Syntax diagrams

The syntax for a command and its options is presented in the form of a syntax diagram called a railroad diagram. Railroad diagrams are a visual format suitable for sighted users. It tells you what options you can supply with the command, how to enter them, indicates relationships between different options, and sometimes different values of an option.

Each railroad diagram begins with a double right arrow and ends with a right and left arrow pair. Lines beginning with a single right arrow are continuation lines. You read a railroad diagram from left to right and from top to bottom, following the direction of the arrows.

Other conventions used in railroad diagrams are:

Table 1. How to read railroad diagrams

Convention	Meaning
	You must specify values A, B, and C. Required values are shown on the main line of a railroad diagram.
	You may specify value A. Optional values are shown below the main line of a railroad diagram.
	Values A, B, and C are alternatives, one of which you must specify.
	Values A, B, and C are alternatives, one of which you might specify.
	This shows that a value (for example, A, or B, or C) must be selected, and if another is to be selected then a comma must be used between the values.
	You might specify value A multiple times. The separator in this example is optional.
	Values A, B, and C are alternatives, one of which you might specify. If you specify none of the values shown, the default A (the value shown above the main line) is used.
Name	The railroad fragment Name is shown separately from the main railroad diagram.
Punctuation and uppercase values	Specify exactly as shown.