Shell scripting built-in variables

When doing scripting in shell, like bash, there are a few built-in variables that we can use to optimize our script. Below are a few useful ones:

• $$ = The PID number of the process executing the shell.
• $? = Exit status variable.
• $0 = The name of the command you used to call a program.
• $1 = The first argument on the command line.
• $2 = The second argument on the command line.
• $n = The nth argument on the command line. n = 0-9
  
• $* = All the arguments on the command line.
• $# = The number of command line arguments.

Hope this can help