[Shell] example of if, for, while flow statement and integer string comparison

[Shell] example of if, for, while flow statement and integer string comparison

Preface:

In fact, Shell is a command interpreter that explains the commands entered by the user and sends them to the kernel. In addition, Shell has its own programming language for editing commands. It allows users to write programs composed of shell commands. The Shell programming language has many features of a common programming language, such as its loop structure and branch control structure. The Shell program written in this programming language has the same effect as other applications.

1. shell flow statement 1, Condition Statement if else if

Sample Code:

[Root @ squid-2 script] # cat s1.sh

#! /Bin/bash

Echo "Please choose project :"

Echo "1: zhu 2: sha"

Read project_no

If [$ project_no = "1"]; then

Echo & quot; 111111 & quot"

Elif [$ project_no = "2"]; then

Echo & quot; 222222 & quot"

Else echo "error"

Fi

[Root @ squid-2 script] #

The execution process is as follows:

[Root @ squid-2 script] # sh s1.sh

Please choose project:

1: zhu 2: sha

1

111111

[Root @ squid-2 script] # sh s1.sh

Please choose project:

1: zhu 2: sha

2

222222

[Root @ squid-2 script] # sh s1.sh

Please choose project:

1: zhu 2: sha

3

Error

[Root @ squid-2 script] #

2, for loop 2.1 for I in

The script is as follows:

[Root @ squid-2 script] # cat host_list.txt

192.168.1.10

192.168.1.11

192.168.1.12

192.168.1.13

[Root @ squid-2 script] #

Test execution result:

[Root @ squid-2 script] # sh s21.sh

The host ip address is: 192.168.1.10

The host ip address is: 192.168.1.11

The host ip address is: 192.168.1.12

The host ip address is: 192.168.1.13

[Root @ squid-2 script] #

2.2 for (assign value; condition; Operation statement ))

Script code:

[Root @ squid-2 script] # cat s22.sh

For (I = 1; I <= 10; I ++); do

Echo "the loop number I: $ I ";

Done;

[Root @ squid-2 script] #

Execution result:

[Root @ squid-2 script] # sh s22.sh

The loop number I: 1

The loop number I: 2

The loop number I: 3

The loop number I: 4

The loop number I: 5

The loop number I: 6

The loop number I: 7

The loop number I: 8

The loop number I: 9

The loop number I: 10

[Root @ squid-2 script] #

3. while loop usage

Condition Statement structure:

While

Do

Action

Done;

Test script:

[Root @ squid-2 script] # cat s3.sh

#! /Bin/sh

I = 10;

While [[$ I-gt 5]; do

Echo "the loop number of while case is: $ I ";

(I --));

Done;

[Root @ squid-2 script]

Execution result:

[Root @ squid-2 script] # sh s3.sh

The loop number of while case is: 10

The loop number of while case is: 9

The loop number of while case is: 8

The loop number of while case is: 7

The loop number of while case is: 6

[Root @ squid-2 script] #

4. until statement

Sample script:

[Root @ squid-2 script] # cat s4.sh

#! /Bin/sh

A = 1;

Until [[[$ a-gt 6]; do

Echo "the until number is: $ ";

(A ++ ));

Done;

[Root @ squid-2 script] #

Execution result:

[Root @ squid-2 script] # sh s4.sh

The until number is: 1

The until number is: 2

The until number is: 3

The until number is: 4

The until number is: 5

The until number is: 6

[Root @ squid-2 script] #

5. shell SELECT statement 5.1 and use case selection statement (case/esac)

Syntax structure:

Case $ arg in

Pattern | sample) # arg in pattern or sample

;;

Pattern1) # arg in pattern1

;;

*) # Default

;;

Esac

Note: pattern1 is a regular expression, which can contain the following characters:

* Any string

? Any character

[Abc] a, B, or c

[A-n] any character from a to n

| Multiple options

 

Code script:

[Root @ squid-2 script] # cat s51.sh

#! /Bin/sh

Case $1 in

Start | begin)

Echo "start something"

;;

Stop | end)

Echo "stop something"

;;

*)

Echo "Ignorant"

;;

Esac

[Root @ squid-2 script] #

PS: execution result. here we need to include the parameter. The parameter value is within the start, begin, stop, and end parameters before). If the parameter is included, "Ignorant" is returned ":

[Root @ squid-2 script] # sh s51.sh start

Start something

[Root @ squid-2 script] # sh s51.sh begin

Start something

[Root @ squid-2 script] # sh s51.sh end

Stop something

[Root @ squid-2 script] # sh s51.sh stop

Stop something

[Root @ squid-2 script] # sh s51.sh test1

Ignorant

5.2. How to Use the select statement (generate menu selection)

Syntax structure:

Select variable name in seq variable

Do

Action

Done

The Code is as follows:

Cat s52.sh

#! /Bin/sh

Select param in "begin" "end" "exit"

Do

Case $ param in

"Begin ")

Echo "start something"

;;

"End ")

Echo "stop something"

;;

"Exit ")

Echo "exit"

Break;

;;

*)

Echo "Ignorant"

;;

Esac

Done;

Execution result:

[Root @ squid-2 script] # sh s52.sh begin

1) begin

2) end

3) exit

#? 1

Start something

#? 2

Stop something

#? 3

Exit

[Root @ squid-2 script] #

PS: During execution, only exit can be entered to exit the execution window.

Note: select is a loop selection, which is generally used with case statements.

2. Comparison operators of shell statements 1, integer comparison

Rule Description:

-Eq equals to if ["$ a"-eq "$ B"]

-Ne is not equal to if ["$ a"-ne "$ B"]

-Gt is greater than if ["$ a"-gt "$ B"]

-Ge is greater than or equal to if ["$ a"-ge "$ B"]

-Lt is less than if ["$ a"-lt "$ B"]

-Le is less than or equal to if ["$ a"-le "$ B"]

<Less than (double parentheses required) ("$ a" <"$ B "))

<= Less than or equal to (...) ("$ a" <= "$ B "))

> Greater than (...) ("$ a"> "$ B "))

> = Equal to or greater than (...) ("$ a"> = "$ B "))

PS: AWK can be used for small data comparison

Sample Code:

[Root @ squid-2 script] # cat com1.sh

A = $1

B = $2

If ["$ a"-eq "$ B"]; then

Echo "a = B true ."

Elif [! "$ A"-gt "$ B"]; then

Echo "a> B true ."

Else echo "a <B true ."

Fi

[Root @ squid-2 script] #

The test results are as follows:

[Root @ squid-2 script] # sh com1.sh 1 1

A = B true.

[Root @ squid-2 script] # sh com1.sh 1 2

A> B true.

[Root @ squid-2 script] # sh com1.sh 1 0

A <B true.

[Root @ squid-2 script] #

2. String comparison 2.1. specification and use

Rule Description:

= Equals to if ["$ a" = "$ B"]

= Equivalent to =

! = Not equal to if ["$ a" = "$ B"]

<Less than, in the order of ASCII letters:

If [["$ a" <"$ B"]

If ["$ a" \ <"$ B"] # Escape <

> Greater

-Z string is null, that is, the length is 0

-N string is not null, that is, the length is not 0

Sample Code:

[Root @ squid-2 script] # cat com2.sh

A = $1

B = $2

#1 the first method to implement

If ["$ a" x = "$ B" x]; then

Echo "a = B"

Elif [! "$ A" x = "$ B" x]; then

Echo "! = B"

Else echo "others"

Fi

#2 the second method to implement

If ["$ a" x = "$ B" x]; then

Echo "a = B"

Elif ["$ a" x! = "$ B" x]; then

Echo "! = B"

Else echo "others"

Fi

Test execution result:

[Root @ squid-2 script] # sh com2.sh ccb aaa

A! = B

A! = B

[Root @ squid-2 script] # sh com2.sh ccb

A = B

A = B

[Root @ squid-2 script] #

[Root @ squid-2 script] #

2.2. Notes

Notes for equal judgment:

The method to compare the two strings is as follows:

If ["$ a" x = "B" x]; then

The key points here are:

1. Use a single equal sign

2 Note that there is a space on both sides of the equal sign: This is a unix shell requirement.

3. Note that the last x of "$ a" x is specially arranged, because when $ test is null, the above expression becomes x = bx, obviously, they are not equal. Without this x, the expression returns the error [: =: unary operator expected

[[$ A = z *] # true if $ a starts with "z" (pattern matching)

[[$ A = "z *"] # If $ a is equal to z * (character matching), the result is true.

[$ A = z *] # File globbing and word splitting will occur

["$ A" = "z *"] # If $ a is equal to z * (character matching), the result is true.

About File globbing:

For a bit of explanation, Fileglobbing is a stenographer of files, for example, "*. c" is, and then ~ Yes.

Comparison of special characters:
Note:In the [] structure, ">" must be escaped.
Note:

2.3, judge whether the string is null

Sample Code:

[Root @ squid-2 script] # cat com3.sh

#! /Bin/bash

A = $1

If [-z "$ a"]; then

Echo "a is empty ."

Else echo "a is a object ."

Fi

[Root @ squid-2 script]

Test execution result:

[Root @ squid-2 script] # sh com3.sh

A is a object.

[Root @ squid-2 script] # sh com3.sh

A is empty.

[Root @ squid-2 script] #

3. File judgment

Rule Description:

-E file exists

F1-nt f2 file f1 is newer than f2

Sample script code:

[Root @ squid-2 script] # cat com4.sh

#! /Bin/bash

A = $1

File = $2

If [-d $ a]; then

Echo "it is a directory ."

Elif [-f "$ a"]; then

Echo "it is a file ."

Else echo "no parameter ."

Fi

The test result is as follows:

[Root @ squid-2 script] # sh com4.sh log1

It is a file.

[Root @ squid-2 script] # sh com4.sh old

It is a directory.

[Root @ squid-2 script] #

References:

Http://www.cnblogs.com/chengmo/archive/2010/10/14/1851434.html

Http://blog.csdn.net/yf210yf/article/category/1475895