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 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 (see the description of the declare builtin in
section Bash Builtin Commands), then value
is subject to arithmetic expansion even if the $((...))
expansion is not used (see section Arithmetic Expansion).
Word splitting is not performed, with the exception
of "$@" as explained below.
Filename expansion is not performed.
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