awk basics (field processing) in Linux CLI - Mini Project: Build & Apply

awk basics (field processing)

📖 Scenario: You have a text file with information about fruits and their prices. You want to use awk to process this data and extract useful information.

🎯 Goal: Learn how to use awk to read fields from a text file and print specific columns.

📋 What You'll Learn

Use awk to process text data

Understand fields and how to access them with $1, $2, etc.

Print specific fields from each line

💡 Why This Matters

🌍 Real World

Processing text files with columns of data is common in system logs, reports, and CSV files. <code>awk</code> helps extract and manipulate this data quickly.

💼 Career

Many IT, data analysis, and system administration jobs require basic <code>awk</code> skills to automate data extraction and reporting.

1

Create the data file

Create a file named fruits.txt with these exact lines:
Apple 100
Banana 50
Cherry 75

Linux CLI

# Create the file fruits.txt with the given lines
# Your code here

Hint

Use echo with -e and redirect output to fruits.txt.

2

Set the field separator

Write an awk command that uses space as the field separator explicitly with -F " " and reads from fruits.txt.

Linux CLI

awk -F " " 
# Your code here

Hint

Use awk -F " " '{print $1, $2}' fruits.txt to print both fields.

3

Print only the fruit names

Modify the awk command to print only the first field (fruit name) from fruits.txt.

Linux CLI

awk -F " " '{print $1}' fruits.txt
# Your code here

Hint

Use {print $1} to print only the first field.

4

Print fruit names with prices doubled

Write an awk command that prints the fruit name and the price multiplied by 2 from fruits.txt.

Linux CLI

awk -F " " '{print $1, $2 * 2}' fruits.txt
# Your code here

Hint

Use {print $1, $2 * 2} to multiply the second field by 2.

Practice

1. What does $2 represent in an awk command?

easy

A. The number of fields in the current line

B. The entire current line

C. The first character of the line

D. The second field (word) in the current line

Solution

1. Step 1: Understand how awk splits lines

   awk automatically splits each input line into fields separated by spaces or tabs.

2. Step 2: Identify what $2 means

   $2 refers to the second field (word) in the current line.

3. Final Answer:

   The second field (word) in the current line -> Option D

4. Quick Check:

   $2 = second field [OK]

Hint: Remember: $1 is first word, $2 second, and so on [OK]

Common Mistakes:

• Confusing $2 with the whole line ($0)
• Thinking $2 is the number of fields (NF)
• Mixing field numbers with character positions

2. Which of the following is the correct syntax to print the third field of each line using awk?

easy

A. awk '{print $3}' filename

B. awk 'print $3' filename

C. awk '{print 3}' filename

D. awk '{print $}' filename

Solution

1. Step 1: Recall the basic awk print syntax

   The print statement must be inside braces: {print $3}.

2. Step 2: Check each option

   awk '{print $3}' filename uses correct braces and $3. Options B, C, and D have syntax errors or wrong usage.

3. Final Answer:

   awk '{print $3}' filename -> Option A

4. Quick Check:

   Print field 3 with braces [OK]

Hint: Always use braces { } around print commands in awk [OK]

Common Mistakes:

• Omitting braces around print statement
• Printing a number instead of a field variable
• Using incomplete or invalid field syntax

3. Given a file data.txt with the following content:

apple 10 red
banana 5 yellow
cherry 20 red

What is the output of awk '{print $1, $3}' data.txt?

medium

A. apple red
banana yellow
cherry red

B. apple
banana
cherry

C. 10 red
5 yellow
20 red

D. apple 10
banana 5
cherry 20

Solution

1. Step 1: Identify fields in each line

   Each line has three fields: $1=fruit, $2=number, $3=color.

2. Step 2: Understand the print command

   awk '{print $1, $3}' prints the first and third fields separated by space.

3. Final Answer:

   apple red banana yellow cherry red -> Option A

4. Quick Check:

   Print first and third fields [OK]

Hint: Match $1, $2, $3 to columns in order [OK]

Common Mistakes:

• Mixing up field numbers
• Printing wrong fields or all fields
• Ignoring spaces between printed fields

4. What is wrong with this command: awk '{print $4}' data.txt if data.txt has only 3 fields per line?

medium

A. It prints the third field instead

B. It causes a syntax error

C. It prints an empty line for each input line

D. It prints the entire line

Solution

1. Step 1: Understand field numbering in awk

   Fields are numbered from 1 to NF (number of fields). If $4 is requested but only 3 fields exist, $4 is empty.

2. Step 2: Predict output behavior

   Printing $4 prints nothing, resulting in empty lines for each input line.

3. Final Answer:

   It prints an empty line for each input line -> Option C

4. Quick Check:

   Missing field prints empty [OK]

Hint: Fields beyond NF print empty, no error [OK]

Common Mistakes:

• Expecting syntax error for missing fields
• Thinking it prints last field instead
• Assuming it prints whole line if field missing

5. You want to print only the lines from file.txt where the second field is greater than 100 using awk. Which command achieves this?

hard

A. awk '{if $2 > 100 print $0}' file.txt

B. awk '$2 > 100 {print $0}' file.txt

C. awk '{print $0 if $2 > 100}' file.txt

D. awk '$2 => 100 {print $0}' file.txt

Solution

1. Step 1: Understand condition syntax in awk

   Conditions go before the action block. The correct comparison operator is >, not =>.

2. Step 2: Check each option

   awk '$2 > 100 {print $0}' file.txt uses correct syntax: $2 > 100 {print $0}. awk '{if $2 > 100 print $0}' file.txt misses parentheses and braces. awk '{print $0 if $2 > 100}' file.txt has wrong syntax. awk '$2 => 100 {print $0}' file.txt uses invalid operator =>.

3. Final Answer:

   awk '$2 > 100 {print $0}' file.txt -> Option B

4. Quick Check:

   Condition before action, use > operator [OK]

Hint: Put condition before braces, use > not => [OK]

Common Mistakes:

• Using invalid comparison operators like =>
• Placing if inside braces without parentheses
• Omitting braces or condition syntax errors