Linux web host - Chapter 2 . Running Commands from the Shell
Wednesday, April 25th, 2007Chapter 2 . Running Commands from the Shell 39 /bin. Then use the man command (for example, man hostnameto see what each command does. There are also administrative commands in /sbin or /usr/sbin directories. Using the Shell in Linux When you type a command in a shell, you can include other characters that change or add to how the command works. In addition to the command itself, these are some of the other items that you can type on a shell command line: . Options Most commands have one or more options you can add to change their behavior. Options typically consist of a single letter, preceded by a dash. You can also often combine several options after a single dash. For example, the command ls -la lists the contents of the current directory. The -l asks for a detailed (long) list of information, and the -a asks that files beginning with a dot (.) also be listed. When a single option consists of a word, it is usually preceded by a double dash (–). For example, to use the help option on many commands, you enter –help on the command line. You can use the –help option with most commands to see the options and arguments that they support. For example, hostname –help. . Arguments Many commands also accept arguments after certain options are entered or at the end of the entire command line. An argument is an extra piece of information, such as a filename, that can be used by the command. For example, cat /etc/passwd displays the contents of the /etc/passwd file on your screen. In this case, /etc/passwd is the argument. . Environment variables The shell itself stores information that may be useful to the user s shell session in what are called environment variables. Examples of environment variables include $SHELL (which identifies the shell you are using), $PS1 (which defines your shell prompt), and $MAIL (which identifies the location of your mailbox). See the Using Shell Environment Variables section later in this chapter for more information. You can check your environment variables at any time. Type declare to list the current environment variables. Or you can type echo $VALUE, where VALUE is replaced by the name of a particular environment variable you want to list. . Metacharacters These are characters that have special meaning to the shell. They can be used to direct the output of a command to a file (>), pipe the output to another command (|), and run a command in the background (&), to name a few. Metacharacters are discussed later in this chapter. To save you some typing, there are shell features that store commands you want to reuse, recall previous commands, and edit commands. You can create aliases that enable you to type a short command to run a longer one. The shell stores previously entered commands in a history list, which you can display and from which you can recall commands. You ll see how this works a little later in the chapter. Tip Note
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