These are special shell variables which are set internally by the shell and which are available to the user. We are not going to cover all these, but this list might be handy someday!
| Variable | Description |
|---|---|
| $0 | The filename of the current script. |
| $n | These variables (where n = an integer) 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. |