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Shell Built-in Variables

$#

the number of arguments

$*

all arguments to shell

$@

similar to $*

$* vs. $@

• $* and $@ expand into the arguments, and are rescanned; blanks in arguments will result in multiple arguments.
• "$*" is a single word composed of all the arguments to the shell file joined together with spaces.
• "$@" is identical to the arguments recevied by the shell file: blanks in arguments are ignored, and the result is a list of words identical to the orginal arguments.

$-

options supplied to the shell

$?

return values of the last command executed

$$

process-id of the shell

$!

process-id of the last command started with &

$HOME

default argument for cd command

$IFS

list of characters that separate words in arguments

$MAIL

file that, when changed, triggers "you have mail" message

$PATH

list of directories to search for commands

$PS1

prompt string, default '$ '

$PS2

prompt string for continued command line, default '> '