Basic Shell Scripting (Ubuntu)

Shell scripting:

A shell is a type of computer program called a command-line interpreter that lets Linux and Unix users control their operating systems with command-line interfaces. Shells allow users to communicate efficiently and directly with their operating systems.


Objectives:

Objectives:

This laboratory aims to learn and practice SHELL scripts by writing small SHELL programs.

The following are the primary objectives of this session:

1. SHELL keywords

2. Arithmetic in SHELL script

3. Control Structures

a. Decision control

b. Repetition control

4. More UNIX commands

5. Executing commands during login time

Handling shell variables:

The shell has several variables which are automatically set whenever you log in. The values of some of these variables are stored in names which collectively are called your user environment. Any name defined in the user environment can be accessed from within a shell script. To include the value of a shell variable into the environment you must export it.

Suppose you have a shell variable called MY_VARIABLE with the value Hello World!, and you want to use this variable in a shell script. You can access the value of a shell variable by putting a dollar sign ($) before the variable name.

Now, if you want to access the value of MY_VARIABLE within a shell script, you need to export it:

Passing arguments to the shell (with predefined variables)

#!/bin/bash
# Usage: SS1 param1 param2 param3 param 4 param5
# Script to accept 5 numbers and display their sum.
echo the parameters passed are : $1, $2, $3, $4, $5
echo the name of the script is : $0
echo the number of parameters passed are : $#
echo the process number of this shell : $$
echo the string containing all the arguments to the shell, starting at s1 All parameters : $*
echo same as above, except when quoted : $@
((sum= $1 + $2 + $3 + $4 + $5))
echo The sum is : $sum

Why need for shift command?

In shell scripting, when you run a script and pass arguments to it, those arguments are stored in special variables called “positional parameters.” You can access these parameters using $1, $2, $3, and so on, where $1 represents the first argument, $2 represents the second argument, and so forth.

However, there’s a limitation: the shell only recognizes positional parameters up to $9. If you pass more than 9 arguments to a script, you can’t access them directly using $10, $11, and so on.

This is where the shift command comes into play. It allows you to shift the positional parameters, effectively “moving” them one position to the left. When you use shift, the value of $2 becomes $1, the value of $3 becomes $2, and so on. The first positional parameter $1 is lost in this process.

A sum of all with more than 9 arguments

#!/bin/bash
# Initialize a variable to hold the sum
sum=0
# Loop through all the arguments
for arg in "$@"; do
    # Add each argument to the sum
    sum=$((sum + arg))
done

# Print the sum
echo "The sum of all arguments is: $sum"

Control Structures:

Every UNIX command returns an exit value that the shell can interrogate. This value is held

in the read-only shell variable $?.

A value of 0 (zero) signifies success; anything other than 0 (zero) signifies failure.

The test command in UNIX is used to check conditions and returns either a 0 (zero) if the condition is true or a 1 (non-zero) if the condition is false. This result indicates success or failure respectively.

Here’s how it works in simpler terms:

1. Purpose of the test Command: The test command helps you evaluate conditions in shell scripts. It checks whether a condition is true or false.
2. Exit Status: After running the test command, it sets its exit status based on whether the condition is true or false. If the condition is true, the test returns a 0. If the condition is false, it returns a non-zero value (typically 1).
3. Integration with if Statement: You can use the exit status of the test command directly within an if statement. If the condition succeeds (exit status is 0), the corresponding block of code within the if statement is executed. If the condition fails (exit status is non-zero), the code within the else block (if present) is executed.

Operators on Numeric Variables used with test command

Operators on Numeric Variables used with test command:
-eq : equal to
-ne : not equals to
-gt : grater than
-lt : less than
-ge : greater than or equal to
-le : less than equal to

#!/bin/bash
# Get two variables from the user
read -p "Enter the First integer " num1
read -p "Enter the second integer " num2
# Check if num1 is equal to num2
if test "$num1" -eq "$num2"
then
echo "$num1 is equal to $num2"
else
echo "$num1 is not equal to $num2"
fi
# Check if num1 is not equal to num2
if test "$num1" -ne "$num2"
then
echo "$num1 is not eual to $num2"
else
echo "$num1 is equal to $num2"
fi
# Check if num1 is greater than num2
if test "$num1" -gt "$num2"
then
echo "$num1 is greater than $num2"
else
echo "$num1 is less than $num2"
fi
# Check if num1 is less than num2
if test "$num1" -lt "$num2"
then
echo "$num1 is less than $num2"
else
echo "$num1 is not less than $num2"
fi

# Check if num1 is greater than or equal to num2
if test "$num1" -ge "$num2"
then
echo "$num1 is greater than or equal to $num2"
else
echo "$num1 is less than $num2"
fi

# Check if num1 is less than or equal to num2
if test "$num1" -le "$num2"
then
echo "$num1 is less than or equal to $num2"
else
echo "$num1 is greater than $num2"
fi

Operators on String Variables used with test command

= : equality of strings
!= : not equal
-z : zero length string (i.e. string containing zero character i.e. null string).
-n : String length is non zero.

#!/bin/bash
# Define two string variables
read -p "Enter the First String " str1
read -p "Enter the second String " str2
empty_str=""
# Check if str1 is equal to str2
if test "$str1" = "$str2"
then
    echo "$str1 is equal to $str2"
else
    echo "$str1 is not equal to $str2"
fi
# Check if str1 is not equal to str2
if test "$str1" != "$str2"
then
    echo "$str1 is not equal to $str2"
else
    echo "$str1 is equal to $str2"
fi
# Check if str1 has zero length
if test -z "$str1"
then
    echo "String str1 is empty"
else
    echo "String str1 is not empty"
fi

# Check if str2 has non-zero length
if test -n "$str2"
then
    echo "String str2 is not empty"
else
    echo "String str2 is empty"
fi

# Check if empty_str has zero length
if test -z "$empty_str"
then
    echo "String empty_str is empty"
else
    echo "String empty_str is not empty"
fi

Operators on files used with the test command

-f : the file exists.
-s : the file exists and the file size is non zero.
-d : directory exists.
-r : file exits and has read permission.
-w : file exists and has write permission.
-x : file exists and has execute permission.

#!/bin/bash

# Set the file path to your drive
mydrive="/home/learn/Desktop/mydrive"
# Check if the file exists
if test -f "$mydrive"
then
    echo "The file $mydrive exists."
else
    echo "The file $mydrive does not exist."
fi
# Check if the file exists and has non-zero size
if test -s "$mydrive"
then
    echo "The file $mydrive exists and has a non-zero size."
Else
    echo "The file $mydrive either does not exist or has zero size."
fi
# Check if the path is a directory
if test -d "$mydrive"
then
    echo "$mydrive is a directory."
else
    echo "$mydrive is not a directory."
fi
# Check if the file exists and has read permission
if test -r "$mydrive"
then
    echo "The file $mydrive exists and has read permission."
else
    echo "The file $mydrive either does not exist or does not have read permission."
fi
# Check if the file exists and has write permission
if test -w "$mydrive"
then
    echo "The file $mydrive exists and has write permission."
else
    echo "The file $mydrive either does not exist or does not have write permission."
fi
# Check if the file exists and has execute permission
if test -x "$mydrive"
then
    echo "The file $mydrive exists and has execute permission."
else
    echo "The file $mydrive either does not exist or does not have execute permission."
fi

Logical Operators used with the test command

Combining more than one condition is done through the logical AND, OR and NOT operators.
-a : logical AND
-o : logical OR
! : logical NOT

#!/bin/bash

# Prompt the user to enter a number
read -p "Enter any integer " num
# Check if the number is greater than 10 and less than 20(-a : logical AND)
if test "$num" -gt 10 -a "$num" -lt 20
then
    echo "$num is greater than 10 AND less than 20"
else
    echo "$num is not greater than 10 OR not less than 20"
fi

# Check if the number is less than 0 OR greater than 100(-o : logical OR)
if test "$num" -lt 0 -o "$num" -gt 100
then
    echo "$num is less than 0 OR greater than 100"
else
    echo "$num is not less than 0 AND not greater than 100"
fi

# Check if the number is NOT equal to 50(! : logical NOT)
if test ! "$num" -eq 50
then
    echo "$num is NOT equal to 50"
else
    echo "$num is equal to 50"
fi

list of users currently logged:

if who | grep -s student > /dev/null
then
echo student is logged in
else
echo student is not available
fi

Explanation:

This lists who is currently logged on to the system and pipes the output through grep to search for

the username student.

The -s option causes grep to work silently and any error messages are directed to the file /dev/null instead of the standard output.

If the command is successful i.e. the username student is found in the list of users currently logged

in then the message student is logged on is displayed, otherwise, the second message is displayed.

The Case Statement

#!/bin/bash

while true; do
    echo "Choose an option: "
    echo "1. Display current date and time"
    echo "2. Display calendar"
    echo "3. List files in current directory"
    echo "4. Display currently logged-in users"
    echo "5. Print current working directory"
    echo "6. Display system information"
    echo "7. Show disk usage"
    echo "8. Display network information"
    echo "9. Show running processes"
    echo "10. Display system memory usage"
    echo "11. Show user information"
    echo "12. Display kernel version"
    echo "13. Show available disk space"
    echo "14. Show system load"
    echo "15. Display system uptime"
    echo "16. List installed packages"
    echo "17. Show hostname"
    echo "18. Show active network connections"
    echo "19. Show file system information"
    echo "20. Show logged-in users and their processes"
    echo "21. Exit"

    read choice

    case $choice in
        1)
            echo "Current date and time: $(date)"
            ;;
        2)
            echo "Calendar:"
            cal
            ;;
        3)
            echo "Files in current directory:"
            ls -l
            ;;
        4)
            echo "Currently logged-in users:"
            who
            ;;
        5)
            echo "Current working directory:"
            pwd
            ;;
        6)
            echo "System Information:"
            uname -a
            ;;
        7)
            echo "Disk Usage:"
            df -h
            ;;
        8)
            echo "Network Information:"
            ifconfig
            ;;
        9)
            echo "Running Processes:"
            ps aux
            ;;
        10)
            echo "System Memory Usage:"
            free -m
            ;;
        11)
            echo "User Information:"
            id
            ;;
        12)
            echo "Kernel Version:"
            uname -r
            ;;
        13)
            echo "Available Disk Space:"
            du -h --max-depth=1 /
            ;;
        14)
            echo "System Load:"
            uptime
            ;;
        15)
            echo "System Uptime:"
            uptime -p
            ;;
        16)
            echo "Installed Packages:"
            dpkg --get-selections
            ;;
        17)
            echo "Hostname:"
            hostname
            ;;
        18)
            echo "Active Network Connections:"
            netstat -tuln
            ;;
        19)
            echo "File System Information:"
            df -T
            ;;
        20)
            echo "Logged-in Users and Their Processes:"
            w
            ;;
        21)
            echo "Exiting program."
            break
            ;;
        *)
            echo "Invalid option. Please select a valid option (1-21)."
            ;;
    esac
done

Example of using for statement

Another Example

#!/bin/bash

# Display a prompt asking the user to enter a command
echo "Please enter a command:"

# Start a loop to continuously read and execute commands
while true; do
    # Read user input into the variable 'response'
    read response
    
    # Check the value of 'response'
    case "$response" in
        'done')
            # If 'done' is entered, exit the loop
            break
            ;;
        "")
            # If no command is entered (empty input), prompt again
            continue
            ;;
        *)
            # For any other command, execute it using 'eval'
            eval "$response"
            ;;
    esac
done

Problem Solving:

Write a shell script that takes a keyword as a command line argument and lists the filenames containing the keyword.

#!/bin/bash

# Check if a keyword is provided as a command line argument
#[ -z "$1" ] checks if the first command line argument ($1) is empty.

if [ -z "$1" ]; then
    echo "Please provide a keyword."
    exit 1
fi

# Loop through each file in the current directory
#* is a wildcard that matches all files in the current directory.

for file in *; do
    # Check if the file contains the keyword
#grep -q "$1" "$file" searches for the keyword $1 in the file $file.
#The -q option is used to suppress grep's output.
# It just checks if the keyword exists without printing any matching lines.
#If the keyword is found in the file, grep exits with a success status,
# and the if statement evaluates to true
    if grep -q "$1" "$file"; then
        echo "$file"
    fi
done

2)Write a shell script that takes a command line argument and reports whether it is a directory, a file, or a link.

#!/bin/bash

# Check if a command line argument is provided
if [ -z "$1" ]; then
    echo "Please provide a filename or directory."
    exit 1
fi

# Check if the provided argument is a directory
if [ -d "$1" ]; then
    echo "$1 is a directory."
    exit 0
fi

# Check if the provided argument is a regular file
if [ -f "$1" ]; then
    echo "$1 is a regular file."
    exit 0
fi

# Check if the provided argument is a symbolic link
if [ -L "$1" ]; then
    echo "$1 is a symbolic link."
    exit 0
fi

# If none of the above conditions are met, the argument is not found
echo "$1 is not found."

3)Write a script to find the number of sub-directories in a given directory.

#!/bin/bash

# Check if a directory path is provided as a command line argument
if [ -z "$1" ]; then
    echo "Please provide a directory path."
    exit 1
fi

# Initialize a counter variable
count=0

# Loop through each entry in the directory
for entry in "$1"/*; do
    # Check if the entry is a directory
    if [ -d "$entry" ]; then
        ((count++))
    fi
done

# Print the number of subdirectories
echo "Number of subdirectories in '$1': $count"

4)Write a menu-driven program that has the following options.

a) Search whether a given file is in the directory or not.

b) Display the names of the users logged in

#!/bin/bash

# Prompt the user to select an option
echo "Menu:"
echo "1. Search a file in the directory"
echo "2. Display names of users logged in"
echo "3. Exit"

# Read user's choice
read -p "Enter your choice: " choice

# Perform action based on user's choice
if [ "$choice" = "1" ]; then
    read -p "Enter the filename to search: " filename
    if [ -e "$filename" ]; then
        echo "File '$filename' exists in the directory."
    else
        echo "File '$filename' does not exist in the directory."
    fi
elif [ "$choice" = "2" ]; then
    if who > /dev/null; then
        echo "Users logged in:"
        who | cut -d' ' -f1 | sort -u
    else
        echo "No users are currently logged in."
    fi
elif [ "$choice" = "3" ]; then
    echo "Exiting program."
    exit 0
else
    echo "Invalid choice. Please enter a number between 1 and 3."
fi

Conclusion:

In this shell scripting lab, we covered the basics of writing Bash scripts, including user input/output, conditional statements, loops, and file operations. We also explored advanced topics such as process substitution and text processing. Through progressively simplified examples, we demonstrated how to create a menu-driven program and automate tasks efficiently.