Shell operator

1 expr

1.1 Introduce

expr yes evaluate expressions Abbreviation , Translated into “ Evaluation expression ”.Shell expr Is a powerful , And more complex commands , In addition to integer calculation , You can also combine some options to process strings , For example, calculate the length of a string 、 String comparison 、 string matching 、 String extraction, etc , Follow up explanation .

1.2 expr grammar

Computational syntax

expr  Arithmetic operator expression 

Be careful : Operation expression

Get the calculation result and assign it to the new variable syntax

result=`expr  Arithmetic operator expression `

1.3 demo

#!/bin/bash
result=`expr 2 + 2`
echo "result :${result}"

Introduction to arithmetic operators

The following table lists the common arithmetic operators , Assumed variable a by 1, Variable b by 2：

Operator	explain	give an example
+	Add	expr $a + $b The result is 3
-	Subtraction	expr $a - $b The result is -1
*	Multiplication	expr $a \* $b The result is 2
/	division	expr $b / $a The result is 2
%	Remainder	expr $b % $a The result is 0
=	assignment	a=$b Will put the variable b The value is assigned to a

If used in the four operations (), It also needs to be escaped \( 1 + 1 \)

#!/bin/bash
read -p " Enter two integers :" a b
echo "a=${a} b=${b}"

rs1=`expr ${
     a} + ${
     b}`
echo "a + b = ${rs1}"

rs2=`expr ${
     a} - ${
     b}`
echo "a - b = ${rs2}"

rs3=`expr ${
     a} \* ${
     b}`
echo "a * b = ${rs3}"

rs4=`expr ${
     a} / ${
     b}`
echo "a / b = ${rs4}"

rs5=`expr ${
     a} % ${
     b}`
echo "a % b = ${rs5}"

2 Integer comparison operators

2.1 grammar

The following table lists the common comparison operators , Assumed variable a by 1, Variable b by 2：

Operator	explain	give an example
-eq	equals Check whether two numbers are equal , Equal return 0, Otherwise return to 1.	[ $a -eq $b ] return 1.
-ne	not equals Check if two numbers are not equal , Unequal return true.	[ $a -ne $b ] return 0.
-gt	greater than Check whether the number on the left is greater than that on the right , Is to return 0, otherwise 1	[ $a -gt $b ] return 1.
-lt	lower than Check if the number on the left is less than the number on the right , Is to return 0, otherwise 1	[ $a -lt $b ] return 0.
-ge	greater equals Check whether the number on the left is equal to or greater than the number on the right , Is to return 0, otherwise 1	[ $a -ge $b ] return 1.
-le	lower equals Check whether the number on the left is less than or equal to the number on the right , Is to return 0, otherwise 1	[ $a -le $b ] return 0.
<	Check if the number on the left is less than the number on the right , Is to return 0, otherwise 1	(($a<$b)) return 0
<=	Check whether the number on the left is less than or equal to the number on the right , Is to return 0, otherwise 1	(($a<=$b)) return 0
>	Check whether the number on the left is greater than that on the right , Is to return 0, otherwise 1	(($a>$b)) return 1
>=	Check whether the number on the left is equal to or greater than the number on the right , Is to return 0, otherwise 1	(($a>=$b)) return 1
==	Check whether the number on the left is equal to the number on the right , Is to return 0, otherwise 1	(($a==$b)) return 1
!=	Check whether the number on the left is not equal to the number on the right , Is to return 0, otherwise 1	(($a!=$b)) return 0

Be careful :

Integer comparison operators only support integers , Decimals and strings are not supported ( Follow up explanation of string comparison ), Unless the value of the string is an integer number .

Each command has a return value , Later, we will explain the exit status and give more details , return 0 On behalf of success , return 1 For failure

2.2 demo

#!/bin/bash
read -p " Enter two integers :" a b

echo "a: ${a}"
echo "b: ${b}"



if (($a > $b));
then
    echo "a  Greater than  b"     
elif (($a == $b)); then
    echo "a  be equal to  b"
else
    echo "a  Less than  b"
fi

3 String comparison operators

You can compare 2 A variable , The type of variable can be a number （ Integers , decimal ） And string

3.1 grammar

The following table lists the common string operators , Assumed variable a by “abc”, Variable b by “efg”：

String comparison can use [[]] and [] 2 Ways of planting

Operator	explain	give an example
== or =	equal . Used to compare two strings or numbers , Same returns 0. have access to =	[ $a == $b ] return 1 [ $a = $b ] return 1 [[ $a == $b ]] return 1 [[ $a = $b ]] return 1
!=	It's not equal . Used to compare two strings or numbers , If not, return to 0.	[ $a != $b ] return 0 [[ $a != $b ]] return 0
<	Less than , Used to compare two strings or numbers , Less than return 0, Otherwise return to 1	[ $a \< $b ] return 0 [[ $a < $b ]] return 0
>	Greater than , Used to compare two strings or numbers , Greater than return 0, Otherwise return to 1	[ $a \> $b ] return 1 [[ $a > $b ]] return 1
-z	Check if the string length is 0, If the length is 0 Return returns 0, Otherwise return to 1.	[ -z $a ] return false.
-n	Check whether the string length is not 0, If the length is not 0 Then return to 0, Otherwise return to 1.	[ -n “$a” ] return true.
$	Check if the string is not empty , Not empty return 0, Empty return 1.	[ $a ] return true.

String comparison no <= Can pass [[ "a" < "b" && "a" == "b" ]]>

3.2 demo

#!/bin/bash
read -p " Enter two strings :" a b

echo "a: ${a}"
echo "b: ${b}"


if [ $a == $b ]
then
   echo "a be equal to b"
else
   echo "a It's not equal to b"
fi

4 Boolean operator

4.1 grammar

Operator	explain	give an example
!	Non operation , Take the opposite , Expression for true Then return to false, Otherwise return to true.	[ ! expression ] Take the opposite .
-o	or Or operations , There is an expression for true Then return to true.	[ expression 1 -o expression 2 ]
-a	and And operation , Both expressions are true To return to true.	[ expression 1 -a expression 2 ]

4.2 demo

#!/bin/bash
a=1 b=2

if [ $a -lt 2 -a $b -gt 10 ]
then
   echo "$a  Less than  2  And  $b  Greater than  10 :  return  true"   
else
   echo "$a  Less than  2  And  $b  Greater than  10 :  return  false"  
fi

if [ $a -lt 10 -o $b -gt 10 ]
then
   echo "$a  Less than  10  or  $b  Greater than  10 :  return  true"  
else
   echo "$a  Less than  10  or  $b  Greater than  10 :  return  false"
fi

if [ ! $a -gt $b ]
then
   echo "$a  Greater than  $b  Take the opposite  :  return  true"
else
   echo "$a  Greater than  $b  Take the opposite  :  return  false"   
fi

5 Logical operators

5.1 command

Operator	explain	give an example
&&	Logical AND	[[ expression 1 && expression 2 ]]
||	Logical OR	[[ expression 1 || expression 2 ]]
!	Logic is not	[[ ! expression ]]

Be careful

Use && and || The operator of must be placed in [[]] or (()) In order to be effective , Otherwise, the report will be wrong

! Can be used in [],[[]] in , Not in (())

5.2 demo

#!/bin/bash

a=1 b=2

if [[ $a -lt 10 && $b -gt 10 ]]
then
   echo " return  true" 
else
   echo " return  false"  
fi

if [[ $a -lt 10 || $b -gt 10 ]]
then
   echo " return  true"   
else
   echo " return  false"  
fi