(bashref.info)Shell Parameters

   A PARAMETER is an entity that stores values.  It can be a `name', a
number, or one of the special characters listed below.  For the shell's
purposes, a VARIABLE is a parameter denoted by a `name'.  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 Note: Bash Builtins).

   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
the `unset' builtin command.

   A variable may be assigned to by a statement of the form
     NAME=[VALUE]

If VALUE 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 `integer' attribute set, then VALUE is
subject to arithmetic expansion even if the `$((...))' expansion is not
used (Note: Arithmetic Expansion).  Word splitting is not performed,
with the exception of `"$@"' as explained below.  Filename expansion is
not performed.  Assignment statements may also appear as arguments to
the `declare', `typeset', `export', `readonly', and `local' builtin
commands.