MiniLesson: An introduction to Linux in ten commands

[AMAZING POWERS OF OBSERVATION]

MiniLesson: An introduction to Linux in ten commands
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This tutorial is the first in a
series of introductory Linux
lessons. This first article will
cover navigating around a Linux
filesystem along with a brief
passage -- with examples -- on using
ten of the most essential GNU/Linux
commands. 

You should have access to a Linux
system in order to perform the
example commands as we progress
through the tutorial. If you don't
have a dedicated Linux box, you can
use a Live Linux CD-ROM-based
distribution such as Knoppix.
Knoppix will let you run Linux
directly from the CD without
modifying anything on your hard
drive.

Once you're logged in to a Linux
system, open a terminal session.
Each of the commands covered here
will be typed directly into a
command line terminal window. Under
Red Hat Linux, terminal is found in
the 'system tools' section of the
menu. (Your system may,
alternatively, have a terminal
program called 'konsole', 'xterm',
or 'shell'. Look around your system
for a menu with 'tools' or
'utilities' in the name if
necessary.)

pwd
The first command we will use is
'pwd' -- which stands for 'print
working directory'. The pwd command
shows you your current position
within the Linux filesystem. The
position is known as your 'current
working directory'. Type pwd now.
The example below shows my command
prompt and the pwd command followed
by the output from the pwd command:

 [rayy at barton0 rayy]$ pwd
 /home/rayy
 [rayy at barton0 rayy]$

>From the output (/home/rayy) we can
tell that I am in my 'home
directory' -- the directory where I
keep my personal files and the
directory where I always start out
in a new session.

ls
The ls command lets you list files.
For example, here is the (shortened)
output of an ls command on my
system:

 [rayy at barton0 code]$ ls
 artdir    countdir   machine
 tsardir   sortdir
 [rayy at barton0 code]$

Alternatively, you can get a 'long
listing' that shows file sizes,
timestamp, ownership, and
permissions as follows:

 [rayy at barton0 code]$ ls -l
 drwxr-xr-x 2 rayy rayy 4096 Feb 3 2002 artdir
 drwxr-xr-x 2 rayy rayy 4096 Feb 3 2002 countdir
 drwxr-xr-x 2 rayy rayy 4096 Feb 3 2002 machine
 drwxr-xr-x 2 rayy rayy 4096 Feb 3 2002 sortdir
 drwxr-xr-x 2 rayy rayy 4096 Feb 3 2002 tsardir
 [rayy at barton0 code]$

You can also supply a target
directory to the ls command. For
example, to view the contents of the
/tmp directory, I enter the
following:

 [rayy at barton0 code]$ ls /tmp
 flp     kde-rayy   mcop-rayy
 [rayy at barton0 code]$

For more information on the ls
command you can reference the manual
page for ls with the following
command:

 [rayy at barton0 code]$ man ls

cd
This next command, 'cd', lets you
change your current working
directory. for example, you can
change your current working
directory to /usr/bin by entering
the following command:

 [rayy at barton0 rayy]$ cd /usr/bin
 [rayy at barton0 bin]$

Note that after I entered the cd
command, my command prompt changed
to reflect the change in the last
node of my current working
directory. Your command prompt may
not be configured to do that.

Change your current working
directory to /usr/bin now and enter
the ls command.

 [rayy at barton0 code]$ cd /usr/bin
 [rayy at barton0 bin]$ ls
 addr2line         mcheck
 addr2name.awk     mcomp
 addresses         mcookie
 .
 .
 .
 [rayy at barton0 bin]$

The preceding is a partial listing.
There are many, many files in the
/usr/bin directory on most Linux
systems.

file
If you have a background in Windows
or are familiar with DOS, you are
used to file extensions that signify
the file type. Linux (and Unix) have
no such requirement. That is, an
executable program can be named
anything. Therefore, a handy command
is supplied with Linux named 'file'.
For example, I have a file named
'sample.c' in my code directory. I
can learn a bit about that file by
entering the following command: 

 [rayy at barton0 code]$ file sample.c
 sample.c: C++ program text
 [rayy at barton0 code]$

Alternatively, I can use the '*'
wildcard -- which represents all
filenames -- to examine all of my
code files at once. The following is
a shortened example:

 [rayy at barton0 code]$ file *
 bbsdir:     directory
 code.tar:   GNU tar archive
 genart.c:   ASCII C program text
 sample.c:   C++ program text
 xor:        ELF 32-bit LSB executable
 [rayy at barton0 code]$

The file command can be very useful
to avoid minor annoyances -- such as
when using one of the following
three commands.

cat
The cat command is useful for
concatenating multiple files -- or
just for dumping a single text file
to the screen. Before you use the
cat command to dump a file to the
screen, use the file command to make
sure it's some variety of text file
such as ascii text, commands/text, C
source code, html/text, etc. The
following is a shortened example of
using file and cat to identify and
dump a text file:

 [rayy at barton0 code]$ file xor.c
 xor.c: ASCII C program text
 [rayy at barton0 code]$ cat xor.c
 main()
 {
     int x;
     unsigned char buff[128],
 .
 .
 .   
 [rayy at barton0 code]$

more
The more command is useful when a
text file is larger than a single
screen. The following is a shortened
example of using more to view a
large C program:

 [rayy at barton0 code]$ more xor.c
 main()
 {
     int x;
     unsigned char buff[128],
 .
 .
 .   
     printf("n%sn",buff1);
     printf("n%sn",buff2);
 }
 --More--(29%)

Note the '--More--(29%)' at the end
of the screen. That means that 29%
of the file is above that line,
implying that another 71% of the
file is below. Press the space bar
to page through the file, a
screenful at a time. Press the b key
to back up. If you finish looking
before reaching the end of the file,
press the q key to quit.

grep
The grep command, short for 'get
regular expression and print', is
useful for finding occurances of a
particular string in a text file. To
find the 'printf' statements in the
example C program above, enter the
following command:

 [rayy at barton0 code]$ grep printf xor.c
 printf("n%sn",buff1);
 printf("n%sn",buff2);
 [rayy at barton0 code]$

The grep command has far more
capability than I describe here and,
as usual, enter 

 [rayy at barton0 code]$ man grep

for more information.

cp
The cp command will let you copy
files. Unlike the commands used
above, this one includes a hazard;
if you copy filename1 to filename2
and filename2 already exists, you
will destroy the original filename2
file. Use cp with caution!

To make a duplicate copy of my xor.c
file I could enter the following
command:

 [rayy at barton0 code]$ cp xor.c xor.c.bak
 [rayy at barton0 code]$ ls xor.c*
 xor.c  xor.c.bak
 [rayy at barton0 code]$

Note that the cp command returned no
output -- I had to enter an ls
command to see the results of the
copy. [By adding the * wildcard to
the original filename, I asked for a
listing of all files that started
with xor.c -- including those with
no additional characters in the
name.]

rm
The rm command is used for removing
files. To remove the duplicate file
I created in the cp command example,
I would enter the following:

 [rayy at barton0 code]$ rm xor.c.bak
 [rayy at barton0 code]$ ls xor.c*
 xor.c
 [rayy at barton0 code]$

Again, note the absense of any
feedback from the rm command. I had
to enter an ls command to verify
that the xor.c.bak file had really
been removed.

As with other commands, rm can
remove multiple files at once when
used with wildcards or with the -r
(recursive) option. See the man page
for more information on rm.

mkdir/rmdir
Ok, this is really two commands, but
they are complementary. Use the
mkdir command to make a new
directory and use the rmdir command
to remove an empty directory. For
example:

 [rayy at barton0 tmp]$ mkdir testdir
 [rayy at barton0 tmp]$ ls
 testdir
 [rayy at barton0 tmp]$ rmdir testdir
 [rayy at barton0 tmp]$ ls
 [rayy at barton0 tmp]$

In the preceding series of commands
I first created a new directory
named 'testdir'. I then used the ls
command to verify its presence.
Then, I removed 'testdir' and
verified that it was gone by using
ls again
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