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

To see if you already have Git installed, open up your terminal application.

Once you've opened your terminal application, type git version. The output will either tell you which version of Git is installed, or it will alert you that git is an unknown command. If it's an unknown command, read further and find out how to install Git.

Install git:

Debian/Ubuntu

Git packages are available using apt.

It's a good idea to make sure you're running the latest version. To do so, Navigate to your command prompt shell and run the following command to make sure everything is up-to-date: sudo apt-get update.

To install Git, run the following command: sudo apt-get install git-all.

Once the command output has completed, you can verify the installation by typing: git version.

To create the github account:

Navigate ​https://github.com and signup with your details

Install source tree on your computer

Create a local Git repo

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.

git init is one way to start a new project with Git. To start a repository, use either git init or git clone - not both.

To initialize a repository, Git creates a hidden directory called .git. That directory stores all of the objects and refs that Git uses and creates as a part of the project's history. This hidden .git directory is what separates a regular directory from a Git repository.

git init: One Person Starting a New Repository Locally
git clone: The Remote Already Exists

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

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”

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

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