Variable reference in Shell (two methods: $ variable name, $ {variable name }).

I. Basics

We define a variable (there cannot be spaces on both sides of the equal sign)

Filepath =/var/home/SSS
Filefilename = test001

When referencing this variable, we can directly use the variable name following $.

For example, $ filepath
You can also: $ {filepath}

The advantage of using $ {} Is that the connection between the two variables can be easily realized, and it is clear at the same time.

$ {Filepath}/$ {filefilename}

This will indicate the full path of the file.

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2. Sometimes we see

$1, $2, which indicates the parameters passed in during shell execution.

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3. We will also see the following forms

1. If the variable meter has a value or is empty, use the value of value.

$ {Variable:-value}

2. If the variable meter has a value or is empty, use the value of value and assign the variable value to value.

$ {Variable: = value}

3. Check whether the variable is empty.

$ {Variable :?}

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Which of the following are not commonly used?

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$0 shell command itself (including the full path)
$1 to $9 indicates the shell Parameters
$ # Number of parameters passed to the script
$ * Display all parameters passed to the script with a single string
$ ID of the script run
$! ID of the last process running in the background
$ @ Is the same as $.
$-Display the current options used by shell.
$? Displays the execution status of the last command. 0 indicates no error.