Getting started with shell Basics

One 、Shell Variable

1.1、 When defining variables , Variable name without dollar sign

• Only English letters can be used for naming , Numbers and underscores , The first character cannot begin with a number .
• No spaces in between , You can use underscores （_）.
• You can't use punctuation .
• Out of commission bash Keywords in （ You can use help Command to view reserved keywords ）

1.2、 The type of variable

local variable

• Local variables are defined in scripts or commands , At present only shell Valid in instance , other shell The program started cannot access Ask local variables .

environment variable

• All procedures , Include shell Program started , Can access environment variables , Some programs need environment variables to ensure their normal operation

Shell Variable

• shell Variables are derived from shell Special variables for program settings .shell Some of the variables are environment variables , Some of them are local variables

# Declaration of variables

name="zhangsan" 

# Variable call

echo $name

echo ${name}

# A read-only variable /bin/sh: NAME: This variable is read only.

url="https://www.google.com"

readonly url

url="https://www.runoob.com"

# Delete variables

unset name

Two 、Shell String

The string is shell The most commonly used and useful data types in programming , Strings can be in single quotes , You can also use double quotes , Or not

Use quotation marks .

Single quotation marks : 

• Any character in a single quotation mark will be output as is , Variables in a single quoted string are invalid ;
• A single quotation mark cannot appear in a single quotation mark string , But in pairs , Use as string concatenation .

Double quotes :  

• You can have variables in double quotes
• Escape characters can appear in double quotes

# Declaration string

str1="hello world 1"

str2='hello world 2'

str3='hello world3

# String splicing -- Double quotes

name='sunwukong'

name1="hello, "$name""  // It can be stitched together

name2="hello, ${name}"  // It can be stitched together

# String splicing -- Single quotation marks

passwd='123456'

passwd1='hello, '$passwd''   // It can be stitched together

passwd2='hello, ${passwd}'  // Can't splice  

echo $passwd2                 # hello, ${passwd} !

# Length of string

email="[email protected]"

echo ${#email}          // Gets the length of the string

echo ${email:1:4}      // obtain email String from 1 Start , take 4 Characters ( The count of strings starts from ) 

3、 ... and 、Shell Array

• bash Supports one dimensional array （ Multidimensional arrays are not supported ）, And there's no limit to the size of the array .

• The subscripts of array elements are 0 Numbered starting . To get the elements in an array, you need to use subscripts , Subscripts can be integers or arithmetic expressions , Its Value should be greater than or equal to 0.

# Define an array Parentheses are used to represent arrays , For array elements " Space " Symbol split

Array name =( value 1 value 2 ... value n)

favs=(" football " " blue ball " " Table tennis " " The bowling ball ")

# Read array ${ Array name [ Subscript ]}

fav=${favs[1]}

# Use @ The symbol can get all the elements in the array

echo ${favs[@]}

# Gets the length of the array

length1=${#favs[@]}

length2=${#favs[*]}

Four 、Shell Notes  

• With # The first line is the comment , Will be ignored by the interpreter .
• Add one... To each line # Number to set multiline comment

#--------------------------------------------

# This is a comment

# author：

# site：

#--------------------------------------------

##### Server configuration -start #####

#

#

#

#

#

##### Server configuration -end #####

# Special multiline comments

:<<EOF

The comment ...

The comment ...

The comment ...

EOF

:<<!

The comment ...

The comment ...

The comment ...

! 

notes : here EOF and ! It can be changed , But make sure the front and the back are the same

5、 ... and 、Shell Parameter passing

perform Shell Script time , Pass parameters to script , The format of obtaining parameters in the script is : $n, n Represents a number . 

#!/bin/bash

echo "Shell Pass parameter instance ！";

echo " File name of execution ：$0";

echo " The first parameter is zero ：$1";

echo " The second parameter is ：$2";

echo " The third parameter is zero ：$3";

# ./hello.sh 11 22 33 44