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This article mainly explains "how to use Shell position parameters". Interested friends may wish to have a look at it. The method introduced in this paper is simple, fast and practical. Let's let the editor take you to learn how to use Shell position parameters.

Shell programming

What are the characteristics of shell variables?

-- weak type

-- default string type (string type cannot directly add, subtract, multiply and divide numeric operations, so when numerical operations are needed, the shell variable needs to be declared as numeric, do not use the default value)

You can write instructions to a file, and shell can perform multiple operations on this script operation at a time. But it can be troublesome to execute one by one, and shell supports conditional control and loop nesting features.

Description:

Specify parser

Be sure to execute the parser on the first line. We are using bash, but we can also use python and ruby to write scripts.

Statement is command

When we learn a command, putting it on the command line is a command, and putting it in a script is a statement.

The commands in the script can be listed directly or included in the function, which can be executed by calling the function name:

Commands are sensitive to spaces, and script statements are abnormally sensitive to spaces!

Position parameter

Scripting language:

When referencing the first sh, use $0 ($for location). $# represents the total number of parameters.

Action:

The script is not executed on the current shell

When we execute any script, we don't execute on this shell, but we open a new shell, although we can't see the shell.

Action:

Note: although you got the purpose of the script, the current directory has not changed, that is, it is not the operation performed in this shell.

The statement in vim is:

To execute the statement on the current shell:

Note: the current working directory has changed.

Declare command

-- used to declare variable types

Format:

Note: it is important to note here that we need to add a declaration with "-" instead of "+".

For example:

Declare a variable as numeric

Note: here we can clearly see that the variable is character by default, so the default output of c is 11 characters 22, if you want to output the result of addition operation, you need to declare c as int type (a, b do not need to be declared as int, because after declaring c, the conversion will be carried out automatically when performing aplimb operation).

You can view variable properties:

Declare array variables

Action:

Note: when viewing the array, you need to add {} after $. The first array element is displayed by default when the array name is given without subscript, and all elements are displayed when you add *.

Declare environment variables

Note: the declaration of environment variables is actually the declare command. -X means declared as an environment variable. If you do this:

Export aa, which means that aa is declared as an environment variable, only aa, not bb, can be seen when executing the two instructions bash-- > set. Bb will only appear in the current directory.

Declare a variable read-only property

Action:

Note: cannot be modified or deleted here. We can't even execute the undo property. In practical application, we should pay special attention to the fact that if the variables of institutional attributes are added to a certain system environment variable, it will start automatically when it is turned on, which will make an error. What we set here is a temporary environment variable, which disappears when the system is rebooted. Therefore, read-only properties are not very easy to use and are used as little as possible in practice.

View variable properties

Numerical operation mode

Numerical operation method 1

Note: this method requires a lot of instructions, trouble.

Method 2:expr or let numerical operation tool

Action:

Note: the operation of direct addition is not performed when there is no space, but as a string operation.

Method 3: "$(operator)" or "[operator]"

Note: it is recommended to write "$((operator))"

Distinguish between the following ways: $(operator)-its function is to execute the system command first, and then assign the result of the command to the variable.

Operator

Note: the higher the priority, the higher the value. In practice, not many numerical operations are used.

Variable test

Brief introduction

It is only effective for shell language and is more complex. In practice, it can be replaced by other ways, with complex format and simple syntax.

Note: when optimizing, such as the web script written by php, the traffic is very large, so it needs to be optimized. In practice, shell is generally used by administrators (because our goal is office automation), so the logic is required to be clear, and this approach is not very applicable.

Example 1: test xdistinct ${y-New value}

Cycle control

Script

However, this script has its limitations, that is, it does not work when another file is introduced and then executed, that is, the file is only for a particular situation. Improvements need to be made:

There is a pilot mark ``above the tab. If you enter a command in the vim: shaped like a 'pwd', pilot mark, you can output the output of the command.

This makes the script general-purpose.

Remote execution

It's still a bit of a hassle to connect to the server every time. We can execute a script locally to get things done on the server.

Note: the statements in quotation marks are executed on the server.

Confirm execution:

The default is y.

But if one day we delete one of the useful files, such as when jim.txt re-executes the script:

At this point, you will be prompted whether to delete the script, which is very good. We can choose not to delete no.

Write scripts in other languages

The grammar of Bash is partial to machine thinking. It's more obscure.

At this point, I believe you have a deeper understanding of "how to use Shell position parameters". You might as well do it in practice. Here is the website, more related content can enter the relevant channels to inquire, follow us, continue to learn!

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