Page 126 - DCAP106_OPERATING_SYSTEM_TOOLS

Unit 7: The Shell

Thus, we can say that the shell acts as an intermediary among the operating system and the user. Notes
The role of shell is to read the command line, interpret its meaning, carry out the command, and
then return the result through the outputs.
The shell is considered as an executable file which is liable for interpreting the commands,
transmitting them to the system, and returning the result. There are several shells. The most
common shell is sh (“Bourne shell”), bash (“Bourne again shell”), csh (“C Shell”), Tcsh (“Tenex
C shell”), ksh (“Korn shell”), and zsh (“Zero shell”). We have discussed these shells in this unit.

Did u know? Generally, the name of the different shells matches the name of the executable.

Every user has a default shell. This default shell will be launched on opening of a command
prompt. We specify default shell in the dans configuration file /etc/passwd in the last field of
the line corresponding to the user. It is possible to change the shell during a session by simply
executing the corresponding executable file.

Example:
/bin/bash
We initialize shell by reading its overall configuration (in a file of the directory /etc/), followed
by reading the user’s own configuration (in a hidden file whose name begins with a dot, situated
in the basic user directory, that is, home/user_name/.configuration_file). Then, a prompt is
displayed as follows:
machine:/directory/current$
By default, the prompt (for most shells) include the machine name, followed by two points (:), the
current directory, and then a character signifying the type of user connected:
z z “$” specifies a normal user

z z “#” specifies the administrator, called “root”
We can define a command line as a chain of characters which represents a command corresponding
to an executable file of the system or rather a shell command and optional arguments (parameters):

ls -al /home/jf/
In the above command, ls signifies the name of the command, -al et /home/jean-francois/ are
arguments. Arguments starting with - are known as options. Generally, for each command, there
is a certain number of options which can be detailed by typing one of the following commands:

command --help
command -?
man command
The shell gives an interface to Linux. In this interface, commands can be typed or entered by
using the keyboard. It is called the command line interface (CLI). To find out your current shell
type following command:
echo $SHELL

ps $$
ps -p $$

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