zsh: Parameters

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## 15 Parameters

### 15.1 Description

A parameter has a name, a value, and a number of attributes. A name may be any sequence of alphanumeric characters and underscores, or the single characters ‘`*`’, ‘`@`’, ‘`#`’, ‘`?`’, ‘`-`’, ‘`$`’, or ‘`!`’. A parameter whose name begins with an alphanumeric or underscore is also referred to as a variable.

The attributes of a parameter determine the type of its value, often referred to as the parameter type or variable type, and also control other processing that may be applied to the value when it is referenced. The value type may be a scalar (a string, an integer, or a floating point number), an array (indexed numerically), or an associative array (an unordered set of name-value pairs, indexed by name, also referred to as a hash).

Named scalar parameters may have the exported, `-x`, attribute, to copy them into the process environment, which is then passed from the shell to any new processes that it starts. Exported parameters are called environment variables. The shell also imports environment variables at startup time and automatically marks the corresponding parameters as exported. Some environment variables are not imported for reasons of security or because they would interfere with the correct operation of other shell features.

Parameters may also be special, that is, they have a predetermined meaning to the shell. Special parameters cannot have their type changed or their readonly attribute turned off, and if a special parameter is unset, then later recreated, the special properties will be retained.

To declare the type of a parameter, or to assign a string or numeric value to a scalar parameter, use the `typeset` builtin.

The value of a scalar parameter may also be assigned by writing:

> name`=`value

In scalar assignment, value is expanded as a single string, in which the elements of arrays are joined together; filename expansion is not performed unless the option `GLOB_ASSIGN` is set.

When the integer attribute, `-i`, or a floating point attribute, `-E` or `-F`, is set for name, the value is subject to arithmetic evaluation. Furthermore, by replacing ‘`=`’ with ‘`+=`’, a parameter can be incremented or appended to. See Array Parameters and Arithmetic Evaluation for additional forms of assignment.

Note that assignment may implicitly change the attributes of a parameter. For example, assigning a number to a variable in arithmetic evaluation may change its type to integer or float, and with `GLOB_ASSIGN` assigning a pattern to a variable may change its type to an array.

To reference the value of a parameter, write ‘`$`name’ or ‘`${`name`}`’. See Parameter Expansion for complete details. That section also explains the effect of the difference between scalar and array assignment on parameter expansion.

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