Shell programming core technology "I"

Today's main introduction Shell Programming core technology , That is, how to write Shell Script .

In our daily work, we often need to write some Shell Logical script to batch process some tasks , For example, read the input data for related processing , Put the task into a script for automatic operation and other scenarios , These all involve Shell Programming , Actually Shell Programming is also very simple , It's like Java、Python These popular programming languages are the same , It also has the characteristics of programming language , Let's see Shell Programming mainly involves several aspects .

• Variable
• Logic control
• Shell Environmental Science
• Script application
• automation

Variable

1

Variable definitions

Learn a programming technique , First, you need to master how to use variables , stay Shell Defining a variable in is very simple , It's related to Python Defining a variable in is very similar , You can define a variable in any position and assign a value to it , and Shell Than Python The more concise part is that you are not forced to enter single quotation marks or double quotation marks to indicate the content .

As shown in the figure above , Define a x=1 The variable of , Then input echo x Instructions , among x Indicates that this variable can be referenced .

As shown in the figure above , Input hello String assignment to variable x, And then print it echo $x.

If input x=hello world , At this time, the system will report an error , because x=hello It will be considered as a variable assignment , and world Will be considered another independent command .

When there are spaces in the variable value , We just need to put single quotation marks or double quotation marks on the string . Then print this variable echo $x That's all right. .

In the definition of variables , There are a few points that need our attention ：

• Equal sign (=) There should be no spaces on the left and right ;
• If there are spaces in the content , Single or double quotation marks are required ;
• Double quotation marks support escape characters ,$ The first variable will be automatically replaced .

There are several methods to pay attention to in the use of variables ,echo a、echo “a” Can represent variable output , But if you want to use it more strictly, it is recommended to use double quotation marks , And when multiple strings are used in series , Use {} Enclose the content to indicate that the content is a variable , Avoid confusion with other characters that follow, resulting in an error .

2

Predefined variables

stay Shell Programming , The system also provides several predefined variables . such as ：

• PWD Represents the current directory ;
• USER Represents the current user ;
• HOME Represents the home directory of the current user ,HOME You can also use ~ Abbreviation ;
• PATH Indicates all current executable programs ;
• RANDOM Can produce a random number .

among , We need to pay special attention to PATH Variable , Such as input echo $PATH Instructions , You can see PATH Variables define all executable programs , It's defined in PATH The program in can directly call the program name to execute without entering all the paths of the program .

For example, use which ls Instructions , You'll find that ls stay /bin/ls Under the table of contents , Print echo $PATH Variable , You will find the corresponding /bin Catalog , But if the program is not defined in PATH variable , You need to enter all the paths to execute the program .

3

Use of special symbols

In addition to these , You also need to master some special symbols , for example ：

• Double quotation marks are used to enclose a string , Support $var Variable substitution of form ;
• Single quotation marks also indicate that its content is a string , But escape is not supported ;
• $’\n’ Express ANSI-C quote ;
• \ Backslash , In some cases, it means escape ;
• ((a=a+3)) Is an integer extension , Variables in double brackets are treated as integers ;
• $(ls) Execute the command and save the result as a variable , Shorthand for ``;
• {1..10} Equivalent to seq 1 10, Express 1~10 Numbers ;
• seq 1 3 10 Means to generate a step of 3 Of 1~10 The number of .

among , We need to focus on integer extension , You can put the mathematical expression in double brackets for corresponding calculation ,

Such as input echo $((100000/3)), Its result is 33333, Why is there no decimal place here ？

Because Shell Currently, only integers are processed in , If you need to be accurate to decimal places, you can use awk Instructions .Shell The integer calculation of does not use It's OK, too , But if you want to quote specific result values , You need to use .

The content shared today is quite practical , If you want to master these knowledge points , You still need to knock orders line by line , In this way, the impression will be more profound . Next issue will continue to share Shell The core technology of programming , Coming soon ~

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