Shell learning notes (latest update: 2022-02-18)

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1. shell brief introduction

shell It's a command line interpreter , Parse user commands into the actual execution of the operating system , Realize the interaction between users and operating system , When you need to repeat several commands , You can assemble commands , Add certain control statements , Become shell Script files , hand shell Batch execution

2. first demo

• create a file vi test.sh

• Write output statement

  echo "hello world"
  echo "first demo"
  

• Authorize scripts chmod +x ./test1.sh

• Execute the script ./test1.sh

• Output

  

3. shell Variable

3.1 Variable definitions

There can be no spaces between variable names and equal signs , This may not be the same as any programming language you are familiar with . meanwhile , The naming of variable names should follow the following rules ：

• The first character must be a letter （a-z,A-Z）.
• 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 ）.

myname="huhaoming"

3.2 Use defined variables

Use a defined variable , Just precede the variable name with a dollar sign , Such as ：

myname="huhaoming"
echo $your_name
echo ${your_name}  #{} not essential , Curly braces are used to help the interpreter identify the boundaries of variables 

Variables defined , Can be redefined

myname="tom"
echo $myname
myname="alibaba"
echo $myname

A read-only variable use readonly modification

myname="tom"
readonly myname
myname="huhaoming"  

Delete variables

Use unset Command to delete variables , Variable cannot be used again after being deleted .unset Command cannot delete read-only variables .

echo "huhaoming hello world"
myname="huhaoming"
readonly myname
echo $myname
unset myname
echo $myname

echo "huhaoming hello world"
myname="huhaoming"
echo $myname
unset myname
echo $myname

3.3 shell character string

Restrictions on single quoted string ：

• 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 （ You can't escape a single quotation mark ）.

The advantages of double quotes ：

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

echo "huhaoming hello world"
myname="huhaoming"
str="Hello, I know your are \"$myname\"! \n"
echo $str

String concatenation

echo "huhaoming hello world"
myname="huhaoming"
greeting="hello, "$myname" !"
greeting_1="hello, ${myname} !"
echo $greeting $greeting_1

Get string length

echo "huhaoming hello world"
myname="huhaoming"
echo ${
    #myname}

Extract substring

echo "huhaoming hello world"
myname="huhaoming"
echo ${myname:1:4} 

3.4 Array

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

stay Shell in , Use parentheses to represent arrays , For array elements " Space " Symbol split . The general form of defining an array is ： Array name =( value 1 value 2 … value n)

array_name=(value0 value1 value2 value3)

Individually define

array_name[0]=value0
array_name[1]=value1
array_name[n]=valuen

Read array

${ Array name [ Subscript ]}

Use @ The symbol can get all the elements in the array , for example ：

echo ${array_name[@]}

The length of the array

#  Get the number of array elements 
length=${
    #array_name[@]}
#  perhaps 
length=${
    #array_name[*]}
#  Get the length of a single element of an array 
lengthn=${
    #array_name[n]}

echo "huhaoming hello world"
myname=(0,1,2,3,4)
echo ${
    #myname[*]}
echo ${
    #myname[@]}
echo ${
    #myname[1]}

4. Shell Pass parameters

In execution Shell Script time , Pass parameters to script , The format of obtaining parameters in the script is ：$n.n Represents a number ,1 For executing the first parameter of the script ,2 For executing the second parameter of the script , And so on ……, among $0 For the filename of the execution ：

echo "huhaoming hello world"
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";

A few special characters are used to handle parameters ：

Processing parameters	explain
$#	The number of parameters passed to the script
$*	Display all the parameters passed to the script in a single string . Such as "$*“ use 「”」 In a nutshell 、 With "$1 $2 … $n" Output all parameters in the form of .
$$	The current process the script runs ID Number
$!	Of the last process running in the background ID Number
[email protected]	And $\ast phaseSame\; as,butyessendusewhenAddleadNumber,andstayleadNumberinreturnreturnEvery\; timeindividualginsengCount.Such\; as"$@“ use 「”」 In a nutshell 、 With "$1" “$2"\dots "$n” Output all parameters in the form of .
$-	Show Shell Current options used , And set Same command function .
$?	Display the exit status of the last command .0 No mistakes , Any other value indicates an error .

5. shell Operator

                            |

| $@|And$* identical , But use quotation marks , And return each parameter in quotation marks . Such as "[email protected]“ use 「”」 In a nutshell 、 With "$1" “$2"\dots "$n” Output all parameters in the form of . |
| $- | Show Shell Current options used , And set Same command function . |
| $? | Display the exit status of the last command .0 No mistakes , Any other value indicates an error . |

5. shell Operator