= 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.