BASH – Default variables for a bash shell

$? => Return code of previous command
$0 => Name of the script itself
$1-9 => Argument number 1 to 9 given to script
${10} => Argument number 10 given to script
$# => Total number of arguments given to script
$@ => All arguments given to script but as an individual string values
“$*” => All arguments given to script as a single string value (must be in double quotes)
$$ => Process id of the script
$! => Process id of last job run in backgroup
$_ => Last argument given to previous command