POSIX shell extensions

The Korn shell provides extensions to the POSIX shell. The following list describes the extensions to the POSIX shell that are available in the Korn shell. If you need to create a completely portable POSIX.2 shell application, however, you must avoid using these extensions:

The Korn shell allows you define a shell function with the syntax:
```
function name {
		command...
}
```
For more information, see function(1).
The Korn shell supports the select(1) control structure.
The Korn shell supports code blocks enclosed in "curly braces": {...}
The Korn shell supports a [[...]] syntax for string and file conditionals. The portable way to test these is with the test(1) utility.
The Korn shell supports a ((...)) syntax for arithmetic conditionals. The portable way to test these is with the test utility.
The Korn shell adds the commands typeset(1) and let(1).