Trying to understand Shell Scripting

In this post, we are trying to understand about shell scripting. As per wikipedia, A shell script is a computer program designed to be run by a Unix shell, a command-line interpreter. A script is command line program that contains a series of commands. The commands contained in the script are executed by interpreter. In case of shell scripts, the shell acts as interpreter and executes the commands listed in the script one after another.

Let’s create our first shell script

Create a new file in your Unix / Linux operating system. I have ubuntu with me, so I will create new file using touch command

touch firstScript.sh

Then open file with your favorite editor, I am using VIM editor.

vim firstScript.sh

it will open file in edit mode, press “I” to go to insert mode

#!/bin/bash
echo "This is my first shell script"

Save the file, and go back to folder where you have saved this file before you try to execute the script, make sure that it is executable and have enough permission to run Enter following command

chmod 755 firstScript.sh 

Now, this is time to run our first shell script Enter file name directly

.\firstScript.sh

We have successfully run our first shell script

Let’s further discuss more about shell scripting

Variables

You can use variables in shell script. Variables simply storage locations which have name. We can assign value to those variable for later use. Example, any operations, manipulation etc. Syntaxt to create variable is very simple

Variable_name="some value"

Once variable created, you can retrieve value inside variable using ${variable_name}

let’s create shell script with variable and try to retrieve that value

Create another shell script and write code as below

#!/bin/bash
MY_VAR="Bash"
echo "This is ${MY_VAR} command, running in shell scirpt"

Grant permission as we have given in previous example

then run this shell script

Conditional Statements

if statement

We can use if statement same as other programming languages in shell script. In shell scripts, we have to use if-then-fi, fi is to tell interpreter that, if condition block has been ended.

below is syntax

if [ condition ]
then
  command 1
  command 2
  ...
fi

lets take an example

#!/bin/bash
firstNumber=10
secondNumber=10

if [ $firstNumber = $secondNumber ]
then
        echo "both numbers are equal"
fi

save the file , grant permission and try to run this shell script you will see output as below

both numbers are equal

NOTE: Remember to add one space in between if and [ otherwise you will get an error

if-else statement

We can also add else statement along with if , below is its syntax

if [ condition ]
then
  command 1
  command 2
  ...
else
  command 3
  command 4
  ...
fi

let’s continue to expand our previous example but here we will change value of variable firstNumber and secondNumber

#!/bin/bash
firstNumber=50
secondNumber=100

if [ $firstNumber = $secondNumber ]
then
        echo "both numbers are equal"
else
        echo "both numbers are not equal"
fi

Run this shell script and you will see results as below

both numbers are not equal

if- else if - else statements

Shell script also allow us for else-if with below syntax

if [ condition 1 ]
then
  command 1
  command 2
  ...
elif [ condition 2 ]
then
  command 3
  command 4
  ...
else
  command 5
  command 6
  ...
fi

lets further expand our previous example again

#!/bin/bash
firstNumber=10
secondNumber=100

if [ $firstNumber = $secondNumber ]
then
        echo "both numbers are equal"
elif [ $firstNumber = 10 ]
then
        echo "first number is 10"
else
        echo "both numbers are not equal"
fi

Now, as firstNumber assigned to 10, it will go to else-if statement block and result will be

first number is 10

Loops

for loop

if you want to perform any action number of times then, we can make use of for loop. Below is syntax for “For” loop

for variable_name in item1 item2 ... itemN
do
  command 1
  command 2
  ...
done

lets take an example of this

#!/bin/bash

for color in red green blue
do
        echo "Selected color in this iteration is: ${color}"
done

like before, you need to grant permission and then execute this shell script. Result will be as below

Comments

We can add comments in shell script using # symbol. Anthing after # symbol is consider as comments and this command will not get executed exmaple

#!/bin/bash
for color in red green blue # define the colors here 
do
        echo "Selected color in this iteration is: ${color}"
done

in above script at second line we have added comment as # define the colors here this is just comment, and this will not be executed

Return codes

Return code can be used in script for error checking, if return code is 0 meaning command executed successfully, if it is non-zero value, that means there is some error We can use special variable $? to check return code if previously executed command.

let’s take an example, if I try ls command for any such path which does not exist , then our script will return non-zero code.

#!/bin/bash
ls /prasad/test
echo "$?"

/prasad/test this directory does not exists in my machine, so when I run script, I will get output as below

As we can see it has return code as 2, as it is not zero, we can say there is some error in command execution.

Let’s take another example, in this example we are going to create some file and try to see what it will return

#!/bin/bash
touch prasad.txt
echo "text file has been created"
echo "$?"

Run this script

as we can see it has returned code as 0 and created new file with name prasad.txt

Exit Command

We can defined our own exit codes in shell script, simply use exit command in script and follow it with number from 0 to 255. We can add exit command anywhere in script. Whenever, script reached to exit command it will stop running.

#!/bin/bash
cat prasad.txt
if [ "$?" -eq "0" ]
then
        echo "Command success"
        exit 0
else
        echo "Command failed"
        exit 1
fi

In above example, we are using cat command to read content from file, if it is success then we are checking with return code. If code returned as 0 that means we have successfully executed previous command and then we are using exit command by setting exit status as 0 , similarly if it is failed to read file then we are exiting with code as 1.

as file with name prasad.txt already present in directory, it will return output as

Command success

Functions

Functions is most importent topic in any programming language, this help to avoid code duplication ana make program well strucured, easy to read, easy to understand.

Below is syntax to create function in shell script

function Function_name() {
# do something
}

We can also write functions without specifying function before it’s name as below

function_name(){
#do something
}

and simply use function name in your script file to call function let’s take an example

function add(){
        n1=1
        n2=2
        addition=$((n1+n2)) # add two numbers
        echo "Addition of two number is : ${addition} "
}

add  #<---function calling

this will return output as below

Addition of two number is : 3

NOTE: To perform arithmetic in Bash, you need to use either $(()) or the expr command.

Positional Parameters

We can pass parameters to our shell script and to our functions and those can be accessible using $ symbol example $1, $2 etc Let’s contunue to our above example of addition.

function add(){
        n1=$1
        n2=$2
        addition=$((n1+n2)) # add two numbers
        echo "Addition of two number is : ${addition} "
}

add 10 20 #<---function calling

This will return output as below Addition of two number is : 30

We also can pass parameters to our shell script directly. lets modify our script and then pass parameters while running script

function add(){
        n1=$1
        n2=$2
        addition=$((n1+n2)) # add two numbers
        echo "Addition of two number is : ${addition} "
}

add $1 $2 #<---function calling

Now, we are going to pass parameters while running script, use below command to run script

This is how we can pass parameter to script and to the function.