awk patterns and actions in Linux CLI - Step-by-Step Execution

Concept Flow - awk patterns and actions

Read line from input

↓

Check pattern match?

No→Skip line

Yes↓

Execute action block

↓

Output result if action prints

↓

Repeat for next line

↓

End of input?

Yes↓

Exit

Awk reads each line, checks if it matches a pattern, runs actions if matched, then moves to next line until input ends.

Execution Sample

Linux CLI

awk '/error/ { print $0 }' logfile.txt

Prints every line from logfile.txt that contains the word 'error'.

Execution Table

Step	Input Line	Pattern Match (/error/)	Action Executed	Output
1	Info: system started	No	No
2	Warning: low disk space	No	No
3	Error: failed to load module	Yes	Print line	Error: failed to load module
4	Info: user login	No	No
5	error: disk not found	Yes	Print line	error: disk not found
6	End of log	No	No

💡 Reached end of input file, awk stops processing.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	After 6	Final
$0 (current line)		Info: system started	Warning: low disk space	Error: failed to load module	Info: user login	error: disk not found	End of log
Pattern matched		No	No	Yes	No	Yes	No
Output				Error: failed to load module		error: disk not found

Key Moments - 3 Insights

Why does awk skip some lines without printing anything?

Awk only runs the action block when the pattern matches the line. Lines without the pattern are skipped, as shown in steps 1, 2, 4, and 6 in the execution_table.

What does $0 represent in awk?

$0 holds the entire current input line. In the example, printing $0 outputs the whole line that matched the pattern, as seen in steps 3 and 5.

Can awk run actions without a pattern?

Yes, if no pattern is given, awk runs the action on every line. Here, the pattern '/error/' filters lines, so only matching lines trigger the action.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of $0 at step 3?

A Error: failed to load module

B Info: system started

C Warning: low disk space

D error: disk not found

💡 Hint

Check the 'Input Line' column at step 3 in the execution_table.

At which step does the pattern '/error/' first match?

A Step 2

B Step 3

C Step 5

D Step 6

💡 Hint

Look at the 'Pattern Match' column to find the first 'Yes'.

If the pattern was changed to '/Warning/', which step would produce output?

A Step 3

B Step 5

C Step 2

D Step 6

💡 Hint

Look for the line containing 'Warning' in the 'Input Line' column.

Concept Snapshot

awk pattern-action basics:
- awk reads input line by line
- For each line, it checks if the pattern matches
- If yes, it runs the action block
- $0 is the whole current line
- Without a pattern, action runs on all lines
- Useful for filtering and processing text streams

Full Transcript

Awk works by reading each line from input. For every line, it checks if the pattern matches. If the pattern matches, awk runs the action block. For example, the pattern '/error/' matches lines containing 'error'. When matched, the action '{ print $0 }' prints the whole line. Lines without the pattern are skipped and produce no output. The variable $0 holds the entire current line. This process repeats until the end of the input file. This way, awk filters and processes text easily.

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