Command-line arguments ($1, $2, ...) in Bash Scripting - Mini Project: Build & Apply

Using Command-line Arguments in Bash Scripts

📖 Scenario: You want to create a simple bash script that takes two numbers as input when you run it from the command line. Then, the script will add these two numbers and show the result.This is like when you use a calculator app and enter two numbers to get their sum, but here you will type the numbers next to the script name in the terminal.

🎯 Goal: Build a bash script that reads two numbers from the command line arguments $1 and $2, adds them, and prints the sum.

📋 What You'll Learn

Create a bash script that uses command-line arguments

Use $1 and $2 to get the first and second input numbers

Add the two numbers inside the script

Print the result to the terminal

💡 Why This Matters

🌍 Real World

Many command-line tools accept inputs as arguments. Learning to use <code>$1</code>, <code>$2</code>, etc., helps automate tasks and create flexible scripts.

💼 Career

Understanding command-line arguments is essential for system administrators, DevOps engineers, and anyone writing shell scripts to automate workflows.

1

Create a bash script file

Create a bash script file named add_numbers.sh and add the first line #!/bin/bash to specify the script interpreter.

Bash Scripting

# Your code here

Hint

The first line in a bash script should be #!/bin/bash to tell the system to use bash to run the script.

2

Assign command-line arguments to variables

Inside add_numbers.sh, create two variables called num1 and num2 and assign them the values of the first and second command-line arguments $1 and $2 respectively.

Bash Scripting

#!/bin/bash

# Assign command-line arguments to variables
# Your code here

Hint

Use num1=$1 and num2=$2 to get the first and second inputs from the command line.

3

Add the two numbers

Add the two variables num1 and num2 using arithmetic expansion and store the result in a variable called sum.

Bash Scripting

#!/bin/bash

num1=$1
num2=$2

# Calculate the sum of num1 and num2
# Your code here

Hint

Use sum=$((num1 + num2)) to add the two numbers in bash.

4

Print the sum

Print the value of sum to the terminal using echo.

Bash Scripting

#!/bin/bash

num1=$1
num2=$2

sum=$((num1 + num2))

# Print the sum
# Your code here

Hint

Use echo $sum to show the result on the screen.

Try running the script with ./add_numbers.sh 7 8 to see the output 15.

Practice

1. What does $1 represent in a bash script?

easy

A. The last command-line argument passed to the script

B. The total number of arguments passed to the script

C. The name of the script file

D. The first command-line argument passed to the script

Solution

1. Step 1: Understand command-line arguments in bash

   In bash scripts, $1 refers to the first argument given after the script name.

2. Step 2: Differentiate from other special variables

   $# is the count of arguments, $0 is the script name, and $@ is all arguments.

3. Final Answer:

   The first command-line argument passed to the script -> Option D

4. Quick Check:

   $1 = first argument [OK]

Hint: Remember $1 is always the first argument after script name [OK]

Common Mistakes:

• Confusing $1 with $# (argument count)
• Thinking $1 is the script name
• Mixing $1 with $0 or $@

2. Which of the following is the correct way to access the third argument in a bash script?

easy

A. $0

B. $2

C. $3

D. $#

Solution

1. Step 1: Recall argument numbering in bash

   Arguments start from $1 for the first, so the third argument is $3.

2. Step 2: Identify incorrect options

   $2 is second argument, $# is count, $0 is script name.

3. Final Answer:

   $3 -> Option C

4. Quick Check:

   Third argument = $3 [OK]

Hint: Count arguments starting at $1, so third is $3 [OK]

Common Mistakes:

• Using $2 instead of $3 for third argument
• Confusing $# with argument value
• Using $0 which is script name

3. What will be the output of this script if run as ./script.sh apple banana cherry?

echo "First: $1"
echo "Second: $2"
echo "Count: $#"

medium

A. First: apple
Second: banana
Count: 3

B. First: apple
Second: cherry
Count: 3

C. First: ./script.sh
Second: apple
Count: 3

D. First: banana
Second: cherry
Count: 2

Solution

1. Step 1: Identify argument values

   Arguments are apple ($1), banana ($2), cherry ($3). Script name is $0.

2. Step 2: Understand output commands

   echo prints $1 as apple, $2 as banana, and $# as 3 (number of arguments).

3. Final Answer:

   First: apple Second: banana Count: 3 -> Option A

4. Quick Check:

   Arguments match output [OK]

Hint: Remember $1, $2 are first and second args; $# is count [OK]

Common Mistakes:

• Mixing script name ($0) with arguments
• Confusing argument order
• Miscounting number of arguments

4. Given this script snippet:

echo "Argument 1: $1"
echo "Argument 2: $2"

What is the error if you run the script without any arguments?

medium

A. Script prints empty lines for $1 and $2

B. No output is printed

C. Script throws a syntax error

D. Script prints the script name instead of arguments

Solution

1. Step 1: Understand missing arguments behavior

   If no arguments are passed, $1 and $2 are empty strings, not errors.

2. Step 2: Check script output

   echo prints "Argument 1: " and "Argument 2: " with nothing after colon.

3. Final Answer:

   Script prints empty lines for $1 and $2 -> Option A

4. Quick Check:

   Missing args = empty $1, $2 [OK]

Hint: No args means $1, $2 are empty, no errors [OK]

Common Mistakes:

• Expecting syntax or runtime error
• Thinking script name prints as argument
• Assuming no output at all

5. You want a bash script to print all arguments passed to it, each on a new line, using only $1, $2, etc. Which approach works best?

hard

A. Use a loop with a counter and access arguments by $1, $2, ... until counter > $#

B. Use shift command in a loop to access each argument and print it

C. Use $0 to print all arguments one by one

D. Use echo $@ to print all arguments on one line

Solution

1. Step 1: Understand how to access all arguments one by one

   Using shift moves arguments left, so $1 becomes next argument each time.

2. Step 2: Compare options

   Use a loop with a counter and access arguments by $1, $2, ... until counter > $# is complex and error-prone; Use echo $@ to print all arguments on one line prints all on one line; Use $0 to print all arguments one by one prints script name; Use shift command in a loop to access each argument and print it correctly loops and prints each argument on new line.

3. Final Answer:

   Use shift command in a loop to access each argument and print it -> Option B

4. Quick Check:

   Shift loops through args one by one [OK]

Hint: Use shift in loop to print each argument separately [OK]

Common Mistakes:

• Using $0 instead of arguments
• Trying to manually count without shift
• Printing all args on one line instead of separate lines