Shell Scripting for Beginners: Automate Your Linux Tasks 🚀

Sparsh JaipuriyarSparsh Jaipuriyar
8 min read

Table of contents

What is Shell Scripting? 🤔

Shell scripting means writing a series of commands, saved in a file, which the computer will carry out one by one. Think of it as creating a set of instructions so your computer can do your boring or repetitive jobs for you, automatically.

Why use it?

  • Automate tasks like file handling and software installs.

  • Manage system settings.

  • Speed up your daily computer chores without clicking around.

  • How to Create and Run a Shell Script

    Every shell script:

    • Is just a text file.

    • Usually ends with .sh

    • Begins with the magic line: #!/bin/bash

Steps:

  1. Write your script (in Notepad, VS Code, etc.).

  2. Make it executable:

     chmod 700 myscript.sh

  3. Run it:

     ./myscript.sh

Basic Shell Script Example 🚦

    #!/bin/bash

    # This is a comment
    echo "Hello, welcome to my Shell Scripting Tutorial!"

    # Show current date and time
    date

    # Create a directory, move in, and create a file
    mkdir my_directory
    cd my_directory
    touch my_file.txt

    # Write text into the file and display it
    echo "This is some example text." > my_file.txt
    cat my_file.txt

    # Remove the file and directory, go back
    rm my_file.txt
    cd ..
    rmdir my_directory

    echo "Script execution complete.!"

Getting User Input 📥

To make your script interactive, ask the user for info:

  •   #!/bin/bash

  # reads input from the user and puts it in the username variable
  echo "Enter Username: "
  read username
  echo $username

  # displays the prompt message
  # -p stand for prompt
  # reads input from the user and puts it in the newusername variable
  read -p "Enter the new username: " newusername
  echo $newusername

  # reads input from the user & hides the text from echoing in the terminal
  # -s stands for silent
  read -sp "Enter Password: " password
  echo ""
  echo $password

  • if you don’t wish to specify the variable name for the read we can use $REPLY to echo the value

      #!/bin/bash

  echo "Enter the username: "
  read
  echo "Read without variable name assignment: "$REPLY

  • Argument Passing

    We can pass arguments that can be used inside the scripts when it is executed. Those arguments can be accessed by the script using special variables like $1 $2 $3 etc.

    • $0 - returns the file name of the shell script.

    • $@ - returns all arguments passed from cli.

    • $# - returns the no of arguments passed from cli.

Let’s say we have a script file named arguement.sh and let's pass 2 arguments to it.

    argument.sh "Sparsh" "DevOps Engineer"

    #!/bin/bash

    # gives the filename of the script itself
    echo "FileName Argument: "$0 # argument.sh

    # gives the first argument passed
    echo "First Argument: "$1 # Sparsh

    # gives the second argument passed
    echo "Second Argument: "$2 # DevOps Engineer

    # displays all arguments passed
    echo "All Arguments: "$@ # Sparsh DevOps Engineer

    # displays number of arguments passed
    echo "No of Arguments: "$# # 2

Variables & Data Types

Variables are where you store data to reuse later.

Assigning and Using Variables

    #!/bin/bash

    length=10
    width=5
    area=$((length * width))
    echo "The area of the rectangle is: $area"  # Shows 50

Strings and Numbers

    #!/bin/bash

    # String variables
    name="Sparsh"
    greeting="Hello"

    # Concatenating strings
    message="$greeting, $name! How are you?"

    # Displaying the message
    echo $message

    #!/bin/bash

    # Integer variables
    num1=10
    num2=5

    # Performing arithmetic operations
    sum=$((num1 + num2))
    difference=$((num1 - num2))
    product=$((num1 * num2))
    quotient=$((num1 / num2))

    # Displaying the results
    echo "Sum: $sum"
    echo "Difference: $difference"
    echo "Product: $product"
    echo "Quotient: $quotient"

The output will be:

    Sum: 15
    Difference: 5
    Product: 50
    Quotient: 2

Using Environment Variables 🌎

Environment variables are special variables predefined by your operating system or set by users. They hold important information about your system's environment.

    #!/bin/bash

    # Accessing environment variables
    echo "Home directory: $HOME"
    echo "Username: $USER"
    echo "Current working directory: $PWD"
    echo "Operating system: $OSTYPE"

In the above example, we use the echo command to display the values of several common environment variables:

  • $HOME: This variable represents the home directory of the current user. It typically stores the path to the user's folder.

  • $USER: This variable holds the username of the current user.

  • $PWD: This variable stores the absolute path of the current working directory. It provides the location where the script is being executed.

  • $OSTYPE: This variable contains the identifier for the operating system type. It specifies the type of operating system being used, such as "linux-gnu" for Linux systems.

When you run this script, it will output something similar to:

    Home directory: /home/username
    Username: username
    Current working directory: /path/to/current/directory
    Operating system: linux-gnu

The actual values will depend on your system configuration.

  • Conditional Statements

      #!/bin/bash

  a=10
  b=20

  # less than using square brackets
  if [[ $a -lt $b ]]
  then
     echo "a is less than b"
  else
     echo "a is not less than b"
  fi

    In shell scripting, [[ ]] or test command can be used for evaluating conditional expressions. Below are certain unary operators that can be used for testing the conditions

    Conditions

    • [[ -z STRING ]] - Empty string

    • [[ -n STRING ]] - Not empty string

    • [[ STRING == STRING ]] - Equal

    • [[ STRING != STRING ]] - Not equal

    • [[ NUM -eq NUM ]] - Equal

    • [[ NUM -ne NUM ]] - Not equal

    • [[ NUM -lt NUM ]] - Less than

    • [[ NUM -le NUM ]] - Less than or equal

    • [[ NUM -gt NUM ]] - Greater than

    • [[ NUM -ge NUM ]] - Greater than or equal

    • [[ ! EXPR ]] - Not

    • [[ X && Y ]] - And

    • [[ X || Y ]] - Or

File Conditions

  • [[ -e FILE ]] - Exists

  • [[ -r FILE ]] - Readable

  • [[ -h FILE ]] - Symbolic link

  • [[ -d FILE ]] - Directory

  • [[ -w FILE ]] - Writable file

  • [[ -s FILE ]] - File size is > 0 bytes

  • [[ -f FILE ]] - File

  • [[ -x FILE ]] - Executable file

    #!/bin/bash

    # -e stands for exists
    if [[ -e ./ifelse.sh ]]
    then
      echo "File exists"
    else
      echo "File does not exist"
    fi

    #!/bin/bash

    read -p "Enter the number between 1 to 3: " number

    if [[ $number -eq 1 ]]
    then
            echo "The number entered is 1"
    elif [[ $number -eq 2 ]]
    then
            echo "The number entered is 2"
    elif [[ $number -eq 3 ]]
    then
            echo "The number entered is 3"
    else
            echo "Invalid Number"
    fi

for

The for loop is used to iterate over a sequence of values and below is the syntax

    #!/bin/bash

    for i in {1..10}
    do
      echo "Val: $i"
    done

while

The while loop is used to execute a set of commands repeatedly as long as a certain condition is true. The loop continues until the condition is false.

    #!/bin/bash

    count=0

    while [ $count -lt 5 ]
    do
      echo $count
      count=$(($count+1))
    done

Arrays

An array is a variable that can hold multiple values under a single name

  • ${arrayVarName[@]} - displays all the values of the array.

  • ${#arrayVarName[@]} - displays the lenght of the array.

  • ${arrayVarName[0]} - displays the first element of the array

  • ${arrayVarName[-1]} - displays the last element of the array

  • unset arrayVarName[2] - deletes the 2 element

    #!/bin/bash

    # Declare an array of fruits
    fruits=("apple" "banana" "orange" "guava")

    # Print the entire array
    echo "All fruits using @ symbol: ${fruits[@]}"
    echo "All fruits using * symbol: ${fruits[*]}"

    # Print the third element of the array
    echo "Third fruit: ${fruits[2]}"

    # Print the length of the array
    echo "Number of fruits: ${#fruits[@]}"

Break Statement

break is a keyword. It is a control statement that is used to exit out of a loop ( for, while, or until) when a certain condition is met. It means that the control of the program is transferred outside the loop and resumes with the next set of lines in the script

    #!/bin/bash

    count=1

    while true
    do
      echo "Count is $count"
      count=$(($count+1))
      if [ $count -gt 5 ]; then
        echo "Break statement reached"
        break
      fi
    done

Continue statement

continue is a keyword that is used inside loops (such as for, while, and until) to skip the current iteration of the loop and move on to the next iteration. It means that when the continue keyword is encountered while executing a loop the next set of lines in that loop will not be executed and moves to the next iteration.

    #!/bin/bash

    for i in {1..10}
    do
      if [ $i -eq 5 ]
      then
        continue
      fi
      echo $i
    done

Functions

Functions are a block of code which can be used again and again for doing a specific task thus providing code reusability.

Normal Function

    #!/bin/bash

    sum(){
            echo "The numbers are: $n1 $n2"
            sum_val=$(($n1+$n2))
            echo "Sum: $sum_val"
    }

    n1=$1
    n2=$2
    sum

Function with return values

To access the return value of the function we need to use $? to access that value

    #!/bin/bash

    sum(){
            echo "The numbers are: $n1 $n2"
            sum_val=$(($n1+$n2))
            echo "Sum: $sum_val"
            return $sum_val
    }

    n1=$1
    n2=$2
    sum

    echo "Retuned value from function is $?"
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Sparsh Jaipuriyar
Sparsh Jaipuriyar