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A positional parameter is a parameter denoted by one or more
digits, other than the single digit 0. Positional parameters are
assigned from the shell’s arguments when it is invoked,
and may be reassigned using the set builtin command.
Positional parameter N may be referenced as ${N}, or
as $N when N consists of a single digit.
Positional parameters may not be assigned to with assignment statements.
The set and shift builtins are used to set and
unset them (see Shell Builtin Commands).
The positional parameters are
temporarily replaced when a shell function is executed
(see Shell Functions).
When a positional parameter consisting of more than a single digit is expanded, it must be enclosed in braces.