sed substitution in Linux CLI - Step-by-Step Execution

Concept Flow - sed substitution

Read input line

↓

Match pattern?

No→Print line unchanged

Yes↓

Replace matched text

↓

Print modified line

↓

Repeat for next line

↓

End

sed reads each line, checks if it matches the pattern, replaces matched text if yes, then prints the line.

Execution Sample

Linux CLI

echo 'apple banana apple' | sed 's/apple/orange/'

Replace first 'apple' with 'orange' in the input line.

Execution Table

Step	Input Line	Pattern Match	Action	Output Line
1	apple banana apple	Yes (apple found)	Replace first 'apple' with 'orange'	orange banana apple
2	No more lines	N/A	End of input	N/A

💡 No more input lines to process, sed ends.

Variable Tracker

Variable	Start	After Step 1	Final
input_line	apple banana apple	apple banana apple	No more lines
pattern_matched	false	true	N/A
output_line		orange banana apple

Key Moments - 2 Insights

Why does sed only replace the first occurrence of 'apple'?

By default, sed's 's/pattern/replacement/' replaces only the first match per line, as shown in execution_table step 1.

What happens if the pattern is not found in the line?

sed prints the line unchanged.

Visual Quiz - 3 Questions

Look at the execution table, what is the output line after step 1?

A apple banana apple

B apple banana orange

C orange banana apple

D orange orange apple

💡 Hint

Check the 'Output Line' column in execution_table row for step 1.

At which step does sed stop processing input lines?

A Step 1

B Step 2

C After Step 1

D Never stops

💡 Hint

Look at the 'Step' and 'Action' columns in execution_table for when input ends.

If we add 'g' flag to sed like 's/apple/orange/g', how would the output line change at step 1?

A orange banana orange

B orange banana apple

C apple banana apple

D orange orange orange

💡 Hint

The 'g' flag replaces all matches per line, not just the first, affecting output_line in variable_tracker.

Concept Snapshot

sed substitution syntax: s/pattern/replacement/ 
By default replaces first match per line.
Add 'g' flag to replace all matches.
Reads input line by line, applies substitution, prints result.
Useful for quick text changes in scripts or command line.

Full Transcript

sed substitution reads each input line, checks if the pattern matches. If yes, it replaces the first occurrence of the pattern with the replacement text and prints the modified line. If no match, it prints the line unchanged. This process repeats for all input lines until none remain. By default, only the first match per line is replaced unless the 'g' flag is added to replace all matches. This makes sed a powerful tool for quick text editing in command line scripts.