Reading files line by line (while read) in Bash Scripting - Mini Project: Build & Apply

Reading files line by line (while read)

📖 Scenario: You have a text file that contains a list of your favorite fruits, one fruit per line. You want to read this file line by line and print each fruit with a friendly message.

🎯 Goal: Build a bash script that reads a file called fruits.txt line by line using while read loop and prints each fruit with the message: "I like <fruit>".

📋 What You'll Learn

Create a file named fruits.txt with the exact fruits listed.

Create a variable to hold the filename.

Use a while read loop to read the file line by line.

Print each fruit with the message: "I like <fruit>".

💡 Why This Matters

🌍 Real World

Reading files line by line is common when processing logs, configuration files, or lists in scripts.

💼 Career

Many automation and system administration tasks require reading files line by line to handle data or perform actions.

1

Create the fruits.txt file

Create a file named fruits.txt with these exact lines:
Apple
Banana
Cherry

Bash Scripting

# Create the file fruits.txt with the fruits listed
# Your code here

Hint

Use cat with a here document to create the file.

2

Set the filename variable

Create a variable called filename and set it to fruits.txt.

Bash Scripting

cat  fruits.txt
Apple
Banana
Cherry
EOF

# Create a variable called filename and set it to fruits.txt
# Your code here

Hint

Use filename="fruits.txt" to assign the variable.

3

Read the file line by line

Use a while read loop with variable fruit to read each line from $filename.

Bash Scripting

cat  fruits.txt
Apple
Banana
Cherry
EOF

filename="fruits.txt"

# Use while read loop to read each fruit from $filename
# Your code here

Hint

Use while read fruit; do ... done < "$filename" to read lines.

4

Print the output

Run the script and print the output lines exactly as:
I like Apple
I like Banana
I like Cherry

Bash Scripting

cat  fruits.txt
Apple
Banana
Cherry
EOF

filename="fruits.txt"

while read fruit; do
    echo "I like $fruit"
done < "$filename"

# Run the script to print the output
# Your code here

Hint

Just run the script to see the output.

Practice

1. What does the command while IFS= read -r line; do echo "$line"; done < file.txt do in a bash script?

easy

A. Deletes the contents of file.txt.

B. Reads the entire file at once and prints it.

C. Writes each line of file.txt to a new file.

D. Reads the file file.txt line by line and prints each line.

Solution

1. Step 1: Understand the while read loop

   The loop reads input line by line from the redirected file file.txt.

2. Step 2: Check the action inside the loop

   Each line read is stored in variable line and printed with echo.

3. Final Answer:

   Reads the file file.txt line by line and prints each line. -> Option D

4. Quick Check:

   Line-by-line reading = A [OK]

Hint: Remember: while read loops read files line by line [OK]

Common Mistakes:

• Thinking it reads the whole file at once
• Confusing input redirection with output
• Ignoring the role of IFS= and -r

2. Which of the following is the correct syntax to read a file line by line in bash?

easy

A. while read line; do echo $line; done < file.txt

B. while read -r line; do echo $line; done > file.txt

C. while read line; do echo $line; done >> file.txt

D. while read line; do echo $line; done | file.txt

Solution

1. Step 1: Identify input redirection for reading

   The correct way to read from a file is using input redirection: < file.txt.

2. Step 2: Check the loop syntax

   while read line; do ... done is the standard loop to read lines.

3. Final Answer:

   while read line; do echo $line; done < file.txt -> Option A

4. Quick Check:

   Input redirection with < = C [OK]

Hint: Use < file.txt to feed file lines into the loop [OK]

Common Mistakes:

• Using output redirection (>) instead of input (<)
• Piping file name instead of redirecting input
• Missing the -r option to prevent backslash escapes

3. What will be the output of this script if file.txt contains:

apple
banana
cherry

Script:

while IFS= read -r line; do echo "Fruit: $line"; done < file.txt

medium

A. Fruit: apple
Fruit: banana
Fruit: cherry

B. apple
banana
cherry

C. Fruit: apple banana cherry

D. Syntax error

Solution

1. Step 1: Understand the loop reading each line

   The loop reads each line from file.txt separately.

2. Step 2: Check the echo command

   Each line is printed prefixed with "Fruit: ".

3. Final Answer:

   Fruit: apple Fruit: banana Fruit: cherry -> Option A

4. Quick Check:

   Line prefixing with echo = B [OK]

Hint: Each loop iteration prints one line with prefix [OK]

Common Mistakes:

• Expecting all lines printed on one line
• Ignoring the prefix in echo
• Confusing input and output redirection

4. Identify the error in this script snippet:

while read line; do echo $line; done file.txt

medium

A. Echo command syntax is wrong

B. Incorrect variable name used

C. Missing input redirection operator < before file.txt

D. Missing semicolon after read

Solution

1. Step 1: Check how the file is provided to the loop

   The file must be redirected as input using < file.txt, not as an argument.

2. Step 2: Confirm syntax correctness

   The rest of the syntax is correct; variable and echo usage are fine.

3. Final Answer:

   Missing input redirection operator < before file.txt -> Option C

4. Quick Check:

   Input redirection needed = D [OK]

Hint: Use < file.txt to feed file lines, not argument [OK]

Common Mistakes:

• Passing filename as argument instead of redirecting input
• Forgetting semicolon after read (optional here)
• Misusing echo syntax

5. You want to read a file line by line and skip empty lines. Which script snippet correctly does this?

hard

A. while read line; do echo "$line"; done < file.txt | grep '^$'

B. while IFS= read -r line; do if [[ -n $line ]]; then echo "$line"; fi; done < file.txt

C. cat file.txt | while read line; do echo "$line"; done

D. while IFS= read -r line; do echo "$line"; done < file.txt

Solution

1. Step 1: Understand skipping empty lines

   We need to check if the line is not empty using [[ -n $line ]].

2. Step 2: Confirm reading and condition

   while IFS= read -r line; do if [[ -n $line ]]; then echo "$line"; fi; done < file.txt reads lines safely and prints only non-empty lines inside the loop.

3. Final Answer:

   while IFS= read -r line; do if [[ -n $line ]]; then echo "$line"; fi; done < file.txt -> Option B

4. Quick Check:

   Condition inside loop filters empty lines = A [OK]

Hint: Use if [[ -n $line ]] inside loop to skip empty lines [OK]

Common Mistakes:

• Using external grep after loop (may lose variable scope)
• Not checking for empty lines inside the loop
• Using unnecessary cat command