13
U S I N G T H E C O M M A N D L I N E
Some might ask, Why would I want to use the command line?
The best answer is that it can save you a lot of time, particu-
larly if you are doing repetitive tasks. For example, sophisticated
editing can be performed in vi or Emacs with many fewer keystrokes
than when performing the same task using a mouse- based GUI editor.
And for repetitive editing, programs such as sed can automate the task for you,
as can awk for report generation.
Before we scare you away, you can be a successful user of your KDE environ-
ment without ever seeing the command line. If you spend most of your time in
an office suite, such as OpenOffice, or running a custom application, you can
skip this chapter confidently and get on with your life with KDE.
If, on the other hand, you need the power of the command line, KDE offers
a lot of advantages:
·
Support for multiple terminal windows on one graphical screen.
·
Selection of various screen and character sizes.
·
Session management, including the ability to send signals.
·
Emulation of different terminal types and support for various character sets.
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Terminal Windows
To enter commands you must open a terminal window. This is a program that
brings up a text- based window and starts a command interpreter (called a shell).
See "The Shell" later in this chapter.
The easiest way to start a terminal window is to click on the terminal icon
on the KDE panel.
Figure 13.1: Terminal Icon
An alternate way of starting a terminal window is by pressing
ALT
-F2 to
bring up a Run Command dialog box and entering the name of a terminal
application, such as Konsole or xterm.
T I P
With the combination of multiple desktops, which you learned about in Chapters 2 and
11, and terminal windows you can have tens or even hundreds of shells running. This
could prove to be very useful if you need to monitor many remote systems.
Konsole
The default terminal application in KDE is Konsole. Another common terminal
application is xterm. By default, the icon will bring up Konsole.
Figure 13.2: Konsole Terminal Window
The Konsole terminal window has menus across the top, a scroll bar on the
right, and icons on the bottom. The first icon, labeled New, makes it possible to
create additional terminals within this single KDE window. Following the New
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269
icon are the icons for the terminals you create. By default you have only one,
but each click on New will create another. You can then click on the terminal
icons to switch between them.
T I P
You can use
CTRL
and the left and right arrow keys to switch between the terminal
windows within Konsole.
The menus allow you to perform various configurations. In the File menu,
you can request various types of terminals or exit Konsole completely.
Figure 13.3: Konsole with Sessions and Signal Menus
The Edit menu allows you to send a signal to the current terminal and to
rename a terminal session. Send Signal allows you to send an asynchronous
event to the program running in your current terminal window. Signals and sig-
nal handling are advanced topics, most of which are beyond the scope of this
book. A simple example, however, is the hangup signal. If you were connected
remotely to a computer system and the connection was dropped (by a call-
waiting interrupt on a phone line, for example), the programs you were running
would be sent a hangup signal. The programs then have the option of deciding
what they want to do with this signal.
You can use the Rename session menu item to change the names of your
terminal windows. By default, they are labeled as Terminal or Shell and are
numbered sequentially. Changing the name can make it a lot easier to remem-
ber which window does what.
T I P
Some distributions include a command-line command, titlebar, or xttitle, that allows you
to change any terminal window title; simply type
titlebar
or
xttitle
followed by a
space and whatever you would like to name the terminal.
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The Settings menu allows you to configure the terminal window.
Figure 13.4: Konsole with Settings Menu
The Schema configuration options let you set a combination of foreground
and background colors, sizes, and fonts. Size allows you to change the number
of columns and lines in the window, and font allows you to change the size of
the characters within the window.
The Show options allow you to turn on and off the various types of ancillary
information around the outside of the window itself. You can turn the bottom
menubar, the top toolbar, and the frame border on and off. Don't panic if you
turn off the toolbar. You can also call up the Settings menu by right- clicking
your mouse while the cursor is within the terminal window.
These settings are only for the current session within Konsole. If you want
them to affect future invocations, use the Save Settings button.
The Help menu gives information about Konsole or, by selecting Contents,
lets you search the online manual.
xterm
A second type of terminal window is xterm. It can be started by entering
xterm
in a Run Command box. You should know about xterm only because there may
be cases where Konsole is not recognized by a remote system. This can happen
if the remote system was, for example, a mainframe running something other
than Linux. Generally, Konsole is a better choice and has more capabilities.
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The Shell
The shell is the program that interprets the commands you enter and invokes
the appropriate programs to perform the requested tasks. In other words, the
shell performs some basic functions that make it easier for you to communicate
with programs. These functions include:
·
Command-line editing: enables you to backspace and edit within the line
you are entering.
·
Job control: allows you to control (start, stop, and terminate) multiple
programs from one shell invocation.
·
Control of input and output files: normally a program receives input from
your keyboard and sends output to your screen. The shell gives you the
ability to change the location of these data files to other programs or files
on your computer system.
·
Command history: lets you call up, edit, and execute previously entered
commands.
Shell Choices
Linux systems come with a choice of shells. Each has its advantages and disad-
vantages. For example, some programmers find the editing features of csh
(called the C shell) to be very appealing. Some see the compact size of ash as an
important consideration, and for some, the standardized nature of ksh (called
the Korn shell) eases their transition between Linux systems and other UNIX
platforms.
The most popular shell among Linux users is Bash, which stands for
Bourne Again Shell. The name comes from UNIX history. The first shell that
offered its own programming language--that is, the ability to make control deci-
sions within a saved set of commands based on conditions that could be tested
for--was written by Steven Bourne of, at the time, AT&T. Bash took these capa-
bilities and has expanded them over the years.
Learning all there is to know about a shell is easily the subject for a whole
book. For the purposes of this chapter, we will use Bash as our shell. Most of the
information presented, however, applies to virtually all available shells.
T I P
The shell tcsh, an enhanced clone of csh from UC Berkeley, is available for those who are
familiar with csh and want to use its advanced history and scripting features.
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Shell Prompt
The character string that the shell displays when it is ready to accept your com-
mand input is called the shell prompt. This prompt can be configured to be as
simple as a single character or it can be generated dynamically each time.
The default shell prompt is typically a combination of useful information
about your current environment. As I write this, my prompt reads
fyl@nicaragua:~/KDE/Command >
This string offers three pieces of dynamic information:
·
fyl is my user name.
·
nicaragua is the name of this computer.
·
~/KDE/Command is the name of my current directory. Note that
~
is
shorthand for my home directory.
Occasionally you may see a single
>
character when you are expecting either
your usual shell prompt or the prompt from an applications program. The
>
character is the default secondary shell prompt and means the shell is expecting
more input from you. Most commonly, this situation occurs when you have
typed one quotation mark and the shell is expecting you to type a matching quo-
tation mark. The quickest way out of this problem is to press
CTRL
-C. This will
end the execution attempt and return you to the shell prompt.
T I P
The shell prompt can be changed by setting the
PS1
environment variable.
Control Characters
The shell interprets various control characters allowing you to edit the lines you
enter and perform other local tasks. Signify that you have completed entering a
command line by pressing the
ENTER
key. Prior to pressing
ENTER
, the two
basic editing keys are
BACKSPACE
and
CTRL
-U. Pressing the
BACKSPACE
key
erases the most recently entered character. Entering
CTRL
-U deletes all the char-
acters you have entered on the current line, allowing you to start over.
Once you have pressed the
ENTER
key, control is turned over to the applica-
tion. For example, if you type in
ls -l
and press
ENTER
, the shell turns over
control to the ls program. Thus, the
BACKSPACE
and line- delete character
sequences can no longer be interpreted by the shell. If you want to terminate
the execution of a program, you can press
CTRL
-C. This will send an interrupt
to the program.
If you want to suspend the execution of a program, press
CTRL
-Z. This sus-
pends the program and returns you to the shell prompt. Entering the
fg
com-
mand (followed by
ENTER
) will resume the suspended program.
You can also enter
bg
(followed by
ENTER
) to resume the suspended pro-
gram in the background. This means the program will be restarted, but you will
receive a shell prompt. This is useful when you want to run a time- consuming
report program and redirect both its input and output streams to files.
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273
Finally, you can use
CTRL
-S to pause the output of a running command.
CTRL
-Q resumes the output. Note that program output may scroll by too fast to
make effective use of these control sequences. Generally, you will be better off
using a pager program such as more or less, both of which allow you to view the
output a screenful at a time. For example, if the output of the ls -l command is
more than a screenful, you can enter
ls -l | more
.
T I P
Some programs require that you send them an End-of-File (EOF) character to signify that
you have completed your input. To send this character, press
CTRL
-D.
Pipes and Redirection
By default, the shell establishes three connections with the terminal window.
These connections are called standard input, standard output, and standard
error. In UNIX documentation you will commonly see them abbreviated as
stdin, stdout, and stderr.
The stdin connection is connected to the keyboard. Programs read your
keyboard input by reading from stdin. The stdout connection is connected to
your screen and is the way that programs send their output back to you. The
stderr connection is also connected to your screen and is the way programs
send output about problems and abnormalities.
There are two output streams (stdout and stderr) so that it is possible to send
error information to your screen, even though the normal output of the com-
mand is being sent somewhere else (to a file or possibly to another program).
The tools to connect these streams to other places are called pipes and redi-
rection. A pipe allows you to connect the output of one program to the input of
another program, that is, connect stdout of one program to stdin of another
program.
For example, say you have a program called bigreport that produces a great
deal of output, and you want to view it a screenful at a time. We previously dis-
cussed the more program that allows viewing by the screenful.
Thus, you want to run the program bigreport and send its output (stdout)
to the input (stdin) of more. The vertical bar character (
|
) is used to represent
the pipe, so the command to perform this task looks like this:
bigreport | more
The preceding example shows spaces around the pipe character. These are
allowed but not necessary. The following command would work also:
bigreport|more
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Of course, you probably don't have a program called bigreport available to
you. So, to practice, enter a command that will produce a lot of output. Try the
following command:
ls /bin | more
You should see output like this:
Figure 13.5: Stdout Piped to more
Note that the word More appears at the bottom of the screen in reverse
video. This tells you that you are still in the more program rather than at the
shell prompt. Pressing the space bar will display the next screenful. Pressing
Q
will terminate more and return you to the shell prompt.
The more program and its more capable cousin less are very handy. To find
out about additional capabilities, enter
?
at the more prompt.
With pipes we connect two programs together. With redirection we can tell
a program to read either input from a file or write output to a file. To redirect
stdout to a new file, use the > character. Going back to the previous example,
we could get the ls command to write its output to the file ls.output by entering
the following command:
ls /bin > ls.output
As with pipes, the spaces around the redirection character are optional.
Now that we have this information in a file, the next trick is actually reading
it. All we need to do is use the more command again and redirect its input
(stdin) from the file we created. The input redirection operator is <, so we enter:
more < ls.output
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The operation of more is virtually identical to the way it was before--that is,
the shell handled piping, and here it handles redirection. The more program
itself knows nothing of the actual source of the data. Contrast this with
more
ls.output
where more itself opens the file.
Besides these two forms of redirection, there are others. The following table
lists other forms of redirection.
Character
What It Does
>
Redirects output (stdout) of the command to the specified file.
>>
Similar to > but will append to a file if it already exists.
2>
Redirects the error output (stderr) of the command to the specified file.
<
Redirects the input of the command (stdin) from the specified file.
|
Joins two commands. Output (stdout) of the command on the left is sent to the
input (stdin) of the command on the right.
Command-Line Format
The previous section introduced some basic command-line formats. Now it's
time to see how command lines really work.
·
Command lines are made up of words.
·
Words are separated by whitespace, which is defined as one or more spaces
or tabs, or a combination thereof.
·
The first word of the line is the command itself. This is the program you
are asking the shell to run. It could be a binary program, an alias, or a
script to be interpreted by the shell, or one of the many other interpreted
languages, such as awk or Python.
·
The remaining words on the command line are called arguments.
·
A special argument, called an option, begins with a hyphen (-). Long
options, a special kind of option, begin with two hyphens (- -). Generally,
options are used to modify the way a command works.
·
Multiple commands can be entered on one line by separating them with a
semicolon (;).
·
You can tell the shell to run a command in the background and thus not tie
up your terminal screen by following it with an ampersand (&).
Examples
Display a long (detailed) listing of all the files in the directory /bin paged
through more:
ls -l /bin | more
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Print the date on the terminal, and then write a long list of all files in the direc-
tory /bin to the file /tmp/testme. Note that the redirection applies only to the
ls command, not to the date command:
date; ls -l /bin > /tmp/testme
Command-Line Editing and Shell History
The shell keeps a record of your recent commands and allows you to retrieve
them, edit them, and re-execute them. Depending on the mode in which your
shell is set, the history access and editing will work much like either the Emacs
editor or the vi editor. That is, all the text-editing features of either vi or Emacs
that make sense to use on a single line are available within the shell. To select vi
editor mode, enter
set -o vi
at your shell prompt. To select Emacs editing
mode, enter
set -o emacs
at your shell prompt.
You can make a permanent change to your editing mode by adding the
appropriate set command to the end of your .bashrc file in your home directory.
Note that the change will not take effect until your next login.
If you are not familiar with either Emacs or vi, you will most likely do better
in the Emacs mode. In this mode, the up and down arrow keys can be used to
move through the history list, and diligent use of the left and right arrows, as
well as the
BACKSPACE
and
DELETE
keys, allow you to edit the command.
Pressing
ENTER
executes the resulting command.
If you are in vi mode, press
ESC
(
ESCAPE
), and then any vi command that
works on a single line can be used. Use the up and down arrows, or J and K, to
move within the lines of the history list.
Command Completion
Besides providing command history, Bash will guess the name you mean if you
are entering the name of a file. Once you have entered enough of a filename to
make it unique, press
TAB
. If it is in fact unique, Bash will fill in the rest of the
name. If it is not, nothing happens. At that point you can, however, press
TAB
again, and the shell will list all the possible matches and wait for you to continue
entering characters.
Shell Expansions
A special shorthand called shell expansions is available to allow you to select
multiple filenames on command lines. This expansion capability is one of the
most powerful capabilities of the shell that is useful on the command line.
Properly used, it can save you many keystrokes and much time.
?
Matches any single character.
*
Matches any number (zero or more) of any characters.
~
Shorthand for the name of a home directory. ~name is replaced with the
pathname of the home directory of user name. If name is omitted, ~ is
replaced by the pathname of your home directory.
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[ ]
Encloses a set of characters for a single- character match. For example,
[aQz] matches a, Q, or z. A hyphen (-) can be used to specify a range. For
example, [am-pZ] matches a, m, n, o, p, or Z. A leading ^ negates the
match string. For example, [^a-z] matches anything except a lowercase
letter.
$
Introduces a shell variable. The variable is replaced with its value as saved
by the shell. For example, $HOME is replaced with the pathname of your
home directory.
\
Turns off the special meaning of the following character. For example, \?
matches a ? instead of any single character, and \ [ matches a [ instead of
introducing a set of matched characters.
T I P
Don't confuse shell expansions with regular expressions. They have some similarities, but
they have different capabilities available in different places.
Quoting
Besides the use of backslash (\ ) to protect a character from shell expansion or
interpretation, there are other quoting characters. Of course, a backslash can be
used to turn off their special meaning.
"
Disables the special meaning of all characters except $ in the enclosed
string. This is commonly used when you want to treat multiple words as a
single argument.
'
Disables the special meaning of all characters in the enclosed string.
`
Executes the enclosed string as a command and replaces it with the output
of the executed command.
Examples
The echo command displays its arguments back to the screen. In this example,
echo displays the string entered, but $HOME will be replaced with the value of
the HOME environment variable, which is the pathname of your home directory.
echo "My home directory is $HOME"
In this example, $HOME is displayed literally instead of being replaced with
the value of the environment variable.
echo 'My home directory is $HOME'
The date command returns the current date and time as a 26 - character
string. In this example, the backquoted reference is replaced by the result of
executing the date command before the line is printed:
echo "The current time is `date`"
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Conventions
The remainder of this chapter uses certain conventions in its examples as a
metalanguage to help you understand the commands.
Monospaced font
Monospaced font is used to indicate examples.
[ ]
Square brackets are used to show optional information that
may be included within a command and shouldn't actually be
typed. Don't confuse this use with the character- matching
operation.
Boldface
Boldface identifies the proper syntax for typing particular
commands-- in other words, the way the commands should be
set up and typed.
italic
Parameters to be replaced are shown in italic. The word used
is intended to be descriptive. The most common example is
file
to represent a filename. Plurals (for example, files) are
used to indicate where multiple instances are permitted.
option
The option keyword can be replaced with any of the available
command options. Using options indicates that more than
one may be used.
File Hierarchy
You learned about files back in Chapter 3, where you could move around the
file hierarchy by pointing and clicking. When using the shell, you need to know
how to move around the hierarchy because you don't get any graphical clues.
T I P
Although Linux allows any character (including control characters) in filenames, it is
best to stick to letters, numbers, and some safe, special characters, such as _ , ., and ,.
Naming the Pieces
By convention, the slash character ( /) is used to separate the levels in the file
hierarchy. The top level of the whole file hierarchy, usually referred to as the
root, is identified by /, and all other files are located relative to it.
The location of a file within the hierarchy is called its pathname. Pathnames
can be specified relative to your current directory--that is, where you are now--
or relative to the root directory. A pathname starting with / signifies that it is
relative to the root and is called a full pathname. If a pathname does not start
with a /, it is relative to your current directory and is called a relative pathname.
Each directory in the hierarchy has two special files in it named . (dot) and
.. (dot dot): . is a synonym for the current directory, and .. is a synonym for the
parent of the current directory. They are always there, so you can use them as
shorthand to help you move around the hierarchy.
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Examples
My home directory is called fyl and lives under the directory home, which is off
the root of the hierarchy. The full pathname of my home directory is
/home/fyl
.
If I am in my home directory, as described in the preceding example, and
your home directory is called jill, also located under home, then
/home/jill
is
the full pathname to your home directory.
../jill
is the relative pathname to
your home directory.
Moving About
To move about the hierarchy and know where you are, there are two commands
you need to know: cd and pwd.
The simplest, pwd, tells you the name of your current directory. It takes no
arguments.
You can use cd in three ways. First, just entering
cd
with no arguments will
move you to your home directory. You can, of course, check the results by enter-
ing
pwd
.
If you know the location of a directory relative to the root, you can follow
cd with the full pathname of the desired directory. For example, many of the
programs you will use from the command line are located in the directory
/usr/bin. Entering
cd /usr/bin
will change /usr/bin to your current directory.
Similarly, entering
cd /var/spool
will make the system spool directory your cur-
rent directory.
File Manipulation Utilities
Now that you can move around the filesystem and know what commands look
like, it's time to try a few.
ls--Listing Directory Contents
You have already seen this command in earlier examples. It produces a list of
names and, if so requested, other information, about the specified files. The
most basic case is
ls
, which lists all files in your current directory whose name
does not start with a dot (.).
The general form of the command is: ls [options] [files]
Common Options
-a
Includes files whose names start with a dot. (Linux treats any file whose
name starts with a dot as hidden, meaning that the names are not displayed
by default.)
-l
Includes file size, ownership, and permission information.
-R
Recursive-- includes subdirectories.
-t
Sorts by last modification time.
-u
Sorts by time of last access instead of last modify.
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Chapter 13
Examples
Lists all files in your current directory sorted by last modify time
ls -lt
Lists files in /usr/bin whose name starts with gif; make it a long list:
ls -l /usr/bin/gif*
cp--Copy Files
The cp command copies one or more files. In the most basic form, you specify a
source filename and a destination filename:
cp
[options] source destination
If the final argument is a directory, then all specified files are copied to that
directory under their same name:
cp
[options] files directory
Common Options
-a
Archives by preserving the file attributes and including subdirectories
but not following symbolic links.
--help
Displays a help message.
-p
If possible, preserves file attributes such as owner, modify time, and
access permissions.
-R
Recursively includes subdirectories.
-v
Explains what is being done.
Examples
Make a copy of harry in your current directory. The copy is given the new name
chest in the directory tmp.
cp harry /tmp/chest
Copy all files whose names start with a or b in /tmp to your current directory,
preserving file attributes:
cp -p /tmp/[ab]* .
Copy all files in the directory /home/bill/secret and any subdirectories into
/tmp/savebill.
cp -R /home/bill/secret/* /tmp/savebill
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mv--Rename or Move Files
The mv command moves or renames files. You need not concern yourself with
the distinction, as mv does what is necessary. Much like the cp command, there
are two formats.
In the most basic form, you just specify a source filename and a destination
filename:
mv
[options] source destination
Common Options
--help
Displays a help message.
-i
Prompts before overwriting any files (interactive).
-u
Moves only older or brand-new nondirectories.
-v
Explains what is being done.
T I P
Some Linux distributions include a shell alias that sets the
-i
option by default. Type
alias
at the shell prompt to see your current list of aliases.
Examples
Move the file harry in your current directory to /tmp, and name the new file
chest in the directory tmp:
mv harry /tmp/chest
Move all files whose names start with a or b in /tmp to your current directory,
prompting before overwriting any files:
mv -i /tmp/[ab]* .
rm--Remove Files
The rm command deletes one or more files. It can also be used to force the
removal of directories.
rm
[options] files
Common Options
-f
Forces removal without prompting.
--help
Displays a help message.
-i
Prompts before any removal (interactive).
-r
Recusively removes the contents of directories.
-R
Recursively includes subdirectories.
-v
Explains what is being done.
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Examples
Remove the file harry from your current directory:
rm harry
Remove all files whose names start with a or b in /tmp:
rm /tmp/[ab]*
Remove the directory called Garbage that is in your current directory, including
all of its files and subdirectories. In this case, the -R option can be either upper-
or lowercase:
rm -r Garbage
T I P
Sometimes you need to remove a file whose name starts with a hyphen. To do this,
preceed the name with ./, which doesn't change the meaning of the file location, or use
the full pathname.
mkdir--Create a Directory
The mkdir command creates directories. This is the same as making a new
folder with Konqueror:
mkdir
[options] dirnames
Common Options
--help
Displays a help message.
-
m mode
Sets directory permissions to mode masked by your umask value. Uses
the symbolic values (e.g.,
a=rwx
) for all permissions for everyone or
octal numeric values (
777
). See the "How Permissions Work" section
in this chapter.
-p
Creates any missing parent directories for each argument. Mode is set
to umask modified by u+wx. Arguments corresponding to existing
directories are ignored.
-v
Explains what is being done.
Examples
Make a directory named Harry in your current directory and a directory named
Chest in /tmp:
mkdir Harry /tmp/Chest
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Make a directory named Harry in your current directory and force the directory
permissions to everything for user, read, and execute (search) for group, and
nothing for others. (See the "How Permissions Work" section in this chapter.)
mkdir -m u=rwx,g=rx,o-rwx Harry
Repeat the preceding directions, only using a numeric mode specification:
mkdir -m 750 Harry
Create the directory Chest as a subdirectory of the directory Harry in your cur-
rent directory. Also, create Harry if it doesn't exist:
mkdir -p Harry/Chest
rmdir--Remove a Directory
The rmdir command is the opposite of the mkdir command. It removes empty
directories.
rmdir
[options] directories
Common Options
--help
Displays a help message.
-p
Removes a directory and then tries to remove each component of the
pathname.
-v
Explains what is being done.
Examples
Remove the directory named Harry in your current directory and a directory
named Chest in /tmp:
rmdir Harry /tmp/Chest
Remove the directory Chest as a subdirectory of the directory Harry in your cur-
rent directory. Then, if Harry is empty, delete it as well:
rmdir -p Harry/Chest
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Text Editors
This section introduces the most common editors used with Linux on the com-
mand line, but an entire book could be written on each editor. In the case of vi
and Emacs, that has already happened.
Don't confuse these text editors with word processors. These are programs
designed to let you enter and edit text. Their text-formatting capabilities are lim-
ited to setting line-length limits. Traditionally, these editors were used in conjunc-
tion with text-processing programs, such as troff, to produce typeset documents.
Emacs
Emacs is a project of the Free Software Foundation. Many people see it as a
work environment rather than only an editor. It includes its own built- in pro-
gramming language, so you can customize its operation.
Joe
Less popular than Emacs or vi, Joe has a lot to offer the beginner. Besides its
easy-to-use default mode with on-screen menu, it can emulate the basics of
Emacs and vi.
vi
vi is the most popular, by far, of the available editors. There are many flavors of
vi, including nvi, vim, vile, elvis, and stevie. All share what is called a moded sys-
tem, where you enter text in one mode and edit in another. The users and devel-
opers of vi pride themselves on minimizing the number of keystrokes required
to complete a task.
lpr--Send File to a Printer
The lpr command can be used to spool a file for printing. This is similar to the
menu choices in GUI applications and the drag-and- drop capability in KDE.
lpr
[options] [files]
If no files are specified, then lpr reads from standard input.
Common Options
-
m [user]
Sends email to user (you as default) when the job has printed.
-P ptr
Sends the file to the printer named ptr instead of your default
($PRINTER).
-r
Removes the file after printing.
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File Attributes and Permissions
Associated with each file and directory are two ownership fields and a set of per-
missions. Ownership and permissions are used to restrict access to files.
How Permissions Work
Each file has an owner, which is the user ID associated with the file. It also
belongs to a group, which by default is set to the default group of the user who
created the file. For example, your system might be set up so that everyone in a
department is in the same group. To share a file with a member of another
group, you could change the group owner to the owner of the other group.
There are a total of nine basic file permissions, divided into three groups.
Those groups are associated with the file owner, the group the file belongs in,
and everyone else.
Within those three groups, permission to read the file, write to the file, and
use the file as an executable program are available. In addition, there are similar
permissions associated with each directory.
Note that when you create a file, you are the owner. A normal user cannot
change that ownership. If such a change is necessary, your system administrator
must log on as the root user to make the change.
You can think of the permissions as a three- digit octal number. The left-
most digit represents the user's permissions. The next one represents the group's
permissions, and the last digit represents the permissions of everyone else.
Within each digit, a 4 represents file- read permission, a 2 represents file-
write permission, and a 1 represents file-execute permission or the ability to
search for directories. It is only necessary to add up these values for each digit
in order to set the permissions. For example, 754 represents read, write, and
execute permission for the file owner (7=4+2+1), read and execute permission
for the group (5=4+1), and only read permission for everyone else.
As an alternative, the permissions can be specified symbolically using a com-
bination of letters (u, g, and o to represent user, group, and other, respectively,
and r, w, and x to represent read, write, and execute permission, respectively).
The letter a is used to represent the combination of u, g, and o.
These letters are combined using punctuation to specify the permissions.
Using an equal sign (=) sets the permissions to the value specified, a minus sign
(-) indicates permissions to be removed, and a plus sign (+) represents permis-
sions to be added.
If more than one of the ownership/punctuation/permissions strings is
required (for example, if you want to add group write and remove read for
other), you combine the sets with a comma (,). If all this sounds harder than the
numeric specification, it is. But it is there if you wish to use it.
To use the example 754 from the preceding numeric explanation, you could
specify these same permissions as
u=rwx,g=rx,o=r
.
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chmod--Change File Permissions
The chmod command is used to change file (and directory) permissions. Specify
the desired permissions symbolically or numerically in octal:
chmod
[options] mode files
Common Options
-c
Like -v but only reports if a change is made.
-f
Suppresses error messages
--help
Displays a help message.
-R
Changes file and directory permissions recursively.
-v
Explains what is being done.
Examples
Change the permissions on the file harry in your current directory to read and
write for the owner, read for the group, and nothing for everyone else:
chmod 640 harry
Change the permissions on the directory Harry and anything it contains to read
and execute for owner only:
chmod -R 500 Harry
T I P
There are three additional permission bits for each file and directory that may be of
interest to the advanced user. Type
man chmod
for details.
chgrp--Change File Group
The chgrp command is used to change the group to which a file (or directory)
belongs:
chgrp
[options] group files
Common Options
-c
Like -v but only reports if a change is made.
--help
Displays a help message.
-R
Changes file and directory permissions recursively.
-v
Explains what is being done.
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Examples
Change the group on harry in your current directory to buenas:
chgrp buenas harry
Change the group ownership on the directory Harry and anything it contains to
buenas:
chgrp -R buenas Harry
T I P
The chown command can change both the owner and group of a file, but only the super-
user is permitted to change file ownership.
Information
file--Guess File Type
Sometimes you need an educated guess as to what is contained in a file. The file
command will do this for you. It uses a combination of innate intelligence and a
magic number file
to come up with a reasonable guess.
file
files
Examples
Guess the type of the file harry in your current directory:
file harry
Guess the type of all files in /tmp:
file /tmp/*
man--Online Documentation
If you are looking for the description of a command or a command that works
with something in particular, the man command can help.
Examples
To display documentation on a specific command, use:
man
[section] command
To search for all commands related to a keyword, use:
man -k
keyword
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Examples
Display information about the more command, defaulting the section number:
man more
Display information about the more command, specifying the section number.
Note that all user commands are in section 1, system calls in section 2, library
functions in section 3, and so on. You will almost always want section 1:
man 1 more
List a synopsis of all commands related to the keyword permissions:
man -k permissions
Sorting and Searching
Linux comes with a whole host of utilities for finding things and manipulating
data. What you see here is a very small subset. For serious data manipulation
and reporting, look into awk, Perl, and Python.
Using pipes to string these commands together and connect them to pager
commands, such as more, is very common. This is where the toolkit aspect of
Linux comes into play.
grep--Searching for Strings in Files
The grep command is used to locate a data pattern within a file. It gets its
strange name from the command you would need to type into the original
UNIX editor in order to perform this task.
In its most basic form, grep only displays lines that match the specified
character string. However, to take full advantage of grep you need to under-
stand regular expressions, a special pattern-specification language.
grep
[options] string files
Common Options
-c
Counts the number of matched lines rather than actually printing
them out.
-i
Ignores the case of the letters in the match string.
-l
Prefixes each output line with the line number within the input file.
-v
Inverts match, only displaying lines that do not match.
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Examples
Display all lines that contain the string
cool
in the file harry in your current
directory:
grep cool harry
Display all the lines that do not contain the string
real cool
in the file harry in
your current directory and page the output through more:
grep -v "real cool" harry | more
Display all the lines in a long list of the files in your current directory that con-
tain the string
Oct
followed by a space. Page the output through more:
ls -l | grep "Oct " | more
find--Locating a File
The find command has a very complicated syntax and a lot of options but,
because of this, is extremely powerful:
find
[paths] expression
The paths portion tells find where to start searching. It then continues
recursively under the specified starting point or points.
The expression part is a series of conditions used to specify the file you want
to find, plus, if you wish, the action to be taken to each match. The default
action is simply to print the name of the file. By default, all the specified condi-
tions must be met. They are matched in order from left to right. For "or" rather
than "and" conditions, use an
-o
connecting expression.
Common Options
-name filename
The filename must match. Note that the shell wildcard charac-
ters can be used in the match, but the match string must be
quoted to prevent the shell, rather than find, from doing the
expansion.
-group group
File belongs to group group.
-name filename
Like - iname but case- insensitive.
-newer file
Desired file was modified more recently than file.
-v
Explains what is being done.
Examples
Look for the file harry, starting in your current directory:
find . -name harry
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Repeat the preceding example, but ignore the case and match any file whose
name starts with harry:
find . -iname "harry*"
Look for any file whose name matches harry or chest in your current directory
and the home directory of bill:
find . ~bill -name harry -o -name chest
locate--Locating a File
Although find will always find a file, it can take a long time. The locate com-
mand uses a database that is created regularly (usually daily) to try to find a
matching file.
It is sometimes desirable to combine locate with grep to narrow down
the match.
locate
[options] pattern
Common Options
-i
Ignores the case of the match.
--help
Displays a help message.
Examples
Locate any file whose pathname contains the string
picture
:
locate picture
Locate any file whose pathname contains both
picture
and
Secret
:
locate picture | grep Secret
System-Related Commands
Here is just a brief introduction to some system- related commands. While you
saw much of this functionality with the KDE-specific graphical commands, there
may be an advantage to using the command line with some of these.
Think about what you have learned about grep, for example. You could use
grep to select specific lines from the output of the ps command.
ps--Process Status
Each active program in Linux is called a process. When you enter a command,
you start a process. If you enter multiple commands, connected by pipes, you
are starting two or more processes.
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In addition, the system has many daemons. These are programs that take
care of the system and perform background processes for you. A good example
is the print spooler--the program that monitors the print queues and interacts
with the printer.
By default, ps shows only processes that are owned by you.
ps
[options]
Common Options
a
Shows all processes associated with a terminal.
T
Shows all processes associated with this terminal.
l
Shows a long display format.
f
Shows the ASCII art process hierarchy.
T I P
There is an amazing array of options available to format and sort the output. See the
man page for details.
Examples
Display all your processes:
ps
Display all processes associated with a terminal in the long format:
ps al
Display a process hierarchy tree for all terminal- related processes.
ps af
top--Ongoing Process Status
If you want to see an ongoing display of current processes, top is the answer.
Processes are sorted with the most active at the top. There are many options
and interactive commands, but 99 percent of all runs are just top alone.
Terminate top by pressing Q.
top
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df--Display Free Disk Space
This command displays the location, mount point, size, amount used, and per-
centage used of all mounted filesystems of nonzero size. Many options exist, but
the default is almost always what you need.
df
du--Display Disk Space Usage
This command estimates file usage recursing downward from the specified loca-
tion or locations.
du
[options] files
Common Options
-a
Writes counts for all files, not just directories.
-h
Prints sizes in human- readable format.
--help
Displays a help message.
-s
Prints summaries for each argument.
Examples
Show usage of all files under your current directory:
du
Show summary usage information for /usr/bin and /bin:
du -s /usr/bin /bin
Conclusion
This chapter has only scratched the surface as far as the capabilities of the com-
mand line. Hundreds of commands are available at the shell prompt and a lot
more capabilities are within the command line of the shell. In addition, the shell
offers a built- in programming language that makes it possible to make decisions
and execute commands selectively.
Complete books have been written on shell programming. If you would like
to know more, we suggest you take a trip to a good technical book store and
find a book that is a good fit for you.
Also, SSC offers two reference cards, one on Bash and the other on the
Korn shell as part of its Linux Library series. You can check them out, as well as
other SSC books and reference cards at http://store.linuxjournal.com.
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