8.2 Entering Commands
As soon as the prompt appears on the shell it is ready to receive and
execute commands. A command can consist of several elements. The first
element is the actual command, followed by parameters or options. You can
type a command and edit it by using the following keys: ←, →, Home,
End, Backspace (Backspace), Del, and Space. You can correct typing
errors or add options. The command is not executed until you press Enter.
IMPORTANT: No News Is Good News
The shell is not verbose: in contrast to some graphical user
interfaces, it usually does not provide confirmation messages when
commands have been executed. Messages only appear in case of problems or
errors —or if you explicitly ask for them by executing a command
with a certain option.
Also keep this in mind for commands to delete objects. Before
entering a command like rm (without any option) for
removing a file, you should know if you really want to get rid of the
object: it will be deleted irretrievably, without inquiry.
8.2.1 Using Commands without Options
In Section 7.3.1, Permissions for User, Group and Others you already got to
know one of the most basic commands: ls, which used to list the contents of a directory. This command
can be used with or without options. Entering the plain
ls command shows the contents of the current
directory:
tux@knox:~> ls
bin Desktop Documents public_html tux.txt
tux@knox:~>
As you already learned in Section 7.2.1, Key Features files
in Linux may have a file extension or a suffix, such as
.txt, but do not need to have one. This makes it
difficult to differentiate between files and folders in this output of the
ls. By default, the colors in the Bash shell give you a
hint: directories are usually shown in blue, files in black.
8.2.2 Using Commands with Options
A better way to get more details about the contents of a
directory is using the ls command with a string of
options. Options modify the way a command works so that you can get it to
carry out specific tasks. Options are separated from the command with a
blank and are usually prefixed with a hyphen. The ls
-l command shows the contents of the same
directory in full detail (long listing format):
tux@knox:~> ls -l
drwxr-xr-x 1 tux users 48 2006-06-23 16:08 bin
drwx---r-- 1 tux users 53279 2006-06-21 13:16 Desktop
drwx------ 1 tux users 280 2006-06-23 16:08 Documents
drwxr-xr-x 1 tux users 70733 2006-06-21 09:35 public_html
-rw-r--r-- 1 tux users 47896 2006-06-21 09:46 tux.txt
tux@knox:~>
This output shows the following information about each object:
drwxr-xr-x
1
tux
users
48
2006-06-23 16:08
bin
Usually, you can combine several options by prefixing only the first
option with a hyphen and then write the others consecutively without a
blank. For example, if you want to see all files in a directory in long
listing format, you can combine the two options -l and
-a (show all files) for the ls
command. Executing ls -la shows also
hidden files in the directory, indicated by a dot in front (for example,
.hiddenfile).
The list of contents you get with ls is sorted
alphabetically by filenames. But like in a graphical file manager, you can
also sort the output of ls -l
according to various criteria such as date, file extension or file
size:
-
For date and time, use ls -lt
(displays newest first).
-
For extensions, use ls -lx
(displays files with no extension first).
-
For file size, use ls -lS
(displays largest first).
To revert the order of sorting, add -r as an option
to your ls command. For example, ls
-lr gives you the contents list sorted in
reverse alphabetical order, ls -ltr
shows the oldest files first. There are lots of other useful options for
ls—in the following section you will learn how to
investigate them.
8.2.3 Getting Help
Nobody is expected to know all options of all commands by heart. If
you remember the command name but are not sure about the options or the
syntax of the command, choose one of the following possibilities:
- --help option
-
If you only want to look up the options of a certain command, try
entering the command followed by a blank and --help.
This --help option exists for many commands. For
example, ls --help displays all the
options for the ls command.
- Manual Pages
-
To learn more about the various commands, you can also use the
manual pages. Manual pages also give a short description of what the
command does. They can be accessed with man followed
by the name of the command, for example,
man ls.
The man pages are displayed directly in the shell. To navigate
them, move up and down with PgUp and PgDn. Move between the beginning and the end of a
document with Home and End.
End this viewing mode by pressing Q. Learn more about
the man command itself with man
man.
- Info Pages
-
Info pages usually provide even more information about commands.
To view the info page for a certain command, enter
info followed by the name of the command, for
example, info ls. You can browse an info page with a
viewer directly in the shell and display the different sections, called
nodes.
Use Space to move forward
and Backspace to move backwards. Within a node,
you can also browse with PgUp and PgDn but only Space and Backspace will take you also to the previous or
subsequent node. Like for the man pages, press Q to
end the viewing mode.
Note that man pages and info pages do not exist for all commands:
sometimes both are available (usually for key commands), sometimes only a
man page or an info page exists, sometimes neither of them.
8.2.4 Bash Shortcut Keys
When having entered several commands, your shell will soon be filled
with all sorts of commands and the corresponding outputs. In the following
table, find some useful shortcut keys for navigating and editing in the
shell which help you to keep overview.
