(bash.info) Shell Parameters
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3.4 Shell Parameters
====================
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* Positional Parameters The shell's command-line arguments.
* Special Parameters Parameters denoted by special characters.
A PARAMETER is an entity that stores values. It can be a `name', a
number, or one of the special characters listed below. 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 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
evaluated as an arithmetic expression even if the `$((...))' expansion
is not used ( Arithmetic Expansion). Word splitting is not
performed, with the exception of `"$@"' as explained below. Filename
expansion is not performed. Assignment statements may also appear as
arguments to the `alias', `declare', `typeset', `export', `readonly',
and `local' builtin commands. When in POSIX mode ( Bash POSIX
Mode), these builtins may appear in a command after one or more
instances of the `command' builtin and retain these assignment
statement properties.
In the context where an assignment statement is assigning a value to
a shell variable or array index ( 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 (
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 (for indexed arrays), or added as additional
key-value pairs in an associative array. When applied to a
string-valued variable, VALUE is expanded and appended to the
variable's value.
A variable can be assigned the NAMEREF attribute using the `-n'
option to the \fBdeclare\fP or \fBlocal\fP builtin commands ( Bash
Builtins) to create a NAMEREF, or a reference to another variable.
This allows variables to be manipulated indirectly. Whenever the
nameref variable is referenced or assigned to, the operation is
actually performed on the variable specified by the nameref variable's
value. A nameref is commonly used within shell functions to refer to a
variable whose name is passed as an argument to the function. For
instance, if a variable name is passed to a shell function as its first
argument, running
declare -n ref=$1
inside the function creates a nameref variable REF whose value is
the variable name passed as the first argument. References and
assignments to REF are treated as references and assignments to the
variable whose name was passed as `$1'.
If the control variable in a `for' loop has the nameref attribute,
the list of words can be a list of shell variables, and a name reference
will be established for each word in the list, in turn, when the loop is
executed. Array variables cannot be given the `-n' attribute.
However, nameref variables can reference array variables and subscripted
array variables. Namerefs can be unset using the `-n' option to the
`unset' builtin ( Bourne Shell Builtins). Otherwise, if `unset'
is executed with the name of a nameref variable as an argument, the
variable referenced by the nameref variable will be unset.
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