What is a Shell & Shell script very basic to learn for freshers

by Ankam Ravi Kumar · Published November 4, 2015 · Updated July 23, 2016

What is a shell ?

The Shell is a command line interpreter that executes commands read from the standard input device such as keyboard or from a file. In other forms, it is a user program or it is an environment provided for user interaction. We can start using the shell by opening the console(terminal). The shell is not a part of system kernel, but it uses the system to execute programs. what is a shell .

The various shells available for Linux are:

BASH (Bourne-Again SHell) – Most common shell in Linux. It is an Opensource software
CSH (C SHell) – The C shell’s syntax and usage are very similar to the C programming language.
KSH (Korn SHell) – Created by David Korn at AT & T Bell Labs. The Korn Shell also was the base for the POSIX Shell standard specifications.
TCSH – It is an enhanced but completely compatible version of the Berkeley UNIX C shell (CSH). what is a shell.

What is a shell script ?

A shell script is series of commands stored in a plain text file.
Shell scripts are a fundamental part of the UNIX and Linux programming environment.
Scripts are useful for automating the processes repeatedly at the command line.
Linux users involved with system administration and troubleshooting often work with the shell scripts.
/etc/profile is the system-wide user login script which runs whenever a user logs into the system.

Each shell script consists of

Shell keywords such as if..else, do..while.
Shell commands such as pwd, test, echo, continue, type.
Linux binary commands such as w, who, free etc..
Text processing utilities such as grep, awk, cut.
Functions – add frequent actions together via functions. For example, /etc/init.d/functions file contains function to be used by most or all system shell scripts in the /etc/init.d directory.
Control flow statments such as if..then..else or shell loops to preform repeated actions.

Each script has purpose

Specific purpose – For example, backup file system and database to NAS server.
Act like a command – Each shell script is executed like any other command under Linux.
Script code usability – Shell scripts can be extended from existing scripts. Also, you can use functions files to
package frequently used tasks.

Why shell scripting?

Shell scripts can take input from a user or file and output them to the screen.
Whenever you find yourself doing the same task over and over again you should use shell scripting, i.e., repetitive
Task automation.
Creating your own power tools/utilities.
Automating command input or entry.
Customizing administrative tasks.
Creating simple applications.
Since scripts are well tested, the chances of errors are reduced while configuring services or system administration tasks such as adding new users.
Real time usage of shell scripts where it is actively used:
Monitoring Linux system.
Data backup and creating snapshots.
Dumping Oracle or MySQL database for backup.
Creating email based alert system.
Find out what processes are eating up our system resources.
Find out available and free memory.
Find out all logged in users and what they are doing.
Find out if all necessary network services are running or not. For example if web server failed then send an alert to system administrator via a pager or an email.
Find out all failed login attempt, if login attempt are continue repeatedly from same network IP automatically block all those IPs accessing your network/service via firewall.
User administration as per our own security policies.
Find out information about local or remote servers.
Configure server such as BIND (DNS server) to add zone entries.

Advantages

Easy to use.
Quick start, and interactive debugging.
Time Saving.
Sys Admin task automation. what is a shell
Shell scripts can execute without any additional effort on nearly any modern UNIX / Linux / BSD / Mac OS X operating system as they are written an interpreted language.

Disadvantages