$0,$?,$! and << ' END ' in the Linux shell

Variable Description:
$$
The PID of the shell itself (ProcessID)
$!
PID of the Shell's last running background process
$?
End code of the last Run command (return value)
$-
Flag at a glance using the SET command
$*
All parameter lists. such as "$*" in the Case of "" ", in the form of" $ $ ... $n "output all parameters.
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All parameter lists. such as "[email protected]" with "" "in the case, with" $ "" $ "... All parameters are output in the form "$n".
$#
Number of arguments added to the shell
$
The name of the shell itself
$1~ $n

Each parameter value added to the shell. $ $ is the 1th parameter, and the $ = is the 2nd parameter ...

<< ' END '

...

END

In fact, end can be replaced with any string, such as ' HAHA ',<< is redirect, generally in front of it will have a shell command, the entire syntax is the content between the end as the input of the shell command, such as

The content between end as input to the Cat command, so the next carriage return will appear

This syntax is called here documentation in the Linux shell

$0,$?,$! and << ' END ' in the Linux shell