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
section ???Using Shell Environment Variables??? 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.
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