BASH SCRIPTING

BASH :

BASH stands for BOURNE AGAIN SHELL (LINUX OR UNIX SHELL).

SCRIPT :

A file containing a sequence of instructions executed by the BASH PROGRAM step by step.

A Bash script is a plain text file which contains a series of commands. These commands are a mixture of commands we would normally type ourselves on the command line (such as ls or cd) and commands we could type on the command line but generally wouldn’t . An important point to remember though is:

Anything you can run normally on the command line can be put into a script and it will do exactly the same thing.

BENEFITS OF BASH SCRIPTING :

1 . AUTOMATION :

Shell scripts allow you to automate repetitive tasks and processes, saving time and reducing the risk of errors that can occur with manual execution - Letting the computer performs tasks without human intervention.

2 . ACCESSIBILITY :

Shell scripts are easy to write and don’t require any special tools or software. They can be edited using any text editor, and most operating systems have a built-in shell interpreter.

STEPS FOR DOING BASH SCRIPTING :

STEP 1 : DETERMINING YOUR SHELL TYPE IN LINUX :

Your shell type in LINUX can be determined through this command :

ps

ps stands for process status.

Mine is bash , you should check yours through this way.

STEP 2 : CREATING A SCRIPT FILE :

Firstly , we’ll create a script file through this command :

nano file.sh

And the file name must end with “sh“ to show that this file is a BASH SCRIPT. This also helps in distinguishing scripts from other files.

nano is a TERMINAL BASED TEXT EDITOR. It allows you to create, view, and edit text files directly from the command line.

STEP 3 : WRITING A SCRIPT :

SHEBANG: SHEBANG tells the shell to execute your file via a BASH SHELL. 1st line of your BASH SCRIPT should be :

#!/bin/bash

/bin/bash shows that bin is a folder that contains important system commands and bash is a file.

bin —> binary because programs are stored as binary executable files and not machine code.

You can write anything in the script.

STEP 4 : EXECUTING THE SCRIPT :

Execute this file through this command :

chmod +x file.sh

chmod refers to change mode

• chmod: Command used to change the permission of the file in LINUX .
• +x: It gives permission to the owner of the file to execute it .

STEP 5 : RUNNING THE BASH SCRIPT :

A script can be run through this command :

./file.sh

./ : looks for the file in the current directory.

STEP 6 : COMMENTS IN BASH SCRIPTING :

Comments in bash scripting starts with a BASH (#) . It is helpful in documenting the code .

STEP 7 : VARIABLES IN BASH SCRIPTING :

VARIABLES :

Variables are used to store data and they also lets you read , access and manipulate data throughout your script.And there are no data types in bash scripting .

In BASH SCRIPTING, a variable is capable of storing NUMERIC VALUES , INDIVIDUAL CHARACTERS or STRING OF CHARACTERS .

STEP 1 : STORING DATA :

We can store data directly in variables but that data should be enclosed in double quotes and there should be no space in between the equal sign and stored data .

such as :

variablename="value"

STEP 2 : ACCESSING DATA :

Retrieving the value that is already stored in a variable , array , file or command output .

OR

Accessing data is the process of reading or using stored information.

$ = It is required to access an existing variable ‘s value .

INPUT:

OUTPUT:

STEP 3 : TAKING INPUT FROM THE USER :

It is done using this command:

read

INPUT:

OUTPUT:

COMMON VARIABLE OPERATIONS :

Variables can be used in various operations such as : concatenation and arithmetic .

CONCATENATION :

INPUT:

OUTPUT:

ARITHMETIC :

For this you must use double brackets like : ((hi))

INPUT:

OUTPUT:

INTEGER COMPARISON :

In integer comparison , we use SQUARE BRACKETS [] , they are called TEST COMMANDS in BASH .

They are used to check conditions like COMPARING NUMBERS , STRINGS OR CHECKING THE EXISTENCE OF A FILE .

• (-eq) = is equal to if [ “$a “ -eq “$b” ]

• (-ne) = is not equal to if [ “$a” -ne “$b” }

• (-gt) = is greater than if [ “$a” -gt “$b” ]

• (-ge) = is greater than or equal to if [ “$a” -ge “$b” ]

• (-lt) = is less than if [ “$a” -lt “$b” ]

• (-le) = is less than or equal than if [ “$a” -le “$b” ]

ARITHMETIC COMPARISON :

Here we would use double brackets

• (<) is less than - ( (“$a” < “$b” ) )

• (<=) is less than or equal - ( ( “$a” <= “$b” ) )

• (>) is greater than - ( (“$a” > “$b” ) )

• (>=) is greater than or equal to - ( ( “$a” >= “$b” ) )

STRING COMPARISON :

• (=) is equal to if [ “$a” = “$b” ]

• (==) is equal to if [ “$a” == “$b” ]

• (!=) is not equal to if [ “$a” != “$b” ]

• (<) is less than , in ASCII alphabetical order if [ [ “$a “ < “$b” ]]

• (>) is greater than , in ASCII alphabetical order if [ [ “$a” > “$b ] ]

• (-z) string is null , that is , has zero length if [-z “$a“]

DECISION CONTROL IN BASH SCRIPTS :

if BLOCK :

• If condition is enclosed in SQUARE BRACKETS [].
• Statements ends with “fi“ which is “if“ spelled backwards marking the end of “if“ block.
• In Bash , spaces around [ ] are compulsory , without them , the if block breaks.

INPUT:

OUTPUT:

IF-ELSE BLOCK :

INPUT:

OUTPUT:

NESTED IF-ELSE :

INPUT:

OUTPUT:

LOOPS :

TYPES OF LOOPS IN BASH :

1. FOR LOOP
2. WHILE LOOP
3. UNTIL LOOP

FOR LOOP :

This loop is used when the ending condition is known . They are useful for repeating tasks a specific number of times.

The “for“ keyword is followed by a variable name, a range of values, “do“ keyword marks the start of the loop block and “done“ keyword indicates the end of the for loop block.

INPUT:

OUTPUT:

WHILE LOOP :

While loops execute a block of code as long as a specified condition is true.

INPUT:

OUTPUT:

UNTIL LOOP :

Until loops are similar to while loops, but they execute until a specified condition becomes true.

INPUT:

OUTPUT:

CRON SCHEDULING SYSTEM :

CRONJOB :

CronJob are scheduled tasks that allows you to run scripts and commands at specific intervals.

WHY USE CRONJOBS :

To automate repetitive tasks and to ensure that they run at scheduled times without manual intervention.

FOR EXAMPLE :

Backing up files and sending email reports etc

HOW DOES CRONJOB WORK ?

CronJob uses a daemon(a background process) called CRON to execute scheduled tasks. And the configuration of these tasks is stored in a file called CRONTAB.

CRON :

• A daemon(a background process) in LINUX.
• It runs automatically in the background.
• Its job is to execute scheduled tasks at specific intervals.

CRONTAB :

A file or command used to schedule CRONJOBS.

CRONTAB COMMANDS :

1. crontab -e –> To edit crontab file by adding CRONJOBS.
2. crontab -l –> To view all your scheduled CRONJOBS.
3. crontab -r –> To delete all scheduled CRONJOBS.

BASIC FORMATE OF CRONJOB :

* * * * * /path/to/script.sh

MINUTES(0 - 59) | HOUR(0 - 23) | DAY OF MONTH(1 - 31) | MONTH(1 - 12) | DAY OF WEEK(0 - 7)

CONCLUSION :

Bash scripting is a simple and powerful way to automate tasks in Linux. It helps save time, reduce effort, and improve efficiency by turning repetitive commands into automated processes.