Introduction to let command and expr command in shell

Let command introduction:

Let command allows the bash shell to perform arithmetic operations, using lets to compare two operational values or perform operations such as subtraction, which are often used in the process control structure of the shell program or perform the required operations, noting that let can only perform the related operations of integers, The result of an operation can only hold integers.

Use the following methods:

Let variable name = variable 1 operator variable 2

Common Arithmetic Operations Categories:

Addition: +

Subtraction:-

Division:/

Multiplication: *

Take remainder:%

Let command use Method Example:

Note: Similar to the C language, let i= $i +1 can be written as lets i++ to simplify writing

Shell Instance 1:

[Root@changerlee operator comparison] #cat let_op.sh 

#!/bin/bash

#some Examples about let

 

 

num1=106

num2=10

Num3=5

#oprating let- 

res1=${num1}+${num2}+${num3} let-

res2=${num1}-${num2}-${num3} let

res3=${ NUM1}*${NUM2}*${NUM3} let-

res4=${num1}/${num2}/${num3} let

res5=${num1}%${num2}%${num3}

#output Results

echo "num1+num2+num3=${res1}"

echo "Num1-num2-num3=${res2}"

echo "Num1*num2*num3=${res3}"

echo "Num1/num2/num3=${res4}"

echo "num1%num2%num3=${res5}"

[Root@changerlee operator comparison] #sh let_op.sh 

num1+num2+num3=121

num1-num2-num3=91

num1*num2*num3=5300

num1/num2/num3=2

num1%num2% Num3=1

Note that num1/num2/num3 gets the result to be an integer 2, which means that let can only hold an integer result and intercept the fractional part

Shell Instance 2:

[Root@changerlee operator comparison] #cat let_op_v1.sh #!/bin/bash # Let and while

 

i=10

num=1 while

 

[1]

do

if [$num-le $i]

then 

echo "$num"

else

break

fi let

num= $num +1

done

[ Root@changerlee operation comparison] #sh let_op_v1.sh 

1

2

3

4

5

6 7 8 9 10

Let does not apply to the operation between decimals:

Shell Instance 3:

<span style= "FONT-SIZE:18PX;" ><strong>[root@changerlee operation Comparer] #let 1+1

[Root@changerlee operator comparison] #let 1.1+1

-bash:let:1.1+1: Syntax error:invalid arithmetic operator (Error token is ". 1+1") </strong></span>

Introduction to the expr command:

Expr is similar to let function in Linux commands when it is arithmetic, can only do integer type operation, cannot save decimal result. Expr can also perform operations between strings.

Expr is used in the following ways:

Expr expression1 Operator Expression2

The operator must be used to escape, and the operator and the two expression must have a space between them (this is different from let) and expr is not suitable for decimal operations.

Common Arithmetic Operations Categories:

Addition: +

Subtraction:-

Multiplication: *

Division:/

Take remainder:%

Examples of how expr is used:

(1) Perform the usual arithmetic operations:

Shell Instance 4:

[Root@changerlee operator comparison] #cat expr_op_v1.sh 

#!/bin/bash

#an exmple of expr

 

num1=56

num2=10

echo "NUM1: $num 1  num2: $num 2  num3: $num 3"

#operating num

res1= ' expr $num 1 \+ $num 2 \+ $num 3 '

res2= ' expr $num 1 \-$num 2 \-$num 3 '

res3= ' expr $num 1 \* $num 2 \* $num 3 '

res4= ' expr $num 1 \/$num 2 \/$num 3 ' 
  
   res5= ' expr $num 1 \% $num 2 \% $num 3 '

#output results

echo ' num1+num2+num3= $res 1 "

echo" num1-num2-num3=$ Res2 "

echo" num1*num2*num3= $res 3 "

echo" num1/num2/num3= $res 4 "

echo" num1%num2%num3= $res 5 "

[ Root@changerlee operation comparison] #sh expr_op_v1.sh

num1:56  num2:10  num3:5

num1+num2+num3=71

num1-num2-num3=41

num1*num2*num3=2800

num1/num2/num3=1

num1%num2%num3=1
  

</pre><p><strong><span style= "Font-family:microsoft yahei;font-size:18px;" >shell instance 5:</span></strong></p><p><strong><span style= "Font-family:Microsoft yahei;font-size:18px; " ></span></strong></p><pre name= "code" class= "HTML" >[root@changerlee operator comparison] #cat Expr_op_ v2.sh 

#!/bin/bash

# Let and then

 

i=10

num=1 while

 

[1]

do

if [$num-le $i]

then 

E Cho "$num"

else

breaks fi

num= ' expr $num \+ 1 '

done

[Root@changerlee operator comparison] #sh Expr_op_ v2.sh 

1

2

3

4

5

6 7 8 9 10

 

Shell Instance 6:

[Root@changerlee operator comparison] #expr 1 \+ 1

2

[Root@changerlee operator comparison] #expr 1.1 \+ 1

expr:non-integer argument

(2) perform common string operations

1. Length of output string

<strong><span style= "FONT-SIZE:18PX;" >[root@changerlee operation comparison] #cat expr_str.sh 

#!/bin/bash

#output length str

 

str1= "This str length is 21"

str2= "Blog.csdn.net/changerjjlee"

 

Echo ' ${#str1} ' ${#str1}

expr length $str 2

[Root@changerlee Operation Comparison] #sh expr_str.sh 

${#str1}21

26</span></strong>

Note: Expr can only output strings that do not contain spaces

2. The operation of the string

Expr substr $string $postion $length location number starting from 1

echo ${$string: $postion: $length} location number starting from 0

Shell Instance 7:

[Root@changerlee operator comparison] #string = "Abcdefghi"

[Root@changerlee operator comparison] #expr substr $string 1 3

ABC

[ Root@changerlee operation comparison] #echo ${string:0:3}

ABC

(3) operation of string concatenation

Shell Instance 8:

<strong>[root@changerlee operation comparison] #cat con_str_v1.sh 

#!/bin/bash

#connection of Strings

 

str1= "abc"

str2= "def ghi"

str3= "${str1} $str 2"

 

echo "str1= $str 1"

echo "str2= $str 2"

echo "str3= $str 3"

[Root@changerlee operation Comparer] #sh con_str_v1.sh 

str1=abc

str2=def ghi

str3=abcdef ghi</strong>

(4) operation of string substitution

Shell Instance 9:

<strong><span style= "FONT-SIZE:18PX;" >[root@changerlee operation comparison] #cat con_str_v2.sh 

#!/bin/bash

 

string= "Blog.csdn.net/changerjjlee"

Echo ${ STRING/C/C} #只替换一次

echo ${string//c/c} #全部替换

[Root@changerlee operator comparison] #sh con_str_v2.sh 

Blog. Csdn.net/changerjjlee

Blog. Csdn.net/changerjjlee
</span></strong>