Predefined special parameters in the Korn shell or POSIX shell

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Some parameters are set automatically by the Korn shell or POSIX shell.

The following parameters are automatically set by the shell:

Item Description
@ Expands the positional parameters, beginning with $1. Each parameter is separated by a space.

If you place " around $@, the shell considers each positional parameter a separate string. If no positional parameters exist, the shell expands the statement to an unquoted null string.
* Expands the positional parameters, beginning with $1. The shell separates each parameter with the first character of the IFS parameter value.

If you place " around $*, the shell includes the positional parameter values in double quotation marks. Each value is separated by the first character of the IFS parameter.
# Specifies the number (in decimals) of positional parameters passed to the shell, not counting the name of the shell procedure itself. The $# parameter thus yields the number of the highest-numbered positional parameter that is set. One of the primary uses of this parameter is to check for the presence of the required number of arguments.
- Supplies flags to the shell on invocation or with the set command.
? Specifies the exit value of the last command executed. Its value is a decimal string. Most commands return 0 to indicate successful completion. The shell itself returns the current value of the $? parameter as its exit value.
$ Identifies the process number of this shell. Because process numbers are unique among all existing processes, this string of up to 5 digits is often used to generate unique names for temporary files.
The following example illustrates the recommended practice of creating temporary files in a directory used only for that purpose: 
temp=$HOME/temp/$$
ls >$temp
.
.
.
rm $temp
! Specifies the process number of the most recent background command invoked.
zero (0) Expands to the name of the shell or shell script.