awk basics (field processing) in Linux CLI - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to print the first field of each line using awk.

Linux CLI

awk '{ print [1] }' file.txt

Drag options to blanks, or click blank then click option'

ANF

B$1

C1

D$0

Attempts:

3 left

💡 Hint

Common Mistakes

Using $0 prints the whole line instead of the first field.

Using NF prints the number of fields, not a field value.

In awk, $1 refers to the first field of the current line.

2fill in blank

medium

Complete the code to print the second and third fields separated by a dash.

Linux CLI

awk '{ print [1]"-"[2] }' file.txt

Drag options to blanks, or click blank then click option'

A$2

B$3

C$1

D$0

Attempts:

3 left

💡 Hint

Common Mistakes

Using $1 instead of $2 for the second field.

Using $0 prints the whole line.

In awk, $2 refers to the second field and $3 refers to the third field.

3fill in blank

hard

Fix the error in the code to print the last field of each line.

Linux CLI

awk '{ print [1] }' file.txt

Drag options to blanks, or click blank then click option'

A$NF

BNF

C$0

D$-1

Attempts:

3 left

💡 Hint

Common Mistakes

Using NF alone prints the number of fields, not the field value.

Using $-1 is invalid syntax.

$NF in awk refers to the last field of the current line.

4fill in blank

hard

Fill both blanks to print the first field and the total number of fields separated by a space.

Linux CLI

awk '{ print [1], [2] }' file.txt

Drag options to blanks, or click blank then click option'

A$1

BNF

C$NF

D$0

Attempts:

3 left

💡 Hint

Common Mistakes

Using $0 instead of $1 for the first field.

Using $NF instead of NF for the number of fields.

The first field is $1 and the total number of fields is NF.

5fill in blank

hard

Fill all three blanks to print the second field, the last field, and the total number of fields separated by commas.

Linux CLI

awk '{ print [1]","[2]","[3] }' file.txt

Drag options to blanks, or click blank then click option'

A$2

B$NF

CNF

D$1

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up $NF and NF.

Using $1 instead of $2 for the second field.

The second field is $2, the last field is $NF, and the total number of fields is NF.

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