Bash Reference Manual
3.4 Shell Parameters
A parameter is an entity that stores values.
It can be a
name, a number, or one of the special characters
For the shell's purposes, a variable is a parameter denoted by a
A variable has a value and zero or more attributes.
Attributes are assigned using the
declare builtin command
(see the description of the
declare builtin in 4.2 Bash Builtin Commands).
A parameter is set if it has been assigned a value. The null string is
a valid value. Once a variable is set, it may be unset only by using
unset builtin command.
A variable may be assigned to by a statement of the form
is not given, the variable is assigned the null string. All
values undergo tilde expansion, parameter and variable expansion,
command substitution, arithmetic expansion, and quote
removal (detailed below). If the variable has its
attribute set, then value
is subject to arithmetic expansion even if the
expansion is not used (see section 3.5.5 Arithmetic Expansion).
Word splitting is not performed, with the exception
"$@" as explained below.
Filename expansion is not performed.
Assignment statements may also appear as arguments to the
local builtin commands.