Variable Substitution

Variable substitution is typically used to pass the value of a variable as an argument to a command. For example: echo $message

The ${...} variations are used to isolate the variable substitution from whatever text (including any colons) may follow.

	Reference	Meaning
	$var	Value of variable var
	${var}	Same as $var except separated from anything following.
	$var[expr]	Value of var, indexed by an arbitrarily complex expression.
	${var[expr]}	Same as $var[expr] except separated from anything following.
	$#var	Number of words in var.
	${#var}	Same as $#var except isolated from anything following.
	$?var	1 if var exists; 0 otherwise.
	${?var}	Same as $?var except isolated from anything following.
	$procname( argument list )	Procedure reference, used as a variable substitution. The arguments are expressions, separated by commas. Any value returned by the procedure will be substituted in place.
	$<	Pseudo-variable result of reading one line from stdin each time it’s evaluated. Same as $getline.
	$*	Same as $argv
	$0 .. $9	Same as $argv[0] .. $argv[9]