awk patterns and actions in Linux CLI - Mini Project: Build & Apply

Learn awk Patterns and Actions

📖 Scenario: You have a small text file listing fruits and their quantities. You want to use awk to find and display fruits with quantities greater than a certain number.

🎯 Goal: Build an awk command that filters and prints fruits with quantities above a threshold.

📋 What You'll Learn

Create a text file named fruits.txt with exact content

Define a variable threshold with a numeric value

Write an awk command using a pattern to filter lines where quantity is greater than threshold

Print the fruit name and quantity for matching lines

💡 Why This Matters

🌍 Real World

Filtering and processing text data files is common in system administration and data analysis.

💼 Career

Knowing how to use awk patterns and actions helps automate log analysis, report generation, and data extraction tasks.

1

Create the data file fruits.txt

Create a file named fruits.txt with these exact lines:
apple 10
banana 5
cherry 12
date 7
elderberry 3

Linux CLI

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

Hint

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

2

Set the quantity threshold variable

Create a shell variable named threshold and set it to 6

Linux CLI

echo -e "apple 10\nbanana 5\ncherry 12\ndate 7\nelderberry 3" > fruits.txt
# Set the variable threshold to 6
# Your code here

Hint

Use threshold=6 to set the variable.

3

Write the awk command with pattern and action

Write an awk command that reads fruits.txt and prints the fruit name and quantity only if the quantity (second column) is greater than the shell variable threshold. Use the pattern $2 > threshold and print both fields.

Linux CLI

echo -e "apple 10\nbanana 5\ncherry 12\ndate 7\nelderberry 3" > fruits.txt
threshold=6
# Write the awk command below
# Your code here

Hint

Use awk -v threshold="$threshold" '$2 > threshold {print $1, $2}' fruits.txt.

4

Run the awk command and display output

Run the awk command from Step 3 and print the output showing fruits with quantity greater than threshold.

Linux CLI

echo -e "apple 10\nbanana 5\ncherry 12\ndate 7\nelderberry 3" > fruits.txt
threshold=6
awk -v threshold="$threshold" '$2 > threshold {print $1, $2}' fruits.txt
# Run the command and print output
# Your code here

Hint

The output should list fruits with quantities above 6.

Practice

1. In awk, what does a pattern do?

easy

A. It selects which lines to process

B. It changes the file content permanently

C. It saves the output to a file

D. It runs a shell command

Solution

1. Step 1: Understand the role of patterns in awk

   Patterns in awk are used to match lines in the input data. Only lines matching the pattern are processed.

2. Step 2: Differentiate patterns from actions

   Actions define what to do with matched lines, but patterns decide which lines to select.

3. Final Answer:

   It selects which lines to process -> Option A

4. Quick Check:

   Pattern = line selector [OK]

Hint: Patterns filter lines before actions run [OK]

Common Mistakes:

• Thinking patterns change file content
• Confusing patterns with output commands
• Assuming patterns run shell commands

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

easy

A. awk '{print $}' file.txt

B. awk 'print $1' file.txt

C. awk '{print 1}' file.txt

D. awk '{print $1}' file.txt

Solution

1. Step 1: Identify correct awk print syntax

   The print statement must be inside braces {} and use $1 to refer to the first field.

2. Step 2: Check each option

   awk '{print $1}' file.txt uses correct braces and $1. awk 'print $1' file.txt misses braces. awk '{print 1}' file.txt prints number 1 literally. awk '{print $}' file.txt uses invalid $.

3. Final Answer:

   awk '{print $1}' file.txt -> Option D

4. Quick Check:

   Print first field = awk '{print $1}' [OK]

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

Common Mistakes:

• Omitting braces around print
• Using $ without field number
• Printing literal numbers instead of fields

3. What is the output of this command on a file with lines:
1: apple 10
2: banana 20
3: cherry 30

awk '$2 > 15 {print $1}' file.txt?

medium

A. apple

B. banana
cherry

C. 10
20
30

D. apple
banana
cherry

Solution

1. Step 1: Understand the pattern condition

   The pattern '$2 > 15' means only lines where the second field is greater than 15 are selected.

2. Step 2: Apply the condition to each line

   Line 1: 10 > 15? No. Line 2: 20 > 15? Yes. Line 3: 30 > 15? Yes.

3. Step 3: Print the first field of matching lines

   For lines 2 and 3, print $1: 'banana' and 'cherry'.

4. Final Answer:

   banana cherry -> Option B

5. Quick Check:

   Fields > 15 print first field = banana, cherry [OK]

Hint: Check pattern filters before printing fields [OK]

Common Mistakes:

• Printing all lines ignoring pattern
• Printing second field instead of first
• Confusing numeric comparison with string

4. Identify the error in this awk command:
awk '$1 == "apple" print $2' file.txt

medium

A. Incorrect field number

B. Wrong comparison operator

C. Missing braces {} around action

D. Quotes around apple should be single quotes

Solution

1. Step 1: Check awk syntax for pattern and action

   Awk requires actions to be inside braces {} after the pattern.

2. Step 2: Identify missing braces

   The command misses braces around 'print $2', causing syntax error.

3. Final Answer:

   Missing braces {} around action -> Option C

4. Quick Check:

   Actions need braces {} [OK]

Hint: Always put actions inside {} in awk [OK]

Common Mistakes:

• Omitting braces around actions
• Using wrong comparison operators
• Misquoting strings inside patterns

5. You want to print the second field only for lines where the first field starts with 'a' or 'b'. Which awk command achieves this?

hard

A. awk '$1 ~ /^[ab]/ {print $2}' file.txt

B. awk '$1 == "a" || $1 == "b" {print $2}' file.txt

C. awk '$1 = "a" || $1 = "b" {print $2}' file.txt

D. awk '$1 ~ /a|b/ {print $2}' file.txt

Solution

1. Step 1: Understand pattern for fields starting with 'a' or 'b'

   The regex /^[ab]/ matches strings starting with 'a' or 'b'. The operator ~ means regex match.

2. Step 2: Check each option's correctness

   awk '$1 ~ /^[ab]/ {print $2}' file.txt uses correct regex and syntax. awk '$1 == "a" || $1 == "b" {print $2}' file.txt compares whole field to 'a' or 'b' exactly, not starts with. awk '$1 = "a" || $1 = "b" {print $2}' file.txt uses assignment '=' instead of comparison '=='. awk '$1 ~ /a|b/ {print $2}' file.txt matches anywhere 'a' or 'b' appears, not just start.

3. Final Answer:

   awk '$1 ~ /^[ab]/ {print $2}' file.txt -> Option A

4. Quick Check:

   Regex match start with a or b = awk '$1 ~ /^[ab]/ {print $2}' file.txt [OK]

Hint: Use regex ^[ab] to match start of field [OK]

Common Mistakes:

• Using = instead of == for comparison
• Matching exact string instead of start
• Using wrong regex without ^ anchor