Linux the command line interpreter is known as the shell. Whatever
you type at the command line is understood and interpreted by
a program and then that program gives you an output after executing
your command. This program that understands what you type is
called the shell.
Linux comes with quite a few shells such as Bourne Shell, Bourne
Again Shell, C Shell, Korn Shell, etc. The default shell for
Redhat Linux is ' bash ' which is very popular since being the
default, most users start by learning bash. I shall talk about
the bash shell only in this article.
Windows users would be familiar with a program called command.com
which had to be present for the OS to boot. Command.com is the
Windows equivalent of the Linux shell.
Typing the following at the shell
$ echo $SHELL
would give you the name of the current shell you are using.
It would most probably be the bash shell in case you are a new
user and have been assigned the default shell.
The bash shell is actually a program that is located at /bin/bash
and is executed by Linux the moment a user successfully logs
in after entering his user-pass. Once this shell starts, it
takes over control and accepts all further user commands. The
bash shell presents a $ prompt by default (for normal user accounts).
You can change this prompt to whatever you like but leaving
it at the default is best. Other shells present different prompts.
Changing the prompt is explained below.
All the programs that run under Linux are called as processes.
Processes run continuously in Linux and you can kill or suspend
different processes using various commands. When you start a
program a new process is created. This process runs within what
is called an environment. This particular environment would
be having some characteristics which the program/process may
interact with. Every program runs in its own environment. You
can set parameters in this environment so that the running program
can find desired values when it runs.
Setting a particular parameter is as simple as typing VARIABLE=value
. This would set a parameter by the name VARIABLE with
the value that you provide.
To see a list of the environment variables that are already
set on your machine, type the following
This would produce a long list. Just go through the list before
reading the next part of the article. Linux by default sets
many environment variables for you. You can modify the values
of most of these variables. A few of the variables that are
would set the home directory to /home/david. This is perfect
in case your login name is david and you have been given a directory
named /home/david . In case you don't want this to be your home
directory but some other one you could indicate so by typing
the new directory name. The HOME directory is always the directory
that you are put in when you login.
There are many advantages of using the HOME variable. You can
always reach your home directory by only typing ' cd ' at the
prompt, irrespective of which directory you are presently within.
This would immediately transfer you to your HOME directory.
Besides in case you write scripts that have $HOME present in
them to refer to the current HOME directory, these scripts can
be used by other users as well since $HOME in their case would
refer to their home directories.
This is a very important environment variable. This sets the
path that the shell would be looking at when it has to execute
any program. It would search in all the directories that are
present in the above line. Remember that entries are separated
by a ' : ' . You can add any number of directories to this list.
The above 3 directories entered is just an example.
Note : The last entry in the PATH command is a ' . '
(period). This is an important addition that you could make
in case it is not present on your system. The period indicates
the current directory in Linux. That means whenever you type
a command, Linux would search for that program in all the directories
that are in its PATH. Since there is a period in the PATH, Linux
would also look in the current directory for program by the
name (the directory from where you execute a command). Thus
whenever you execute a program which is present in the current
directory (maybe some scripts you have written on your own)
you don't have to type a ' ./programname ' . You can only type
' programname ' since the current directory is already in your
Remember that the PATH variable is a very important variable.
In case you want to add some particular directory to your PATH
variable and in case you try typing the following
This would replace the current PATH value with the new value
only. What you would want is to append the new directory to
the existing PATH value. For that to happen you should type
This would add the new directory to the existing PATH value.
Always a $VARIABLE is substituted with the current value of
PS1 is the shell prompt. It defines what you want your shell
prompt to look like. By default it looks like a ' $ ' in bash
shell. The above case would replace the default ' $ ' with a
new 'boss' . Hence an ls command would look something like
All your commands would now be typed at a ' boss ' prompt instead
of a ' $ ' prompt.
This tells where the program that represents your shell is to
be found. In case you typed /bin/ksh in the above, then your
bash shell would be replaced with the ksh shell (korn shell).
So in case you are not happy with the bash shell, you could
replace the bash with some other shell.
The LOGNAME is automatically set for you as the same as your
login name. This variable is used in case you want to use your
own login name in any script. This is the simplest way of getting
your login name from within a script. Thus in case you use $LOGNAME
in any script the script would work for all users since the
LOGNAME always holds the name of the current user.
A good use is in case you have been given a temporary directory
to work with and to make temporary files then you would want
to delete the files that you created. You could use a command
with the $LOGNAME in it to locate files that were created by
you and then you could pass the result of this command to a
rm command. This would be a neat way to delete all the files
at one go, rather than find them one at a time.
Note : Finding files based on a particular criteria
and then passing those files to another program is explained
in Article No. 21
There are more environment variables then the ones that are
mentioned here. But most users would find the ones given here
to be useful.
Important : To make the above changes permanent (that is
it should work every time you login) make the changes to the
.profile file that exists in your HOME directory. Simply
type the required commands one line for each. And there you
go. It will be available every time you login. You could check
the variables using 'env' command.