Page 251 - DCAP403_Operating System

Operating System

Notes Shell Penal

The command interpreter is the interface between the user and the operating system, hence its
name “shell”.
Users
Shell

Kernel

The shell therefore acts as an intermediary between the operating system and the user using
command lines entered by the latter. Its role consists of reading the command line, interpreting
its meaning, executing the command, and then returning the result via the outputs.

The shell is an executable file responsible for interpreting commands, transmitting them to the
system, and returning the result. There are several shells, the most common being sh (called the
“Bourne shell”), bash (“Bourne again shell”), csh (“C Shell”), Tcsh (“Tenex C shell”), ksh (“Korn
shell”),and zsh (“Zero shell”). Their name generally matches the name of the executable.
Each user has a default shell, which will be launched when a command prompt is opened.

The default shell is specified in the confi guration file /etc/passwd in the last field of the line

corresponding to the user. It is possible to change the shell during a session simply by executing
the corresponding executable file, for example:

/bin/bash
Command Prompt Window (Prompt)

The shell is initialized by reading its overall configuration (in a file of the directory /etc/),

followed by reading the user’s own configuration (in a hidden file the name of which starts with

a dot, located in the basic user directory, i.e. /home/user_name/.confi guration_file). Then, a

command prompt window or prompt is displayed as follows:
machine:/directory/current$
By default, for most shells, the prompt consists of the name of the machine, followed by a colon
(:), the current directory, then a character indicating the type of user connected:

1. “$” specifies a normal user

2. “#” specifies the administrator, called “root”
Command Line Concept

A command line is a character string representing a command corresponding to an executable
system file or shell command along with optional arguments (parameters):

ls -al /home/jf/
In the above command, ls is the name of the command, -al and /home/jean-francois/ are
arguments. Arguments beginning with - are called options. Generally, for each command
there are a certain number of options which can be detailed by entering one of the following
commands:
command --help
command -?
man command

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