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.