1.5. Interactive Shell Use

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. ^[5] 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.

^[5] 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.

1.5.1. Commands, Arguments, and Options

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.