Getting started with shell

Preface

Shell It's a use. C A program written in a language , It is used by users Linux The bridge .Shell It's a command language , Another programming language .
generally speaking , We will be in Liuux The system uses shell Access the system and carry out some interactive operation of data .
This blog , According to his own experience at ordinary times , It summarizes some commonly used Shell Instructions . Writing is not good , I would appreciate your comments. .

common shell Instructions

first shell The procedure to be carried out

The first program to execute Compared with the output you guessed hello,world 了 . ad locum , In order to ensure the integrity of the entry Foundation , Here I will briefly describe how to use shell Output one helloworld.

1. vi Create a .sh The file of （ for example ,vi hello.sh）
   echo “Hello, World. I come from China”
2. Give the script executable permissions
   chmod +x ./hello.sh
3. Execute the script （ Here is ./hello.sh）
   Show “Hello, world. I come from China”

echo The command is used to output text... To the window

Using variables ： Use a defined variable , Just prefix the variable name with $ Symbol , You can also put curly braces on variables {}, Curly braces outside variable names are optional , Add it or not , Curly braces are used to help the interpreter identify the boundaries of variables . Curly brackets are recommended for all variables , It's a good programming habit .

your_name="qinjx"
echo $your_name
echo ${your_name}

for skill in Ada Coffe Action Java; do
    echo "I am good at ${skill}Script"
done

If not skill Variable with curly bracket , It's written in echo “I am good at $skillScript”, The interpreter will $skillScript As a variable （ Its value is empty. ）, The result of code execution is not what we expect .

Use unset Command to delete variables

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

#!/bin/sh
myUrl="http://www.baidu.com"
unset myUrl
echo $myUrl

In the include file

A read-only variable ： Use readonly Commands can define variables as read-only variables , The value of a read-only variable cannot be changed .

#!/bin/bash
myUrl="http://www.google.com"
readonly myUrl
myUrl="http://www.baidu.com"

Run script , The result is ： /bin/sh: NAME: This variable is read only.

Variable type

Variable type ： function shell when , There will be three variables at the same time
• 1) local variable Local variables are defined in scripts or commands , At present only shell Valid in instance , other shell Started program cannot access local variables .
• 2) environment variable All procedures , Include shell Program started , Can access environment variables , Some programs need environment variables to ensure their normal operation . When necessary shell Scripts can also define environment variables .
• 3) 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 , These variables guarantee shell Normal operation of

Shell How to use strings in

Get string length

string="abcd"
echo ${#string} # Output  4

Extract substring

The following example starts from the string No 2 Characters begin to intercept 4 Characters ：

string="runoob is a great site"
echo ${string:1:4} #  Output  unoo

Shell Array

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

array_name=(value0 value1 value2 value3)
 perhaps 
array_name=(
value0
value1
value2
value3
)
 You can also define the components of an array separately ：
array_name[0]=value0
array_name[1]=value1
array_name[n]=valuen
 Can not use consecutive subscripts , And there is no limit to the scope of subscripts .

Be careful ： Use @ or * You can get all the elements in the array

Read array

The general format for reading array element values is ：
${ Array name [ Subscript ]}
for example ：
valuen=${array_name[n]}
Use @ The symbol can get all the elements in the array , for example ：
echo ${array_name[@]}

Gets the length of the array

The way to get the array length is the same as the way to get the string length , for example ：

Get the number of array elements
length=KaTeX parse error: Expected '}', got '#' at position 2: {#̲array_name[@]} …{#array_name[*]}

Get the length of a single element of an array
lengthn=${#array_name[n]}

Shell Basic operators

Shell Basic operators
Shell Like any programming language , Support for multiple operators , Include ：
Arithmetic operator
Relational operator
Boolean operator
String operators
File test operators

Native bash Simple mathematical operations are not supported , But you can do it with other commands , for example awk and expr,expr The most commonly used .

expr Is an expression calculation tool , It can be used to evaluate expressions .
for example ： Add two numbers ： sum=`expr 2 + 2` #sum=4

Two points attention ：
• Space between expression and operator , for example 2+2 It is not right , Must be written as 2 + 2, This is different from most programming languages we are familiar with .
• The complete expression is to be ’ ’ contain , Note that this character is not a common single quotation mark , stay Esc Key bottom .

Show variable
read Command to read a line from standard input , And assign the value of each field of the input line to shell Variable
#!/bin/sh
read name
echo “$name It is a test”
The above code is saved as test.sh,name Receive standard input variables , The result will be :
[root@www ~]# sh test.sh
OK # The standard input
OK It is a test # Output

Reference material ：
1.Shell Novice tutorial