Bash Scripting

What is bash scripting

Generally bash scripts are used by Developers, System Administers, Network Engineers, Computer Scientists and almost every one who uses Unix/Linux systems. Bash script is a simple text file with series of commands, usually we type on command line(commands that we've discussed in the previous post). Although .sh extension is commonly used with bash scripts it is not mandatory since Linux is an extensionless system.

Contents of a simple script

The Shebang (#!)

#!/bin/bash

First line of the above script, #! character sequence referred to as shebang and then it is the path to the interpreter that should be required to run the next lines in the script.   

Executing the script

Before we can run the script we have to grant the permission, otherwise you'll get an error message saying “Permission denied”

 

Variables

Variable is a temporary storage for information.

Use appropriate names for variables so that you can easily identify those without having further description. Setting a value for a variable can be done by directly or by user input. When referring to or reading a variable we place a $ sign before the variable name.

 

Setting a value for a variable by user input

 

If statements

If statements allow users to make decisions in bash scripts. If statements decide which part of the code has to be run based on set criteria.

Following code is a simple example to show how if-else blocks are work

Loops

Loops allow user to do some task repeatedly until given condition is satisfied.

There are three basic loops structures in bash scripting :

• While
• Until
• For 

While loop
Using while loop we can perform some task repeatedly until given condition is true.

Here while loop runs 5 times before it terminates. Since counter starts with 1 and it is incremented by 1 inside while block, after 5 rounds the value of counter will be 6. But the condition satisfies  only with the values less than ( refereed by 'lt' ) 6. So the this will print the numbers 1-5.

Until loop

Until loop is fairly similar to while loop, but this works until given condition is false where as while loop runs until condition is true.

This loop will run until the counter is greater than ( indicates by '-gt' ) 6. That is when counter become 7, loop will terminate.

For loop
In for loop we can set starting and ending point to repetition. In situations where we know the two extremes we can use this loop.

This will print all the numbers in the sequence (indicates by `seq 1 10`). Here starting and ending points will be 1 and 10 respectively.

Case

when the user input is 1-4 this code will print respective month name, otherwise it will display a error message.