awk patterns and actions 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'

A1

BNF

C$1

D$0

Attempts:

3 left

💡 Hint

Common Mistakes

Using 1 instead of $1 prints the number 1 literally.

Using $0 prints the whole line, not just the first field.

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

2fill in blank

medium

Complete the code to print lines where the second field equals 'error'.

Linux CLI

awk '$2 == [1] { print $0 }' logfile.txt

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

A"error"

Berror"

C'error'

Derror

Attempts:

3 left

💡 Hint

Common Mistakes

Not using quotes causes awk to treat error as a variable.

Using single quotes inside the awk script breaks syntax.

String values in awk patterns must be enclosed in double quotes.

3fill in blank

hard

Fix the error in the awk command to print lines where the third field is greater than 100.

Linux CLI

awk '$3 [1] 100 { print $0 }' data.txt

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

A==

B<

C=>

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using => causes syntax error in awk.

Using == checks for equality, not greater than.

The correct operator for 'greater than' is >, not =>.

4fill in blank

hard

Fill both blanks to print the first and third fields separated by a space.

Linux CLI

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

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

A$1

B$2

C$3

D"-"

Attempts:

3 left

💡 Hint

Common Mistakes

Using $2 instead of $3 prints the wrong field.

Using quotes instead of field variables prints literal text.

Use $1 for the first field and $3 for the third field to print them separated by a space.

5fill in blank

hard

Fill all three blanks to create a dictionary of words and their lengths for words longer than 3 characters.

Linux CLI

awk '{ if (length([1]) [2] 3) dict[[3]] = length($1) } END { for (w in dict) print w, dict[w] }' words.txt

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

A$1

B>

Cw

Dlength

Attempts:

3 left

💡 Hint

Common Mistakes

Using w before it is defined causes errors.

Using length as a key is incorrect.

Using < instead of > changes the condition.

Use $1 to get the word, > to check length, and $1 again as the dictionary key.

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