The article directories

• I. About shell
• • 1. What is shell
  • 2. Introduction to shell
• Ii. Advanced Shell (key points)
• • 1. Variables (key points)
  • • 1.1. Meanings of variables
    • 1.2 Definition and Use of variables (emphasis)
    • 1.3. Read-only variables (Understand)
    • 1.4 Receiving user input (key)
    • 1.5. Delete variables (Understanding)
  • 2. Conditional statement
  • 3. Operators
  • • 3.1 Arithmetic operators
    • 3.2. Relational operators
    • 3.3. Logical operators
    • 3.4. String operators
    • 3.5. File test operators (emphasis)
  • 4. Optional shell scripts (key)

I. About shell

1. What is shell

2. Introduction to shell

• Write specifications:
• Code specification:

#! /bin/bash [specifies the shell interpreter to tell the system to use for the current script] shell-related instructionsCopy the code

• File naming conventions:

Sh is the default suffix of bash shell in LinuxCopy the code

• Use process:
  
  ① Create the. Sh filetouch/vim
  
  ② Write shell code
  
  ③ Executing shell scripts Scripts must have execution permission
• Case 1: Createtest.shTo implement the first shell script, outputhello world.
  
  Output command:# echo 123
• Note: If the output contains letters and symbols (not variables), it needs to be enclosed in quotes. If it is a pure number, it may or may not be included.
  
  
  
  
• Pay attention to: this must be written at runtime./test.shRather thantest.shRun other binary programs as well, write directlytest.shLinux will goPATH(Environment variables) look for callstest.shBut only/bin./sbin./usr/bin./usr/sbinSuch as inPATHIn, your current directory is usually not therePATHIn, so it’s written astest.shYou won’t find the order. Use it./test.shTell the system to look in the current directory.
• Case 2: UserootThe user account is created and executedtest2.shImplementation creates oneshelltestUser and create a new file in their home directorytry.html.
  
  
• Another way to execute a script:/bin/bash Specifies the path of the bash script(understand)
  
  
• Shell scripts are divided into simple (stacking of simple commands) and complex (programming)

Ii. Advanced Shell (key points)

1. Variables (key points)

1.1. Meanings of variables

1.2 Definition and Use of variables (emphasis)

• Variable, defined before use.
• The definition looks like:class_name="yunwe "
• Such as:echo $class_name
• A variable consists of two parts: the name of the variable (left) and the value of the variable (right)
• What is the relationship between a variable name and its value? Variable names and variable values are used and used relationships; Our variable name uses the variable value;
• When using a variable, always add one in front of the variable name$Symbol, a requirement that exists in other languages (such as PHP).

Note that there can be no Spaces left or right after an equal sign, unlike in any programming language you’re familiar with. In addition, the naming of variable names must follow the following rules:

• The name must contain only letters, digits, and underscores (_). The first character cannot start with a number.
• There can be no Spaces between them. Instead, use the underscore “_”.
• Do not use punctuation marks.
• You can’t usebashThe key words in thehelpCommand to view reserved keywords.

Case 1: Rewrite the first shell script in the getting started script using variables.





Question about single and double quotation marks:

• Double quotes recognize variables, and double quotes can be escaped (similar to”\“).
• Single quotes do not recognize variables and only output as is. Single quotes cannot be escaped

Case 2: Define a variable and output the current time in the format of “year-month-day hour: minute: second”.

• Note: The backquotes (escThe key below the key) is used when executing instructions in a script and assigning the result to a variable.The quotation marks“.
  
  

1.3. Read-only variables (Understand)

1.4 Receiving user input (key)

• Grammar:Read-p Indicates the variable name of the prompt
• Example: Write a scripttest6.shTo prompt the user to enter the name of the file (path) after execution, and then automatically create the file for the user
  
  
  
  

1.5. Delete variables (Understanding)

2. Conditional statement

• Syntax 1 (one condition) :

if condition
then
    command1 
    command2
    ...
fi
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• Single-line (usually executed on the command line) :if [ condition ]; then command; fi
• Syntax 2 (two conditions) :

if condition
then
    command1 
    command2
    ...
else
    command
fi
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• Syntax 3 (multiple conditions) :

if condition1
then
    command1
elif condition2 
then 
    command2
else
    commandN
fi
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3. Operators

3.1 Arithmetic operators

• The following table lists the commonly used arithmetic operators, assuming variable A is 10 and variable B is 20:
  
  
• Note: Conditional expressions should be placed between square brackets with Spaces, for example:[$a==$b]Is wrong, must be written
  
  [ $a == $b ].
• nativebashSimple math is not supported, but can be done with other commands, such asawk 和 Expr, expr
  
  The most commonly used.
• exprIs an expression calculation tool, using it to complete the expression of the evaluation operation.
• For example, add two numbers (note the use of back quotes) :

#! /bin/bash
val=`expr 2 + 2`
echo "The sum of the two numbers is:$val"
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Two points to note:

• There must be a link between an expression and an operatorThe blank space, e.g.2 + 2That’s not true. It has to be written2 + 2Unlike most programming languages we are familiar with.
• The full expression is to beThe quotation marksContains, note that this character is not the usual single quotation mark, inEscButton below.
  
  
  
  

3.2. Relational operators

-eq: equal -ne: not equal -gt: great than -lt: less than -ge: great than or equal -le: less than or equalCopy the code

3.3. Logical operators

• The following table lists the common Boolean operators, assuming that variable A is 10 and variable B is 20:
  
  
• Or operation: if one is true, it is true; if all are false, it is false
• And operation: if one is false, it is false; if all are true, it is true
  
  

3.4. String operators

3.5. File test operators (emphasis)

• File test operators are used to detect various properties of Unix/Linux files.
• Attribute detection is described as follows:
  
  
  
  
• Case study: Test the effect of the above color green
  
  
• Note: Permission several judgments, if only one part of the match, it is considered to have permission.

4. Optional shell scripts (key)

The tail instruction system is called to pass the subsequent options to tail, which first opens the specified file and fetches the last 10 linesCopy the code

#./test.sh  a  b  c
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A. # user-add user name 【 add user 】 b. # user-del user name 【 delete user and home directory 】Copy the code