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 are assigned using the
declare builtin command
(see the description of the
declare builtin in 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 (see Shell Parameter Expansion).
If the variable has its
attribute set, then value
is evaluated as an arithmetic expression even if the
expansion is not used (see Arithmetic Expansion).
Word splitting and filename expansion are not performed.
Assignment statements may also appear as arguments to the
local builtin commands (declaration commands).
When in POSIX mode (see Bash POSIX Mode), these builtins may appear
in a command after one or more instances of the
and retain these assignment statement properties.
In the context where an assignment statement is assigning a value
to a shell variable or array index (see Arrays), the ‘+=’
operator can be used to
append to or add to the variable’s previous value.
This includes arguments to builtin commands such as
accept assignment statements (declaration commands).
When ‘+=’ is applied to a variable for which the
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 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
local builtin commands
(see Bash Builtin Commands)
to create a nameref, or a reference to another variable.
This allows variables to be manipulated indirectly.
Whenever the nameref variable is referenced, assigned to, unset, or has
its attributes modified (other than using or changing the nameref
attribute itself), the
operation is actually performed on the variable specified by the nameref
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
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, and changes to its attributes,
are treated as references, assignments, and attribute modifications
to the variable whose name was passed as
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
Array variables cannot be given the nameref attribute.
However, nameref variables can reference array variables and subscripted
Namerefs can be unset using the -n option to the
(see Bourne Shell Builtins).
unset is executed with the name of a nameref variable
as an argument, the variable referenced by the nameref variable will be unset.