Why file I/O is core to scripting in Bash Scripting - See It in Action

Why file I/O is core to scripting

📖 Scenario: You work as a helper for a small shop. The shop keeps a list of products and their prices in a text file. You want to write a script that reads this file, finds products that cost more than a certain amount, and saves those products to a new file. This helps the shop owner quickly see expensive items.

🎯 Goal: Build a bash script that reads a file with product prices, filters products above a price limit, and writes them to a new file.

📋 What You'll Learn

Create a file named products.txt with exact product and price lines

Create a variable price_limit with a specific value

Use a while loop to read products.txt line by line

Write products with price greater than price_limit to expensive.txt

Print the contents of expensive.txt at the end

💡 Why This Matters

🌍 Real World

Scripts often read and write files to automate tasks like filtering logs, processing data, or generating reports.

💼 Career

Knowing file input/output is essential for automation roles, system administration, and any scripting job to handle real data.

1

Create the product list file

Create a file named products.txt with these exact lines:
Apple 50
Banana 20
Cherry 75
Date 30
Elderberry 90

Bash Scripting

# Create the file products.txt with the exact lines
# Your code here

Hint

Use cat with a here-document to create the file with exact lines.

2

Set the price limit variable

Create a variable called price_limit and set it to 40.

Bash Scripting

cat  products.txt
Apple 50
Banana 20
Cherry 75
Date 30
Elderberry 90
EOF

# Set price_limit to 40
# Your code here

Hint

Use price_limit=40 to set the variable.

3

Filter products above the price limit

Use a while loop with read to read products.txt line by line. For each line, split into product and price. If price is greater than price_limit, append the line to a file named expensive.txt. Clear expensive.txt before the loop starts.

Bash Scripting

cat  products.txt
Apple 50
Banana 20
Cherry 75
Date 30
Elderberry 90
EOF

price_limit=40

# Clear expensive.txt
# Use while read loop to filter and save products
# Your code here

Hint

Use while read product price; do ... done < products.txt and test price with [ "$price" -gt "$price_limit" ].

4

Display the filtered products

Print the contents of expensive.txt using cat.

Bash Scripting

cat expensive.txt
# Your code here

Hint

Use cat expensive.txt to show the filtered products.

Practice

1. Why is file input/output (I/O) important in bash scripting?

easy

A. It makes scripts run faster by using less memory.

B. It allows scripts to read from and write to files, automating tasks.

C. It helps scripts connect to the internet automatically.

D. It prevents scripts from crashing on errors.

Solution

1. Step 1: Understand file I/O purpose

   File I/O lets scripts handle data stored in files, such as logs or reports.

2. Step 2: Connect file I/O to automation

   By reading and writing files, scripts can automate many tasks involving data processing.

3. Final Answer:

   It allows scripts to read from and write to files, automating tasks. -> Option B

4. Quick Check:

   File I/O = Automate file data handling [OK]

Hint: File I/O means reading/writing files in scripts [OK]

Common Mistakes:

• Confusing file I/O with internet access
• Thinking file I/O speeds up script execution
• Assuming file I/O prevents all script errors

2. Which of the following is the correct way to redirect the output of a command to a file in bash?

easy

A. command |> output.txt

B. command >> output.txt <

C. command < output.txt

D. command > output.txt

Solution

1. Step 1: Identify output redirection syntax

   In bash, using > after a command sends its output to a file, overwriting it.

2. Step 2: Check other options for correctness

   > is correct; >> appends, < reads input, | pipes output, so only > fits the question.

3. Final Answer:

   command > output.txt -> Option D

4. Quick Check:

   Output redirection uses > [OK]

Hint: Use > to write output to a file, overwriting it [OK]

Common Mistakes:

• Using < instead of > for output
• Confusing >> (append) with > (overwrite)
• Using invalid symbols like |>

3. What will be the content of the file output.txt after running this script?

echo "Hello" > output.txt
echo "World" >> output.txt

medium

A. Hello
World

B. World

C. Hello

D. HelloWorld

Solution

1. Step 1: Analyze first echo command

   The first echo writes "Hello" to output.txt, overwriting any existing content.

2. Step 2: Analyze second echo command

   The second echo appends "World" to output.txt, adding it on a new line.

3. Final Answer:

   Hello World -> Option A

4. Quick Check:

   First overwrite, then append with newline [OK]

Hint: > overwrites file, >> appends with newline [OK]

Common Mistakes:

• Thinking >> overwrites instead of appends
• Ignoring newline added by echo
• Assuming output is just 'Hello' or 'World'

4. Identify the error in this script that tries to read from a file and print its content:

cat < input.txt

medium

A. The command should be 'cat >> input.txt' to append content.

B. The file name should be after the command without <, like 'cat > input.txt'.

C. The input redirection < is unnecessary; use 'cat input.txt' instead.

D. The script needs quotes around the file name.

Solution

1. Step 1: Understand cat command usage

   cat reads and prints file content when given the filename as an argument.

2. Step 2: Check input redirection usage

   Using < input.txt is valid but redundant here; 'cat input.txt' is simpler and standard.

3. Final Answer:

   The input redirection < is unnecessary; use 'cat input.txt' instead. -> Option C

4. Quick Check:

   cat filename is simpler than cat < filename [OK]

Hint: Use 'cat filename' to read files, not input redirection [OK]

Common Mistakes:

• Confusing input redirection with output redirection
• Using > or >> instead of < for input
• Adding unnecessary quotes around filename

5. You want to create a script that reads a list of usernames from users.txt and writes a greeting for each user into greetings.txt. Which script snippet correctly does this?

hard

A. while read user; do echo "Hello, $user!" >> greetings.txt; done < users.txt

B. for user in users.txt; do echo "Hello, $user!" > greetings.txt; done

C. cat users.txt | echo "Hello, $user!" > greetings.txt

D. echo "Hello, $(cat users.txt)!" > greetings.txt

Solution

1. Step 1: Understand reading lines from a file

   The 'while read user; do ... done < users.txt' reads each line (username) from users.txt correctly.

2. Step 2: Check writing greetings

   Inside the loop, echo appends a greeting line to greetings.txt for each user.

3. Final Answer:

   while read user; do echo "Hello, $user!" >> greetings.txt; done < users.txt -> Option A

4. Quick Check:

   Loop reads lines and appends greetings [OK]

Hint: Use while read loop with input redirection and append >> [OK]

Common Mistakes:

• Using for loop incorrectly on filename instead of lines
• Overwriting greetings.txt inside loop instead of appending
• Misusing cat with echo without looping