dollar variables

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  1. $1, $2, $3, ... are the positional parameters.
  2. "$@" is an array-like construct of all positional parameters, {$1, $2, $3 ...}.
  3. "$*" is the IFS expansion of all positional parameters, $1 $2 $3 ....
  4. $# is the number of positional parameters.
  5. $- current options set for the shell.
  6. $$ pid of the current shell (not subshell).
  7. $_ most recent parameter (or the abs path of the command to start the current shell immediately after startup).
  8. $IFS is the (input) field separator.
  9. $? is the most recent foreground pipeline exit status.
  10. $! is the PID of the most recent background command.
  11. $0 is the name of the shell or shell script.

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