sed (stream editor) basics in Linux CLI - Step-by-Step Execution

Concept Flow - sed (stream editor) basics

Start with input text

↓

sed reads line

↓

Apply sed command

↓

Output modified line

↓

Repeat for next line

↓

End of input?

No→sed reads next line

Yes↓

Finish

sed reads input line by line, applies the editing command, outputs the result, and repeats until all lines are processed.

Execution Sample

Linux CLI

echo "apple\nbanana\ncherry" | sed 's/a/A/'

Replace the first 'a' with 'A' on each line of the input text.

Execution Table

Step	Input Line	sed Command Applied	Output Line	Notes
1	apple	s/a/A/	Apple	First 'a' replaced with 'A'
2	banana	s/a/A/	bAnana	Only first 'a' replaced
3	cherry	s/a/A/	cherry	No 'a' found, line unchanged
4	End of input	-	-	sed finishes processing all lines

💡 All input lines processed, sed ends.

Variable Tracker

Variable	Start	After Line 1	After Line 2	After Line 3	Final
Current Line	apple	banana	cherry	End	End
Output Line		Apple	bAnana	cherry	All lines output

Key Moments - 2 Insights

Why does sed only replace the first 'a' on each line, not all?

By default, sed's 's' command replaces only the first match per line, as shown in execution_table rows 1 and 2.

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

sed outputs the line unchanged, as seen in execution_table row 3 where 'cherry' has no 'a'.

Visual Quiz - 3 Questions

Look at the execution table, what is the output line for the second input line?

A banana

B Banana

C bAnana

D bAnAna

💡 Hint

Check execution_table row 2 under 'Output Line'

At which step does sed finish processing all lines?

A Step 3

B Step 4

C Step 2

D Step 1

💡 Hint

Look at the 'End of input' step in execution_table

If we add the 'g' flag to the sed command (s/a/A/g), how would the output for 'banana' change?

A bAnAnA

B bAnana

C banana

D Banana

💡 Hint

The 'g' flag replaces all matches per line, not just the first (see key_moments about default behavior)

Concept Snapshot

sed reads input line by line
Applies editing commands like substitution
's/pattern/replacement/' replaces first match per line
Add 'g' flag to replace all matches
Outputs modified lines in order
Stops after last input line

Full Transcript

sed is a tool that reads text line by line. For each line, it applies a command like substitution. For example, 's/a/A/' changes the first 'a' to 'A' on each line. sed then outputs the changed line. It repeats this until all lines are done. If a line has no match, it stays the same. By default, only the first match per line changes. Adding 'g' after the command makes all matches change. This process helps quickly edit text streams.