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.