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NOTE: CentOS Enterprise Linux is built from the Red Hat Enterprise Linux source code. Other than logo and name changes CentOS Enterprise Linux is compatible with the equivalent Red Hat version. This document applies equally to both Red Hat and CentOS Enterprise Linux.
4.3. Basics
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:
[[email protected]
username]$
- shell
-
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 (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>.
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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:
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/
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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]
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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.
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Tip |
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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.
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For example, to find a file called "sneaksomething.txt," enter:
The shell lists every file that matches that pattern:
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:
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.
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