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Shell script quick start-01

2022-07-29 02:38:00 【51CTO】

1 Shell summary

1.1 Why study shell

1.2 shell summary

2 Shell Parser

3 Shell Script entry

3.1 Script format

3.2 Create a Shell Script

3.3 Multi command processing

4 Shell The variables in the

4.1 System variables

4.2 Custom variable

4.3 Special variables :$n

4.4 Special variables :$#

4.5 Special variables $*

4.6 Special variables $?

5 Operator

6 conditional

1 Shell summary

1.1 Why study shell？

Need to understand the preparation of operation and maintenance personnel shell Programming
Occasionally, it needs to be written shell Management cluster , So as to improve the development efficiency

1.2 shell summary

Shell Script Quick start -01 _linux

Shell Be situated between Linux Between the kernel and the outer application

Shell It's a command line interpreter , It receives applications / User command , And then invoke the kernel of the operating system

Shell Is a powerful programming language , Easy to write , Easy to debug , Strong flexibility .

2. Shell Parser

Linux Provided Shell The interpreter has

sh and bash The relationship between ,sh yes bash The soft links

[[email protected] bin]# cd /bin/
[[email protected] bin]# ll | grep bash
-rwxr-xr-x. 1 root root     964608 Oct 31  2018 bash
lrwxrwxrwx. 1 root root         10 Aug 21  2021 bashbug -> bashbug-64
-rwxr-xr-x. 1 root root       6964 Oct 31  2018 bashbug-64
lrwxrwxrwx. 1 root root          4 Aug 21  2021 sh -> bash

     
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Centos The default interpreter is bash

3 Shell Script entry

3.1 Script format

With #!/bin/bash start

3.2 Create a Shell Script

Create a shell Script ,, And the output helloworld

[[email protected] ~]$ mkdir datas
[[email protected] ~]$ cd datas
[[email protected] datas]$ pwd
/home/fenfen/datas
[[email protected] datas]$ touch helloworld.sh
[[email protected] datas]$ vim helloworld.sh 
[[email protected] datas]$ ll
total 4
-rw-r--r-- 1 fenfen happy 38 Jul 26 11:07 helloworld.sh

     
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Write script content

perform shell Script

use sh

use bash

Take an absolute path to see

use ./ Give it a try , I can't find out , The reason is that it was originally through sh perhaps bash The parser executes , But now it's your permission to execute , No, x jurisdiction

[[email protected] datas]$ ./helloworld.sh
-bash: ./helloworld.sh: Permission denied
[[email protected] datas]$ ll
total 4
-rw-r--r-- 1 fenfen happy 38 Jul 26 11:07 helloworld.sh

     
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Therefore, modify permissions

[[email protected] datas]$ chmod 755 helloworld.sh 
[[email protected] datas]$ ll
total 4
-rwxr-xr-x 1 fenfen happy 38 Jul 26 11:07 helloworld.sh

[[email protected] datas]$ ./helloworld.sh 
helloworld fenfen

     
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3.3 Multi command processing

stay /home/fenfen/ Create one in the directory banzhang.txt stay banzhang.txt Add to file “I love lili”

establish shell Script

shell The script content

Execute the script and see the results

[[email protected] datas]$ bash batch.sh 
[[email protected] datas]$ cd /home/fenfen
[[email protected] ~]$ ll
total 4
-rw-r--r-- 1 fenfen happy 24 Jul 26 12:28 banzhang.txt
drwxr-xr-x 2 fenfen happy 43 Jul 26 12:28 datas
-rw-r--r-- 1 fenfen happy  0 Jul 25 03:49 ok.txt
[[email protected] ~]$ cat banzhang.txt 
I love lili

     
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4 Shell The variables in the

4.1 System variables

Common system variables

$HOME: View the user's home directory

$PWD： View current directory

$SHELL: Check the default parser

#USER: View the current user

The principle is to encapsulate these as variables

4.2 Custom variable

Basic grammar ： Variable = value

Be careful ： The value of a variable can be changed , The assignment will change
A=1 No space in the middle

Revoke variables ：unset Variable

Declare a static read-only variable ：readonly Variable

But not to unset

Variable definition rules

Variable names may consist of letters 、 Numbers and underscores , But it can't start with a number , Environment variable name is recommended to be capitalized
No spaces on both sides of equal sign
Variable default type is string type , No direct numerical operation

Value of variable if there is a space , You need to use double quotation marks and single quotation marks

Global variables

If you promote a variable to a global variable export, For other purposes Shell Program usage

shell Add... To the script

But because it doesn't belong to the same process , also D It's a local variable , So use shell Can't print out

So upgrade to global variables

4.3 Special variables :$n

$n：n Is the number , also 0 Indicates the name of the script ,1-9 Represents the first to ninth parameters , More than ten parameters need to be enclosed in braces
${10}, Generally not more than 5 individual
example

Create a sh file

shell Script a

Under execution , Will come out $0

Give a parameter to execute

[[email protected] datas]$ bash parameter.sh banzhang
parameter.sh banzhang 

[[email protected] datas]$ bash parameter.sh banzhang love lili
parameter.sh banzhang love lili


     
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4.4 Special variables :$#

Basic grammar ：$# Get the number of all input parameters , Often used in loops
example

Shell Script

Execution includes $# Script results for

Give me a parameter

4.5 Special variables $*

Basic grammar

$*： This variable represents all the parameters on the command line , Treat all parameters as a sort

[email protected]： This variable also represents all the parameters on the command line , Just treat each parameter differently

example

write in shell Script

Take a look at it , In the end, although the result is the same , But [email protected] Indicates that the kernel is still one by one

[[email protected] datas]# bash parameter.sh banzhang loves lili
parameter.sh banzhang loves lili
3
banzhang loves lili
banzhang loves lili

     
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4.6 Special variables $?

Basic grammar ： The return status of the last command executed . If the value of this variable is 0, The last command on the table is executed correctly , If the value of this variable is not 0, Then the last command is incorrect
example

[[email protected] datas]# ./helloworld.sh 
helloworld fenfen

[[email protected] datas]# echo $?
0

[[email protected] datas]# $?
-bash: 0: command not found
[[email protected] datas]# echo $?
127

     
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5 Operator

Basic grammar

$(( Arithmetic expression ))
$[ Arithmetic expression ]
expr +,-,*,/,% Addition, subtraction, multiplication, division and remainder expr Space between operators

example

[[email protected] datas]# expr 3 + 2
5
[[email protected] datas]# expr 3 \* 2
6
[[email protected] datas]# expr 4 / 2
2

     
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2+3 First calculate and remember ` once

Use [] Calculation

6 conditional

Basic grammar ：[ condition ] condition There should be a space before and after

If the condition is not empty, it is true,[] Returns the false

Common judgment conditions

String comparison
- -lt Less than
- -le Less than or equal to
- -eq be equal to
- -gt Greater than
- -ge Greater than or equal to
- -ne It's not equal to
File permission judgment
- -r Have read permission
- -w Have the right to write
- -x Have the authority to execute
Judge according to the document type
- -f The file exists and is a regular file file
- -e File exists existence
- -d The file exists and is a directory directory
Multi condition judgment
- && It means the previous one is successful , Only then implements the latter
- || Indicates that the previous one failed , Only then implements the latter

example

23 Greater than or equal to 22

[[email protected] datas]# [ 23 -ge 22 ]
[[email protected] datas]# echo $?
0

[[email protected] datas]# [ 23 -le 22 ]
[[email protected] datas]# echo $?
1

     
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helloworld Whether you have write permission

Whether the file of the directory exists

Multi condition judgment

[[email protected] datas]# [ 2 -gt 1 ] && echo OK || echo notok
OK

[[email protected] datas]# [ 2 -gt 1 ] && [ ] || echo notok
notok

     
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