When you use the shell interactively, you engage in a login session that begins when you log in and ends when you type exit or logout or press CTRL-D.  During a login session, you type in command lines to the shell; these are lines of text ending in RETURN that you type in to your terminal or workstation.
 The shell can be set up so that it ignores a single CTRL-D to end the session. We recommend doing this, because CTRL-D is too easy to type by accident . See the section on options in Chapter 3 for further details.
By default, the shell prompts you for each command with an information string followed by a dollar sign, though as you will see in Chapter 3 , the entire prompt can be changed.
Shell command lines consist of one or more words, which are separated on a command line by blanks or TABs. The first word on the line is the command . The rest (if any) are arguments (also called parameters ) to the command, which are names of things on which the command will act.
For example, the command line lp myfile consists of the command lp (print a file) and the single argument myfile . lp treats myfile as the name of a file to print. Arguments are often names of files, but not necessarily : in the command line mail cam , the mail program treats cam as the username to which a message will be sent.
An option is a special type of argument that gives the command specific information on what it is supposed to do. Options usually consist of a dash followed by a letter; we say "usually" because this is a convention rather than a hard-and-fast rule. The command lp -h myfile contains the option -h , which tells lp not to print the "banner page" before it prints the file.
Sometimes options take their own arguments. For example, lp -d lp1 -h myfile has two options and one argument. The first option is -d lp1 , which means "Send the output to the printer (destination) called lp1 ." The second option and argument are the same as in the previous example.