We’ll describe many Linux commands in this book. Each
description begins with a standard heading about the command; Figure 1-1 shows one for the
ls (list files) command. This heading
demonstrates the general usage in a simple format:
which means you’d type “ls” followed, if you choose, by options and then filenames. You wouldn’t type the square brackets “[” and “]”: they just indicate their contents are optional; and words in italics mean you have to fill in your own specific values, like names of actual files. If you see a vertical bar between options or arguments, perhaps grouped by parentheses:
This indicates choice: you may supply either a filename or directory name as an argument.
The special heading also includes six properties of the command printed in black (supported) or gray (unsupported):
Figure 1-1. Standard command heading
The command reads from standard input, i.e., your keyboard, by default. See Input and Output.
The command writes to standard output, i.e., your screen, by default. See Input and Output.
When given a dash (-) argument in place of an input
filename, the command reads from standard input; and likewise,
if the dash is supplied as an output filename, the command
writes to standard output. For example, the following
wc command line reads the files
file1 and file2, then standard input, then ...