[sed Part-2]Mastering sed Pattern Space, Hold Space, and Multiline Processing

Introduction

Most Linux users learn sed through simple substitutions and line filtering. However, the real power of sed appears when you understand Pattern Space, Hold Space, and multiline processing. These features allow sed to remember previous lines, process multiple lines together, reverse files, swap records, and perform transformations that many users assume require awk or Perl.

In this article, we’ll explore the advanced commands that make sed a true text-processing language: h, H, g, G, x, n, N, P, and D. Don't worry if these commands look intimidating at first—by the end, you'll understand exactly how they work and when to use them.

Understanding Pattern Space and Hold Space

Before learning the commands, let’s understand the two most important concepts in advanced sed.

Pattern Space

The Pattern Space is sed’s current working area.

For example, if sed is processing:

apple
banana
orange

When sed reaches the second line:

Pattern Space = banana

All commands operate on the Pattern Space.

Hold Space

The Hold Space is a temporary storage area.

Think of it as a variable that can save text for later use.

Pattern Space = Current line
Hold Space    = Saved line(s)

The Hold Space survives across processing cycles, which allows sed to remember information from previous lines.

1. h — Copy Pattern Space to Hold Space

Meaning

Copies the entire Pattern Space into the Hold Space, replacing anything already stored there.

Input

apple
banana
orange

Command

sed '1h' fruits.txt

Output

apple
banana
orange

What Happened?

When sed processes line 1:

Pattern Space = apple
Hold Space    = empty

After h:

Pattern Space = apple
Hold Space    = apple

The output remains unchanged, but Hold Space now contains a copy of line 1.

When to Use

Store a line for future comparison or reuse.

2. H — Append Pattern Space to Hold Space

Meaning

Appends the current Pattern Space to the Hold Space instead of replacing it.

Input

apple
banana
orange

Command

sed 'H' fruits.txt

Internal Result

After line 1:

Hold Space =
apple

After line 2:

Hold Space =
apple
banana

After line 3:

Hold Space =
apple
banana
orange

What Happened?

Unlike h, which overwrites Hold Space, H keeps adding new content.

When to Use

Build a multiline buffer containing several lines.

3. g — Copy Hold Space to Pattern Space

Meaning

Replaces the current Pattern Space with the contents of Hold Space.

Input

apple
banana
orange

Command

sed '1h;2g' fruits.txt

Output

apple
apple
orange

What Happened?

Line 1:

Hold Space = apple

Line 2:

Pattern Space = banana

After g:

Pattern Space = apple

The original line is replaced with Hold Space contents.

When to Use

Restore previously saved content.

4. G — Append Hold Space to Pattern Space

Meaning

Appends Hold Space to the current Pattern Space.

Input

apple
banana
orange

Command

sed '1h;2G' fruits.txt

Output

apple
banana
apple
orange

What Happened?

Before G:

Pattern Space = banana
Hold Space    = apple

After G:

Pattern Space =
banana
apple

When to Use

Combine current and previously saved data.

5. x — Swap Pattern Space and Hold Space

Meaning

Exchanges Pattern Space and Hold Space.

Input

apple
banana
orange

Command

sed '1h;2x' fruits.txt

Output

apple
apple
orange

What Happened?

Before swap:

Pattern Space = banana
Hold Space    = apple

After swap:

Pattern Space = apple
Hold Space    = banana

When to Use

Temporarily exchange saved and current data.

6. n — Read Next Line

Meaning

Reads the next input line and replaces the current Pattern Space.

Input

A
B
C

Command

sed 'n' file.txt

Output

A
B
C

What Happened?

For every cycle:

Current line discarded
Next line loaded

Important Difference

n = Replace current line
N = Append next line

Many beginners confuse these commands.

7. N — Append Next Line

Meaning

Reads the next line and appends it to Pattern Space.

Input

A
B
C

Command

sed 'N' file.txt

Pattern Space

After first N:

A
B

What Happened?

Instead of replacing the current line, N keeps it and appends the next one.

When to Use

Multiline processing.

Reading Three Lines Together

Input

A
B
C
D

Command

sed '
N
N
'

Pattern Space

A
B
C

What Happened?

Each N adds one more line.

N     -> 2 lines
N N   -> 3 lines
N N N -> 4 lines

8. P — Print First Line of Pattern Space

Meaning

Prints only the first line from a multiline Pattern Space.

Input

A
B

Command

sed -n '
N
P
'

Output

A

What Happened?

Pattern Space contains:

A
B

P prints only:

A

When to Use

Process multiline buffers one line at a time.

9. D — Delete First Line of Pattern Space

Meaning

Deletes only the first line from a multiline Pattern Space.

Input

A
B
C

Command

sed '
N
D
'

Internal Processing

Before:

A
B

After D:

B

What Happened?

Unlike d, which removes the entire Pattern Space, D removes only the first line.

Important Difference

d = Delete everything
D = Delete first line only

Real-World Example: Print Previous Line

One of the most common Hold Space examples.

Input

INFO
ERROR
DEBUG

Command

sed -n '
/ERROR/{
g
p
}
h
'

Output

INFO

What Happened?

When sed finds ERROR:

Hold Space contains previous line

g restores it.

p prints it.

Real-World Example: Reverse a File

Input

A
B
C
D

Command

sed '1!G;h;$!d'

Output

D
C
B
A

How It Works

G -> Append previous lines
h -> Save current result
d -> Suppress intermediate output

This is one of the most famous sed one-liners ever written.

Real-World Example: Swap Adjacent Lines

Input

A
B
C
D

Command

sed -E 'N;s/(.*)\n(.*)/\2\n\1/'

Output

B
A
D
C

What Happened?

N creates:

A
B

Regex swaps:

B
A

The process repeats for the next pair.

Mental Models

Remember these shortcuts:

CommandThink OfhSaveHAppend SavegRestoreGAppend RestorexSwapnNext LineNAppend Next LinePPrint First LineDDelete First Line

Conclusion

Understanding Pattern Space and Hold Space is the moment when sed starts to feel like a programming language rather than a simple text editor. The commands covered here — h, H, g, G, x, n, N, P, and D—form the foundation of advanced sed scripting.

In the next part, we’ll explore labels, branching, flow control, and advanced regular expressions. That’s where sed gains the ability to loop, make decisions, and perform surprisingly sophisticated text transformations.