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$0 –> The filename of the current script.
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$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 $2, and so on).
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$# –> The number of arguments supplied to a script.
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$* –> All the arguments are double quoted. If a script receives two arguments, $* is equivalent to $1 $2.
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$@ –> All the arguments are individually double quoted. If a script receives two arguments, $@ is equivalent to $1 $2.
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$? –> The exit status of the last command executed.
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$$ –> The process number of the current shell. For shell scripts, this is the process ID under which they are executing.
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$! –> The process number of the last background command.