awk patterns and actions in Linux CLI - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the basic structure of an awk command using patterns and actions?

An awk command has the form: awk 'pattern { action }' filename. The pattern selects lines, and the action tells awk what to do with those lines.

beginner

In awk, what happens if you omit the pattern but provide an action?

If the pattern is omitted, awk applies the action to every line of the input file.

beginner

What does the pattern /error/ mean in awk?

The pattern /error/ matches any line that contains the word 'error'. It uses regular expression matching.

beginner

How do you print the first field of each line using awk?

Use the action { print $1 }. For example: awk '{ print $1 }' filename prints the first word of each line.

intermediate

What does the pattern-action pair NR == 3 { print } do in awk?

It prints only the third line of the input. NR is the current line number, so when NR == 3, the action print runs.

What does this awk command do? awk '/fail/ { print $2 }' file.txt

A Prints all lines containing 'fail'

B Prints the second word of lines containing 'fail'

C Prints the second line of the file

D Prints the entire file

The pattern '/fail/' selects lines with 'fail'. The action '{ print $2 }' prints the second word of those lines.

What happens if you run awk '{ print $0 }' file.txt?

A Prints lines containing 'print'

B Prints only the first word of each line

C Prints nothing

D Prints the entire file line by line

$0 means the whole line. Without a pattern, awk prints every line.

Which pattern matches lines where the first field equals 'John'?

A NR == John

B /John/

C $1 == "John"

D John == $0

The pattern '$1 == "John"' matches lines where the first word is exactly 'John'.

What does this command do? awk 'NR > 5' file.txt

A Prints lines after the fifth line

B Prints the first five lines

C Prints lines containing '5'

D Prints nothing

NR > 5 is a pattern that matches lines after line 5. The default action is to print those lines.

How do you print the number of fields in each line using awk?

A { print NF }

B { print NR }

C { print $NF }

D { print $0 }

NF is a built-in variable holding the number of fields in the current line.

Explain how awk uses patterns and actions to process text files.

Think about how you pick certain lines and then decide what to do with them.

Describe how to print the third field of lines containing the word 'error' using awk.

Combine a pattern that finds 'error' and an action that prints a specific field.

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