Mac OS X Tiger in a Nutshell

Variable Substitution

In the following substitutions, braces ({ }) are optional, except when needed to separate a variable name from following characters that would otherwise be considered part of the name.

Table 5-22. Substitution variables

Variable	Meaning
${var}	Value of variable var.
$0	Name of the program.
${n}	Individual arguments on command line (positional parameters); 1 n 9.
$#	Number of arguments on command line.
$*	All arguments on command line.
$@	Same as $*, but contents are split into words when the variable is enclosed in double quotes.
$$	Process number of current shell; useful as part of a filename for creating temporary files with unique names.
$?	Exit status of last command (normally 0 for success).
$!	Process number of most recently issued background command.
$-	Current execution options (see the set built-in command). By default, hB for scripts and himBH for interactive shells.
$_	Initially set to name of file invoked for this shell, then set for each command to the last word of the previous command.

Tables 5-23 through Table 5-25 show various types of operators that can be used with bash variables.

Table 5-23. Substitution operators

Operator	Substitution
${varname:-word}	If varname exists and isn’t null, return its value; otherwise, return word.
	Purpose: Returning a default value if the variable is undefined.
	Example: ${count:-0} evaluates to 0 if count is undefined.
${varname:=word}	If varname exists and ...