04. Positional Parameters and Special Characters $1, shift, $#, $@, $0, $$

Positional parameters are used to access the command-line arguments. These are the trailing words that come after the command line call.

$ myShellScript posParam1 posParam2 posParam3                  

The first argument is denoted by $1, the second by $2, and so on. Any two-digit values are to be wrapped around in braces such as ${10}.

The $0 variable is the name of the command itself. This can be useful when printing out error messages.

Unset positional parameters

If the user does not input any arguments, but you still call a positional parameter, then the shell takes it in as null.

Shifting parameters over

The shift command is used to shift the arguments over one. This removes the first argument, and advances the rest of the input arguments forward. We'll revisit this when using loops, where this command can be useful.

Other Positional Parameters & Special Characters

Here is a list of other important parameters.

$#
Number of arguments.
Print the number of arguments passed in.
Can be used with the shift variable within a loop.
$@
All arguments.
Access all arguments at once.
Useful when invoking a command within the script, and require the user-specified options in original command.
$$
Holds the process ID of the shell.
$?
Exit status of previous command.
$!
Process ID of last background command.

Take your Linux skills to the next level!

System Admin Handbook

Take your Linux skills to the next level! Try Linux & UNIX

This book approaches system administration in a practical way and is an invaluable reference for both new administrators and experienced professionals. It details best practices for every facet of system administration, including storage management, network design and administration, email, web hosting, scripting, and much more.

$ Check price
74.9974.99Amazon 4.5 logo(142+ reviews)

More Linux & UNIX resources

Aching back from coding all day?

Foam Seat Cushion

Aching back from coding all day? Try Back Problems

This foam seat cushion relieves lowerback pain, numbness and pressure sores by promoting healthy weight distribution, posture and spine alignment. Furthermore, it reduces pressure on the tailbone and hip bones while sitting. Perfect for sitting on the computer desk for long periods of time.

$ Check price
99.9599.95Amazon 4.5 logo(9,445+ reviews)

More Back Problems resources

Ad