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.
`$proc( arglist )`	Procedure reference, used as a variable substitution. The arguments are expressions, separated by commas. Any value returned by the procedure will be substituted in place.
`$<`	Pseudo-variable result of reading one line from stdin each time it’s evaluated. Same as `$getline`.
`$*`	Same as `$argv`
`$0` through `$9`	Same as `$argv[0]` through `$argv[9]`