These are special shell variables which are set internally by the shell and which are available to the user. Visit here for better understanding Linux shell system variables.
| Variable | Description |
|---|---|
| $0 | The filename of the current script. |
| $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 process ID of the current shell. For shell scripts, this is the process ID under which they are executing. |
| $# | The number of arguments supplied to a script. |
| $@ | All the arguments are individually double quoted. If a script receives two arguments, $@ is equivalent to $1 $2. |
| $* | All the arguments are double quoted. If a script receives two arguments, $* is equivalent to $1 $2. |
| $? | The exit status of the last command executed. |
| $! | The process ID of the last background command. |
| $_ | The last argument of the previous command. |