the right and, if so, returns True.	[$a-lt $b] returns TRUE.
-ge	Detects if the number on the left is large equal to the right, and returns true if it is.	[$a-ge $b] returns false.
-le	Detects if the left number is less than or equal to the right, and returns true if it is.	[$a-le $b] returns TRUE.

Boolean operator

The following table lists the commonly used Boolean operators, assuming that variable A is 10 and variable B is 20:

operator	Description	Example
!	Non-operation, the expression is true returns False, otherwise true.	[! false] returns TRUE.
-O	Or operation, there is an expression of true to return true.	[$a-lt 20-o $b-GT 100] returns TRUE.
-A	With an operation, two expressions are true to return true.	[$a-lt 20-a $b-GT 100] returns FALSE.

logical operators

Suppose the variable A is 10 and the variable B is 20:

operator	Description	Example
&&	Logical AND	[[$a-lt && $b-GT 100]] returns false
||	Logical OR	[[$a-lt | | $b-GT 100]] returns True

String operators

The following table lists the commonly used string operators, assuming that variable a is "ABC" and Variable B is "EFG":

operator	Description	Example
=	Detects whether two strings are equal and returns true for equality.	[$a = $b] returns FALSE.
!=	Detects whether two strings are equal and returns true if they are not equal.	[$a! = $b] Returns TRUE.
-Z	Detects whether the string length is 0 and returns true for 0.	[-Z $a] returns false.
-N	Detects whether the string length is 0 and does not return true for 0.	[-N $a] returns true.
Str	Detects whether the string is empty and does not return true for null.	[$a] returns TRUE.

File Test Operators

A variety of properties used to detect Unix files.

operator	Description	Example
-B File	Detects if the file is a block device file, and returns True if it is.	[-B $file] returns FALSE.
-C file	Detects if the file is a character device file, and returns True if it is.	[-C $file] returns false.
-D File	Detects if the file is a directory, and returns True if it is.	[-D $file] returns false.
-F File	Detects if the file is a normal file (neither a directory nor a device file), and returns True if it is.	[-F $file] returns TRUE.
-G file	Detects if the file has a SGID bit set, and returns True if it is.	[-G $file] returns false.
-K File	Detects if the file has a sticky bit set (Sticky bit), and returns True if it is.	[-K $file] returns false.
-P File	Detects if the file is a named pipe, and returns True if it is.	[-P $file] returns false.
-U file	Detects if the file has a SUID bit set, and returns True if it is.	[-U $file] returns false.
-R File	Detects if the file is readable and returns true if it is.	[-R $file] returns TRUE.
-W File	Detects if the file is writable and returns true if it is.	[-W $file] returns TRUE.
-X File	Detects if the file can be executed and, if so, returns True.	[-X $file] returns TRUE.
-S file	Detects whether the file is empty (the file size is greater than 0) and does not return true for null.	[-S $file] returns TRUE.
-E File	Detects whether the file (including the directory) exists and, if so, returns True.	[-e $file] returns TRUE.

Shell Basic Operators