Variable declaration and some special variables in shell

There are several points to observe or pay attention to when declaring variables ：

• stay SHELL The variables defined in are more direct , No type difference .
• There can be no spaces between variable names and equal signs .
• The first character of a variable name must be a letter .
• You can use underscores in variable names .
• There can be no spaces in the variable name , Dollar symbol $, Punctuation .
• Variable names cannot use keywords .

Now use mind map , More intuitive

$0： File name of the current script

$n： Parameters passed to a script or function .n It's a number , Indicates the number of parameters . for example , The first parameter is $1, The second parameter is $2.

$#： Number of arguments passed to script or function .

$*： All parameters passed to a script or function .

[email protected]： All parameters passed to a script or function . By double quotes (" ") Inclusion time , And $* not quite the same , I'll talk about .

$?： Exit status of last command , Or the return value of the function .

$$： At present Shell process ID. about Shell Script , It's the process of these scripts ID