Shell scripts from getting started to complex six (conditional judgment and arithmetic operations)

First, condition judgment

Classification:

Condition test Type

Numerical test

String test

File test

1. Conditional test Expressions

Format:

[Expression]

[[Expression]]

Test expression

2. Numerical test

Parameters	Description
-eq	Equals is True
-ne	Not equal to True
-gt	is greater than true
-ge	Greater than or equal to true
-lt	is less than true
-le	is less than or equal to true

Case 1:

Determine if there is a user's default shell for bash on the current system, and if so, how many of these users are displayed; otherwise, no such user is shown

# VI determine1.sh

num = ' awk-f: ' ($7== "/bin/bash" {print $}) '/etc/passwd | Wc-l '

if [[$num-gt 0]];then

echo "System sum $num user Have/bin/bash"

Else

echo "System no user Have/bin/bash"

Fi

Note that there is a space between the contents of the [[]] inside and the brackets

Case 2:

Given a user, determine if the user ID and group ID are the same

# VI determine2.sh

#!/bin/bash

Read-p "Enter Username:" Name

Userid= ' Id-u $name '

Groupid= ' Id-g $name '

If [$userid-eq $groupid];then

echo "Yes,is some"

Else

echo "No,is defferent"

Fi

3. String test

Parameters	Description
=	Equals is True
!=	Not Equal is true
-Z String	True if the length of the string is zero
-N String	True if the length of the string is nonzero

Case:

Given two strings, determine whether two strings are the same

# determine3.sh

#!/bin/bash

Read-p "Enter The string:" Str1 str2

if [$str 1 = $str 2];then

echo "The string is Same"

Else

echo "The string is Defferent"

Fi

4. File test

Parameters	Description
-E File name	True if the file exists
-R file name	True if the file exists and is readable
-W file name	True if the file exists and is writable
-X file name	True if the file exists and is executable
-S file name	True if the file exists and has at least one character
-D file name	True if the file exists and is a directory
-F file name	True if the file exists and is a normal file
-C file name	True if the file exists and is a character-specific file
-B file Name	True if the file exists and is a block special file

Case:

Given a filename, determine if the file exists

# VI determine4.sh

#!/bin/bash

Read-p "Enter filename:" Name

If [-e $name];then

echo "File is exits"

Else

echo "File is not exits"

Fi

5. The relation character

In addition, the shell is provided with (-a), or (-O), non (!) Three logical operators are used to connect test conditions with a priority of: "!" Highest, "-a" followed, "-O" the lowest.

Case:

Determine if the number of parameters is greater than 1 and less than 3

If [$#-gt 1-a $#-le 3]

Whether the number of parameters is greater than 1 or less than 3

If [$#-gt 1] && [$#-le 3]

Second, arithmetic operation

Operator	Description	Example
+	Addition	' Expr $a + $b ' result is 30
-	Subtraction	' Expr $a-$b ' result is 10
*	Multiplication	' Expr $a \* $b ' result is 200
/	Division	' Expr $b/$a ' result is 2
%	Take surplus	' Expr $b% $a ' result is 0
=	Assign value	A= $b will assign the value of variable B to a
==	Equal. Used to compare two numbers, the same returns true	[$a = = $b] returns false
!=	Not equal. Used to compare two numbers, not the same returns true	[$a! = $b] Returns True

Case summation:

There are several ways to assign values

Num1=5

Num2=6

1) Let-arithmetic expressions

Let sum= $num 1+ $num 2

2) $[arithmetic expression]

sum=$[$num 1+ $num 2]

3) $ (())

sum=$ (($num 1+ $num 2))

4) An arithmetic expression of expr, with spaces between the operands and operators in the expression, and the use of a command reference

sum= ' expr $num 1+ $num 2 '

Shell scripts from getting started to complex six (conditional judgment and arithmetic operations)