|GNU Bash Reference Manual|
by Chet Ramey and Brian Fox
Paperback (6"x9"), 180 pages
RRP £19.95 ($29.95)
"An essential resource .... the most detailed coverage available for all aspects of Bash" --- Linux User and Developer Magazine (Issue 37, Mar 2004) Get a printed copy>>>
3.4 Shell Parameters
A parameter is an entity that stores values.
It can be a
name, a number, or one of the special characters
A variable is a parameter denoted by a
A variable has a value and zero or more attributes.
Attributes are assigned using the
declare builtin command
(see the description of the
declare builtin in section 4.2 Bash Builtin Commands).
A parameter is set if it has been assigned a value. The null string is
a valid value. Once a variable is set, it may be unset only by using
unset builtin command.
A variable may be assigned to by a statement of the form
is not given, the variable is assigned the null string. All
values undergo tilde expansion, parameter and variable expansion,
command substitution, arithmetic expansion, and quote
removal (detailed below). If the variable has its
attribute set, then value
is evaluated as an arithmetic expression even if the
expansion is not used (see section 3.5.5 Arithmetic Expansion).
Word splitting is not performed, with the exception
"$@" as explained below.
Filename expansion is not performed.
Assignment statements may also appear as arguments to the
local builtin commands.
In the context where an assignment statement is assigning a value to a shell variable or array index (see section 6.7 Arrays), the ‘+=’ operator can be used to append to or add to the variable's previous value. When ‘+=’ is applied to a variable for which the integer attribute has been set, value is evaluated as an arithmetic expression and added to the variable's current value, which is also evaluated. When ‘+=’ is applied to an array variable using compound assignment (see section 6.7 Arrays), the variable's value is not unset (as it is when using ‘=’), and new values are appended to the array beginning at one greater than the array's maximum index. When applied to a string-valued variable, value is expanded and appended to the variable's value.
|ISBN 0954161777||GNU Bash Reference Manual||See the print edition|