This article covers everything you need to know about user input, conditional statements (if-else), case statements, loops (for and while), functions, and passing arguments in shell scripting.

We will introduce essential commands like date, whoami, uptime, disk and RAM space checks, and last login details, which will be used in our mini project. The project includes welcoming users with their usernames, displaying the current date and time, showing server uptime and the last two or three login information (based on user input), providing server utilization statistics, and presenting details about the top two or three CPU processes(based on user input).

Create a file name whatever you want. But, make sure it ends with .sh. Open that file in your favorite text editor and write the first line to begin your shell script journey.

We execute .sh file and we will provide execute permission by below commands.

Chmod 700 ./filename.sh

#!/bin/bash

The #! /bin/bash line is called the shebang or hashbang, and it is used to specify the interpreter to be used for executing the script. In this case, #!/bin/bash indicates that the script should be interpreted and executed using the Bash shell.

Open your terminal and go to the directory where you created the script file. Then run the below command to execute the script

./my_script.sh

At this stage, you won't see any output since we have not written anything other than the shebang line. Let's add some fun stuff
You can use the echo command to print statements or messages in the terminal. For example:

echo welcome to shell scripting

You will see “ welcome to shell scripting ” in your terminal

You can assign values to variables in Bash using the assignment operator (=). Here is example

name="nahid"
echo "my name is $name"

Now to execute the script and look at the ouput

User input:

To read user input in Bash, you can use the read command. It prompts you to enter a value and stores it in a variable. For example:

echo "Enter your name: "
read name
echo "Hello $name, nice to meet you bro! "

Conditional Statements:

Bash supports various conditional statements to control the flow of your script. Here are a few examples:

Simple If Statement:

name="nahid"
if [ "$name" == "nahid" ];
then
   echo "Your name is nahid"
fi

#!/bin/bash
echo "Enter your name: "
read name
if [ "$name" == "nahid" ];
then
   echo "Your name is nahid"
else
        echo "Your name is not nahid"
fi

If Statement with Else:

name="nahid"
if [ "$name" == "nahid" ];
then
   echo "Your name is nahid"
else
        echo "Your name is not nahid"
fi

Else If Statement:

#!/bin/bash

echo "Enter your name: "
read name
if [ "$name" == "nahid" ]; then
   echo "Your name is nahid"
elif [ "$name" == "jannat" ]; then
   echo "Your name is Jannat"
else
   echo "Your name is not nahid or Jannat"
fi

Below is an example of comparing two numbers using -gt operator in the shell script

NUM1=3 
NUM2=5
if [ "$NUM1" -gt "$NUM2" ] 
then 
    echo "$NUM1 is greater than $NUM2"
else 
    echo "$NUM1 is less than $NUM2"
fi

You can use any of the following operators to do comparisons in a shell script.

Equal to: == or =
- Example: [ "$var1" == "$var2" ]
- Checks if $var1 is equal to $var2.
Not equal to: !=
- Example: [ "$var1" != "$var2" ]
- Checks if $var1 is not equal to $var2.
Greater than: >
- Example: [ "$var1" -gt "$var2" ]
- Checks if $var1 is greater than $var2.
Greater than or equal to: -ge
- Example: [ "$var1" -ge "$var2" ]
- Checks if $var1 is greater than or equal to $var2.
Less than: <
- Example: [ "$var1" -lt "$var2" ]
- Checks if $var1 is less than $var2.
Less than or equal to: -le
- Example: [ "$var1" -le "$var2" ]
- Checks if $var1 is less than or equal to $var2.

Following is the example to check whether a file exists with the given or not

FILE="test.txt"
if [ -e "$FILE" ]
then
  echo "$FILE exists"
else
  echo "$FILE does not exist"
fi

Case Statement:

The case statement allows you to evaluate the value of a variable against multiple patterns and execute different blocks of code based on the matching pattern.

Following is the example:

#!/bin/bash

echo "Enter a fruit: "
read FRUIT

case "$FRUIT" in
    "apple")
        echo "You selected: Apple"
        ;;
    "banana")
        echo "You selected: Banana"
        ;;
    "orange")
        echo "You selected: Orange"
        ;;
    *)
        echo "Invalid fruit selection"
        ;;
esac

For loop

The for loop allows you to iterate over a list of values and perform a set of actions for each value.

#!/bin/bash

echo "Enter your name: "
read NAME

for ((i=1; i<=5; i++))
do
    echo "Hello, $NAME!"
done

While Loop:

A while loop in a shell script allows you to repeatedly execute a block of code as long as a certain condition is true. Here is an example of for loop.

#!/bin/bash

echo "Enter your name: "
read NAME

counter=1
while [ $counter -le 5 ]
do
    echo "Hello, $NAME!"
    counter=$((counter+1))
done

Functions:

In shell scripting, you can define and use functions to encapsulate a block of code that can be executed multiple times throughout your script.

Function with parameters:

#!/bin/bash

greet() {
    echo "Hello, I am $1 and I am $2 years old"
}

# Call the function and pass arguments
greet "nahid" "27"