1. Introduction

UNIX shell programs executes the user command. User can enter like a command directly or executes a set of commands from a file called as a script file or program. Shell scrips are not compiled, it is interpreted executes line by line. In this chapter we are going to see shell script basics

2. Advantages and features using Shell script

Bash is a free software means no restriction to use and develop. bash incorporates features of Korn shell and C shell.
Features of bash are command history, unlimited size, control over job, shell functions, interactive, case statements, loops, conditional statements etc., Since it is free we have freedom to copy, modify and redistribute

a. The startup files in the Linux are based on Bash, the start up files are scripts that are read and executed when the operating starts
b. the important files that are activated during the start up or at the user logon
. /etc/profile
. ~/.bashrc
. ~/.bash_profile
. ~/.bash_history
. ~/.bash_logout

3. Commonly used Shells in Linux

UNIX has lot of shell types. each one is used for different purposes, namely sh,bash,csh,tcsh,ksh,zsh

sh or Bourne shell – It is used in UNIX system, it is a basic shell and will less features
bash – Bourne again shell – It is standard GNU shell probably most used shell in Linux it is flexible. It is advanced and user friendly shell , most of the user prefer the bash shell.
csh – C shell, it is used when a programmer executes C language codes.
tcsh or TENEX C shell – It is a superset of C shell, enhanced features for user friendly also called as Turbo C Shell
ksh – Korn shell , it is a superset of Bourne shell
zsh – Extended bourne shell, it includes features of bash, ksh and tcsh

The /etc/shell shows the available shells on a Linux machine

[root@control ~]# cat /etc/shells 
 /bin/sh
 /bin/bash
 /usr/bin/sh
 /usr/bin/bash
 [root@control ~]

The shell is set for the user and the default shells are set in the /etc/passwd file

[root@control ~]# grep devops /etc/passwd
 devops:x:1000:1000:DevOps:/home/devops:/bin/bash
 [root@control ~]#

To check the current shell execute the below command to check the current user shell

[devops@control ~]$ echo $SHELL
 /bin/bash

However user can switch between the shells

[devops@control ~]$ sh
 sh-4.4$ 

4. How to execute shells

Shell script can be executed in various ways, we are going to see shell script basics

sh filename.sh
for i in cat file; do commands; done

Shell has built in comamnds used for variuos purpose depending on the logic and scnerios

break, eval, exit, exec, export,getopsts, hash, pwd, set, test,trap, umask,unset

alias, bind, command, echo, enable, logout, printf, shopt, read typeset, ulimit

Executing commands:

A command is being executed, Success will return value 0 and other than zero means it is wrong comamnd or unsuccessful, The executed command can be validated by using echo $?

Succesfull Command:

We could see the value 0

[root@control ~]# date
 Sun Dec  6 15:52:14 IST 2020
[root@control ~]# echo $?
0

Unsuccessful command

We could see the value 127 which is unsuccessful

 [root@control ~]# data
 -bash: data: command not found
 [root@control ~]# echo $?
 127
 [root@control ~]# 

5. Example shell scripts

In this section we will see basic scripts that will be useful for the start up

A good script will be without errors and perform the task, Script should be resuable scnerio and avoid unnecessary write ups

use vim editor for the color difference, vim enhanced useful to verify whether it is a keyword or variable or a command

 [root@control ~]# vim test.sh 
 !/bin/bash
 echo "Welcome to computer carriage"
 [root@control ~]# sh test.sh 
 Welcome to computer carriage
 [root@control ~]# 

In the above example #!/bin/bash is the interpreter, usually every script file start with the interpreter, so that the OS or the interpreter will know what language it is written
in this we have used a simple echo command to display, sh filename.sh will executes the script file

Example 2

[root@control ~]# vim test.sh 
 !/bin/bash
 echo "Welcome to computer carriage"
 echo "Todays date and time is $(date)"

 [root@control ~]# sh test.sh 
 Welcome to computer carriage
 Todays date and time 

In the second example we have used date command inside the echo command, $(command) we can execute any command inside the parenthesis

Inputting the variable at run time

We are going to give the variable NAME while executing the script

[root@control ~]# vim test.sh 
 !/bin/bash
 echo "Welcome to computer carriage"
 echo "Todays date and time is $(date)"
 echo "Enter the name"
 read NAME
 echo "Hello $NAME"

 [root@control ~]# sh test.sh 
 Welcome to computer carriage
 Todays date and time is Sun Dec  6 16:07:17 IST 2020
 Enter the name
 Bala                                         <--- Input at runtime
 Hello Bala
 [root@control ~]# 

In the above example read is the keyword used which is used to get input variable at runtime.

Define variables in the file

We can set the standard variable in the file itself

[root@control ~]# cat test.sh 
 !/bin/bash
 NAME=Bala
 echo "Welcome to computer carriage"
 echo "Todays date and time is $(date)"
 echo "Hello $NAME"
 [root@control ~]# sh test.sh 
 Welcome to computer carriage
 Todays date and time is Sun Dec  6 16:15:30 IST 2020
 Hello Bala
 [root@control ~]# 

In the above example we have set the NAME value is BALA, instead of asking inputs at runtime.

6. Loops and conditions

In bash scripts we can use loops and conditions like other programming languages

Loops

The below example i have created a file called cmds, which has the set of commands that has to be executed

[root@control ~]# cat cmds 
 date
 ls
 uptime

In the file cmds to get executed the command at a time, for loop is used, cat cmds will list the file, the variable for i is the input of the cmds file. it is executed one by one line until the end of the file.

[root@control ~]# for i in cat cmds
   do
   $i
   done
   Sun Dec  6 16:10:44 IST 2020 
   anaconda-ks.cfg              awx   splunk-8.0.6-152fb4b2bb96-linux-2.6-x86_64.rpm       test.sh
   ansible-2.8.0-1.el8ae.noarch.rpm  cmds    splunkforwarder-8.0.5-a1a6394cc5ae-linux-2.6-x86_64.rpm
    16:10:44 up 42 min,  2 users,  load average: 0.10, 0.04, 0.16
   [root@control ~]#  

Conditional statements

We are going to see simple if condition, lets consider A=10 and B=20, with simple If condition we can get which bigger using the shell script basics

[root@control ~]# cat test.sh 
 !/bin/bash
 A=10
 B=20
 if [[ A > B ]];then
 echo "A is bigger"
 else
 echo "B is bigger"
 fi
 [root@control ~]# sh test.sh 
 B is bigger
 [root@control ~]# 

7. Conclusion

hope you have understood the shell script basics , let us know you comments on the topic shell script basics

Refer: TLDP Shell script basics – https://tldp.org/LDP/abs/html/

Also refer: CentOS Minimal install – https://computercarriage.com/2020/09/23/centos-8-minimal-install/