This chapter starts by introducing some general information about
working with the command line. From there, it explains how to:
locate yourself in the file system, move and manipulate files,
browse to other directories, and view their contents. Finally,
it moves on to more complicated issues including re-direction and
permissions.
4.3.1. Shell Prompt Terms
Below are a few terms to know before beginning the discussion of the shell prompt.
command line
The command line is where options to a command are
placed. The following is an example of a command
line:
command -options
<filename>
shell prompt
The shell prompt is the marker on the screen that
shows where the command line should be placed. The
following is an example of a shell prompt:
The shell is the program that interprets commands so
that the operating system can understand them.
terminal window
The terminal window is the window that contains the
shell prompt, command line, and output from the
shell.
4.3.2. Opening and using a Shell Prompt
Recall from Section 1.5 Opening a Shell Prompt that a terminal window is opened from the Applications (the main menu on the
panel), or by right-clicking on
the desktop and selecting Open Terminal.
Entering or running a command refers to typing a given
command and pressing [Enter]. To close a
terminal window, either click on the [X]
in the upper right corner of the screen or enter the
exit command at the shell prompt.
The shell prompt within a terminal window looks something
like this:
There are any number of symbols that can be used to
indicate the end of the shell prompt, and you can
customize what your prompt looks like. However, there are two
symbols that you will see more often than any others,
"$" and "#". The first symbol,
"$", is the last character in the prompt when
you are logged in as a normal user. The shell prompt for a
normal user looks something like this:
The second symbol, "#", is the last character in
the prompt when you are logged in as root. This is true
whether you logged in as root from the initial screen or if
you executed the su - command to become
root. The shell prompt for root looks something like this:
This slight difference can help remind you what privileges you
currently have.
4.3.3. Structure of shell prompt commands
In general, a command run from the shell prompt will have the
following format:
command -options <filename>.
Both -options and
<filename> are optional: the
command may not require either one, or it may require multiple
options and files. When specifying multiple options, list
them as a group. For example, to see a long listing of
information (-l) about all files (-a) in a directory, you
would run the command:
ls -la
There are many variations in required information types
for individual commands. If you aren't sure about a command
and how it is used, you can do one of three things:
Enter the command alone at a shell prompt and press
[Enter]. For example, entering
cp alone returns a brief description
of the command and its usage. Other commands, such as
cat, require no arguments or files to run as normal. To
quit such a command, press
[Ctrl]-[D].
If that does not work, press
[Ctrl]-[C].
Enter man command at a shell
prompt. This opens the manual
page for that command. A man page is a
manual written by the command's developer explaining how
it is used and what each option means. You can also
enter man man at a shell prompt
for more information on man pages. Navigate through the
man page using the directional keys on your keyboard.
The [Space] bar moves you down a page,
[B] moves you up a page. To quit, press
[Q]. If the man page for a command is
either unavailable or does not provide enough
information, the info system may provide additional
information.
Enter info command at a shell
prompt. Some info pages have the same information as
man pages, but navigation is slightly different. For
more information, enter info info
at a shell prompt.
4.3.4. Useful tips for the bash shell
Below are a few useful features of the bash shell that reduce
the amount of typing required at a shell prompt. The first of
these is tab completion, the second is command history, and
the third is useful keystrokes.
4.3.4.1. Tab Completion
Tab completion is one of the most useful shortcuts available
at the command line. Red Hat Enterprise Linux has the ability to
"guess" what command, directory, or filename you
are entering at the shell prompt. Press the
[Tab] key, and the shell offers possible completions
for partial words. The more letters typed before pressing
[Tab], the closer the shell comes to the intended command.
If there are multiple solutions to the partial text on the
command line, the shell presents them as a list. If the
list is very long, the shell will first ask if you would
like to display all matches. Navigate long lists of
potential matches by pressing the [Space] bar
to go down one page, the [B] key to go back
one page, the directional (or "arrow") keys to
move one line at a time, and [Q] to
quit.
The shell assumes that the first word entered at the prompt
is a command. The possible completions it offers are the
names of commands or applications. This can be helpful if
you are not sure of the exact spelling of a command or if
you are searching for a certain command. It can also serve
to help a new user become familiar with the available commands.
For example:
Type the letter g at a prompt and
press [Tab] twice.
The shell asks if you want to see all 379
possibilities. This means that there are 379 commands
that start with the letter "g". Searching
through this list would take too much time.
Press [N] for no.
Entering more of the command name will produce a shorter
list of possible matches. For this example, type
gnome and press
[Tab] twice. A list of every command that
starts with "gnome" appears. This is a much
shorter list, and can be scrolled through using the same
keys as man pages. Scroll to the end of the list to
return to the shell prompt. The letters
"gnome" are still entered.
To finish entering a command with tab completion, enter
just a few more characters, "-ca", and press
[Tab] twice. The shell returns a match
of gnome-calculator, and if you then press
[Enter], the GNOME
Calculator application starts.
Tab completion also works on filenames and directories. The
shell prompt assumes that the second word on the command
line is a filename or directory. Typing a partial word and pressing
[Tab] twice will generate possible
completions according to the files and sub-directories in
your current working directory. The command line
"knows" your directory structure. You can use tab
completion to enter a long string of sub-directories by
typing the first few letters of each directory and pressing
[Tab] instead of navigating to one
subdirectory at a time.
For example, reaching the sub-directory
example/sub-dir1/sub-dir2/sub-dir3/
would take a great deal of repetitive typing. However,
with tab completion, a user would only have to enter a few
keystrokes:
$ cd ex[Tab]su[Tab]su[Tab]su[Tab]
4.3.4.2. Command History
It is unnecessary to type the same command over and over. The
bash shell remembers your past commands. These commands
are stored in the .bash_history file in
each user's home directory. To use the history, press
the up arrow to scroll backward through the commands you have
already entered. The [Ctrl]-[R]
shortcut searches through your previous commands. Press
[Ctrl]-[R]
and type the beginning of the command you previously
issued. The command history stops at the most recent
version of that command.
Commands that you only typed partially and did not
follow with [Enter] are not saved into your
command history file. To clear your command history, type
history -c.
By default, Red Hat Enterprise Linux stores 1000 commands. Each terminal
window or shell prompt stores a separate set of commands. If
you gain root privileges by using the command su
-, the history file (and thus the commands)
you access are root's, not the user's.
4.3.4.3. [Ctrl]-[Z]
and running processes in the background
Applications and processes can be started from the command
line. When an application starts from the command line, that
particular shell is taken up with standard output for
that application until the application closes. The screen
fills with gibberish or messages that can be ignored. To continue to use the current shell while running
an application from the same shell, add the
ampersand, "&", to the end of the
command line. For example, oowriter
& starts OpenOffice.org
Writer and allows you to continue entering
commands on the command line. This is known as running a
process in the background.
If you have started an application or process and forgotten
to add the &, first press
[Ctrl]-[Z]
— this suspends the application. To allow it to
continue running without displaying standard output, type
bg and press [Enter].
This is referred to as running the application in the
background.
4.3.4.4. Wildcards
Wildcards are place holders used
to allow users to search for or use multiple files with
similar names. The subject of wildcards is part of the
larger subject of regular expressions. Two of the most
common wildcards are "*" and
"?".
The asterisk, "*", represents any character or
string of characters. The entry
a*.txt could refer to
ab.txt as well as
aardvark.txt.
The question mark represents a single character. The
entry a?.txt could refer to
ab.txt and a1.txt,
but not aardvark.txt.
What if you forget the name of the file you are looking
for? Using wildcards or regular expressions, you can perform
actions on a file or files without knowing the complete file
name. Type out what you know, substituting a wildcard for the remainder.
Tip
To read more about wildcards and
regular expressions, take a look at the
bash man page (man
bash). You can save the file to a
text file by typing man bash | col -b >
bash.txt. Then, you can open and read the file
with less or with an editor such as
vi (vi bash.txt). If
you want to print the file, be aware that it is quite long.
For example, to find a file called "sneaksomething.txt," enter:
ls sneak*.txt
The shell lists every file that matches that pattern:
sneakers.txt
Regular expressions are more complex than the
straightforward asterisk or question mark.
When an asterisk, for example, just happens to be part of
a file name, as might be the case if the file
sneakers.txt was called
sneak*.txt, that is when regular
expressions can be useful.
Using the backslash (\), you can
specify that you do not want to search out
everything by using the asterisk, but you
are instead looking for a file with an asterisk in the name.
If the file is called sneak*.txt,
type:
sneak\*.txt
Here is a brief list of wildcards and regular expressions:
* — Matches all characters
? — Matches one character
\* — Matches the * character
\? — Matches the ? character
\) — Matches the ) character
4.3.4.5. Useful Keystrokes
While working from the command line, there are a few useful
keystrokes that can help you with your session.
The following keystrokes are useful shortcuts at a
shell prompt.
Keystroke
Action
[Ctrl]-[K]
While editing a command on the command line,
this key combination deletes everything that has
been typed in from the cursor's current position
forward.
[Ctrl]-[D]
Pressing this key combination once ends the
current application or process. Pressing it twice
exits the shell.
[Ctrl]-[R]
At the command line,
[Ctrl]-[R]
searches through the command history to find the
entry that starts with the letters you type.
[Ctrl]-[Z]
Suspends the current application. Entering
bg after
[Ctrl]-[Z]
causes a program to run in the background.
[Ctrl]-[C]
"Kills" a program. This should be a
last resort. Try stopping a program with other methods
first.
[Ctrl]-[L]
Clears the terminal window.
Table 4-1. Useful shell prompt keystrokes
4.3.4.6. Clearing and Resetting the Terminal
The terminal window begins to look crowded very
quickly. You can always exit from the terminal window and
open a new one, but there is a quicker and easier way to
remove the contents displayed in the terminal.
To clear the terminal, enter the command
clear at the shell prompt. The
clear command clears the terminal,
leaving only a new shell prompt at the top of the window.
You can also clear the screen using the keystroke
[Ctrl]-[L].
Sometimes, you may accidentally open a program file or
some other non-text file in a terminal window. When you
close the file, you could find that the text you are typing
does not match the output on the monitor.
In such cases, enter reset to return
the terminal window to its default values.
