Hands-On

Linux programming (scripting)

Shell Scripting is the language of the linux terminal. Shell scripts are sometimes referred to as “shebang” which is derived from the “#!” notation. Shell scripts are executed by interpreters present in the linux kernel. Interpreters include: bash, csh, zsh e.t.c. Most popular of which is bash.

Typical operations performed by shell scripts include file manipulation, program execution, and printing text.

How to Write Shell Script in Linux/Unix

• Create a file using a vi editor(or any other editor). Name script file with extension . sh.
• Start the script with #! /bin/sh.
• Write some code.
• Save the script file as filename.sh.
• For executing the script type bash filename.sh.

File Color : Executable scripts appear in a different colour from rest of the files and folders.

Let's start to have some experience on scripting

Simple Script for newbies

First we have to create a text file

$cat > hello.sh

And create the content in it

#!/bin/sh 
echo 'Hello World'

#!/bin/sh is "how to run" the program. echo 'Hello World' is "what to do" in the script

Then make the file executable

$ chmod +x hello.sh

And run the executable file

$./hello.sh

Output:

Hello world

Make yourself familiar with man man and man apropos on the shell. It will need for your self-tutoring.

Let's make this little advanced

How to define variables

Variables can be defined using the syntax variable_name=value. To get the value of the variable, add $ before the variable

#!/bin/bash
# A simple variable example
greeting=Hello
name=world
echo $greeting $name

Arithmetic Expressions

These are the operstors supported by bash for mathematical calculations.

+	addition
-	subtraction
*	multiplication
/	division
**	exponentiation
%	modulus

Numerical expressions can also be calculated and stored in a variable using the syntax below:

value=$((expression))

#!/bin/bash

var=$((3+9))
echo $var

How to read user input

Sometimes user input are needed to perform relevant operations.

In bash, user inputs are taken using the read command.

read variable_name

To prompt the user with a custom message, use the -p flag.

read -p "Enter your age" variable_name

Numeric Comparison logical operators

Comparison is used to check if statements evaluate to true or false.

Equality   	                num1 -eq num2	is num1 equal to num2
Greater than equal to	num1 -ge num2	is num1 greater than equal to num2
Greater than	                num1 -gt num2	is num1 greater than num2
Less than equal to	        num1 -le num2	is num1 less than equal to num2
Less than	                num1 -lt num2	is num1 less than num2
Not Equal to	                num1 -ne num2	is num1 not equal to num2

Syntax:

if [ conditions ]
    then
         commands
fi

Conditional Statements (Decision Making)

Conditions are expressions that evaluate to a boolean expression (true or false). To check conditions, we can use if, if-else, if-elif-else and nested conditionals.

Looping and skipping

For loops allow you to execute statements a specific number of times.

Looping with numbers:

#!/bin/bash

for i in {1..5}
do
    echo $i
done

Looping with strings:

#!/bin/bash

for X in cyan magenta yellow  
do
	echo $X
done

While loop

While loops check for a condition and loop until the condition remains true. We need to provide a counter statement that increments the counter to control loop execution.

#!/bin/bash
i=1
while [[ $i -le 10 ]] ; do
   echo "$i"
  (( i += 1 ))
done

Reading files:

Suppose we have a file sample_file.txt as shown below:

#!/bin/bash

LINE=1

while read -r CURRENT_LINE
	do
		echo "$LINE: $CURRENT_LINE"
    ((LINE++))
done < "sample_file.txt"

How to execute commands with back ticks

If you need to include the output of a complex command in your script, you can write the statement inside back ticks.

syntax:

var= ` commands `

Example: Suppose we want to get the output of a list of mountpoints with tmpfs in their name. We can craft a statement like this: df -h | grep tmpfs.

#!/bin/bash

var=`df -h | grep tmpfs`
echo $var

How to Automate Scripts by Scheduling via cron Jobs

Cron is a job scheduling utility present in Unix like systems. Jobs can be scheduled to execute daily, weekly, monthly or in a specific time of the day. Automation in Linux heavily relies on cron jobs.

Below is the syntax to schedule crons:

# Cron job example
* * * * * sh /path/to/script.sh

Here, * represent represents minute(s) hour(s) day(s) month(s) weekday(s), respectively.

Github administration

Install Git and create a Github account

Install source tree on your computer

Create a local Git repo(Cloning)

Add a new file to the repo

Add a file to the staging environment

Create a commit

Create a branch

Create a new repo on Github

Push a branch to GitHub

Create a pull request

Merge a pull request

Get changes on your Github back your computer

Creating a repository

Create a project directory and cd into it. Execute the following git command from the directory to create a git repository.

git init

Checking out a repository

You can create a copy of your git repository using the clone command. Execute the following command to clone your project directory.

git clone /path/to/project-repository

Every git repository has three trees. A working directory, Index and Head. ====

• Working directory: It contains the actual project files.

• Index: It is the staging area where you add the project files that needs to be committed.

• Head: Head is where the reference to you previous commit exists.

Adding file to the staging area: (add)

git add <filename>

Let’s say you want to add all the files in your project directory to the staging area. Execute the following command to do the same

git add --all

Committing new changes to the repository (commit):

Once you have added all the files to the staging area, you can commit the changes with a reference message using the “-m” flag as shown below.

git commit -m “my first commit”