记录一些特殊的linux shell 参数，在编写shell的过程中会用得上。

linux shell 预定义参数
名称 功能
$0 The name of the command being executed. For shell scripts, this is the path with which it was invoked.
$n These variables correspond to the arguments with which a script was invoked. Here n is a positive decimal number corresponding to the position of an argument (the first argument is $1,the second argument is $2, and so on).
$# The number of arguments supplied to a script.
$* All the arguments are double quoted. If a script receives two arguments, $* is equivalent to $1 $2.
$@ All the arguments are individually double quoted. If a script receives two arguments, $@ is equivalent to $1 $2.
$? The exit status of the last command executed.
$$ The process number of the current shell. For shell scripts, this is the process ID under which they are executing.
$! The process number of the last background command.

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