Special Parameters in the shell (eg: ksh, bash and sh)

For example

$ testscript one two three

would search the user's PATH and invoke the first program or script called testscript, and the first positional parameter would be one and called $1, the second would be two and called $2 and the last one would three and called $3. $0 is the name of the script itself, in this case, testscript

$#     Expands to the number of positional parameters in decimal

$?     Expands to the exit status of the most recently executed foreground pipeline (usually 0 for success)

$$     Expands to the process  ID  of the shell. In a () subshell, it expands to the PID of the current shell, not the subshell.

$!     Expands to the process ID of the most recently executed background (asynchronous) command.

$-     Expands to the current option flags as specified  upon  invocation,  by  the set  builtin command,

       or those set by the shell itself (such as the -i option).

$*      Expands to the positional parameters, starting from  $1.

        When the expansion occurs within double quotes, it expands to a single word

$@      Expands to the positional parameters, starting from  $1.

                 When  the  expansion  occurs within double quotes, each parameter expands to a separate word.

The difference between $* and $@ is subtle, and indeed, to all intents and purposes, they often behave the same way as each other.

In the case of $*, with the expansion happening between double quotes, the individual parameters are separated by the first character of the IFS shell variable.

The shell variable $IFS is the Internal Field Separator. The usual value for this is a three character sequence of <SPACE><TAB><NEW-LINE> and any of its characters will act as a field separator to the shell unless some form of quoting is in place, usually with single quotes, double quotes or backslashes.

You can see the value of $IFS using this little trick:

peach-gaz:/# echo A"$IFS"B | od -a
0000000    A  sp  ht  lf   B  lf
0000006

We are echoing $IFS in between an A and a B and then using od -a (Octal dump with named character output) to display the characters.

You can see that between the A and the B, there is a sp or a <SPACE> an ht which is a horizontal <TAB> and an lf which is a line-feed or <NEW-LINE>. The final lf is the <NEW-LINE> at the end of the command.

So, if these special parameters are expanded between double quotes,

$*      Expands to  "$1c$2c$3c..." where c is the first character of $IFS (usually a <SPACE> ie:  "$1 $2 $3..."

$@      Expands to "$1" "$2" "$3" ...

and in our example above:

$ testscript one two three

$*      Expands to  "one two three"

$@      Expands to "one" "two" "three"

Whether or not the difference is important will depend on how the special parameter is then used.