Typing in commands
After you have logged in, and a command window is available, the
system will prompt you to type in a command. The
prompt is usually $ (dollar), though
many systems are able to change this, and you yourself are able to
alter it.
Beware, however, if you get a prompt that terminates in
% (percent) - this is usually an indication that the
shell you will talk to is either the C shell or a derivative of it,
and not a POSIX shell. Although most of the next couple of chapters
will still be valid for such a shell, there are significant
incompatibilities, and much of the next three Chapters will not be
correct. In such a circumstance, it may be possible for your login
shell to be changed to a POSIX shell (such as bash).
Again, consult your system administrator. Note that Linux
distributions always have a POSIX shell set up as the default.
Try now typing date (remembering
to press the Return key at the end). You should see on the
screen something like
$ date
Tue Dec 4 20:10:39 GMT 2001
$
By typing date you have instructed
the machine to obey the command called date; it has
executed the command and has printed on your screen a message (as
instructed by the command date). When that command
completes, you are then given another prompt. Try now typing
nonsense - you should get something like
$ qwerty
qwerty: command not found
$
telling you that it doesn't understand what you've typed in.
The command date is the name of a program - it's
written in machine code, and you don't need to know the details of
how it works, just what it does. To describe a
program such as date being obeyed, we use the words
running or executing.
You will have been notified of the command you must type for
changing your password; this is usually passwd,
although some system administrators prefer to install their own
command. On some systems passwd can work very slowly,
and your new password may not take effect until a few minutes after
you have entered it, especially if your UNIX system is a network of
machines rather than a single computer.
Note that passwd is not a standard POSIX command,
since the shell does not specify how to authenticate
users.
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