Shell special variables
$1 - $9
these variables are the positional parameters.
$0
the name of the command currently being executed.
$#
the number of positional arguments given to this invocation of the shell.
$?
the exit status of the last command executed is given as a decimal string. When a command completes successfully, it returns the exit status of 0 (zero), otherwise it returns a non-zero exit status.
$$
the process number of this shell - useful for including in filenames, to make them unique.
$!
the process id of the last command run in the background.
$-
the current options supplied to this invocation of the shell.
$*
a string containing all the arguments to the shell, starting at $1.
$@@
same as above, except when quoted.
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