Linux CLI 48 🐧 nl, fold and fmt commands - KandZ

nl filename.txt
nl -n rz filename.txt
nl -n ln filename.txt
nl -n rn filename.txt
nl -i 2 filename.txt

fold longtext.txt
fold -w20 longtext.txt
fold -s  longtext.txt

cat format.txt
fmt format.txt
fmt -w20 format.txt
fmt -w20 -c format.txt

nl, fold, and fmt are three small but useful text-formatting tools. nl numbers lines; fold breaks long lines at a fixed width; fmt reflows paragraphs to a target width. They’re the kind of commands you reach for when you need to make raw text readable — for reports, citations, or terminal output.

Key point: fold and fmt both wrap text, but differently. fold is a hard wrap — it breaks at exactly the width you give, no matter what. fmt is a soft wrap — it reflows whole paragraphs, moving words to fit the width naturally. Use fold for code or unbreakable strings; use fmt for prose.

a – nl command

nl is used to number lines of text. It’s useful when referencing or citing specific lines in a document. It’s like cat -n but with more control over formatting.

Syntax:

nl [options] [file...]

Common options:

Option	Purpose
`-b a`	Number all lines (default)
`-b t`	Number only non-blank lines
`-b n`	Number no lines
`-n ln`	Left-aligned numbers
`-n rn`	Right-aligned numbers (default)
`-n rz`	Right-aligned, zero-padded
`-i N`	Increment by N (default 1)
`-s STR`	Use STR as the separator (default TAB)
`-w N`	Use N columns for the number
`-v N`	Start numbering at N

Examples:

# Look at the file
$ cat filename.txt
first line
second line
third line
fourth line

# Default numbering — right-aligned, tab separator
$ nl filename.txt
     1	first line
     2	second line
     3	third line
     4	fourth line

# Zero-padded numbers
$ nl -n rz filename.txt
000001	first line
000002	second line
000003	third line
000004	fourth line

# Left-aligned numbers
$ nl -n ln filename.txt
1     	first line
2     	second line
3     	third line
4     	fourth line

# Right-aligned (explicit)
$ nl -n rn filename.txt
     1	first line
     2	second line
     3	third line
     4	fourth line

# Increment by 2
$ nl -i 2 filename.txt
     1	first line
     3	second line
     5	third line
     7	fourth line

# Skip blank lines
$ cat mixed.txt
first

second

third

$ nl -b t mixed.txt
     1	first

     2	second

     3	third

Comparing nl formats:

Flag	Example output
`-n ln`	`1 \tfirst line`
`-n rn`	`1\tfirst line`
`-n rz`	`000001\tfirst line`

More examples:

# Use a custom separator
$ nl -s ': ' filename.txt
     1: first line
     2: second line
     3: third line
     4: fourth line

# Wider number column
$ nl -w 4 filename.txt
   1	first line
   2	second line
   3	third line
   4	fourth line

# Start numbering at 100
$ nl -v 100 filename.txt
   100	first line
   101	second line
   102	third line
   103	fourth line

# Number all lines, zero-padded, custom separator
$ nl -b a -n rz -s ': ' filename.txt
000001: first line
000002: second line
000003: third line
000004: fourth line

Tip: nl is often used with sed or awk for numbering, but nl is simpler and faster when you just want line numbers. For plain numbering, nl is the dedicated tool.

b – fold command

fold wraps input text into a specified width. It’s useful for formatting long lines of text into readable chunks — particularly when you have a long string that needs to fit a terminal or a fixed-width column.

Syntax:

fold [options] [file...]

Common options:

Option	Purpose
`-w N`	Wrap at N columns (default 80)
`-s`	Break at spaces instead of mid-word
`-b`	Count bytes instead of columns
`-c`	Count characters (respects multibyte)

Examples:

# Look at the long line
$ cat longtext.txt
This is a very long line of text that will not fit on a single terminal line and needs to be wrapped.

# Default wrap (80 columns)
$ fold longtext.txt
This is a very long line of text that will not fit on a single terminal line a
nd needs to be wrapped.

# Wrap at 20 columns
$ fold -w20 longtext.txt
This is a very long 
line of text that wi
ll not fit on a sing
le terminal line and
 needs to be wrapped
.

# Wrap at 20 columns, breaking at spaces
$ fold -s -w20 longtext.txt
This is a very long 
line of text that 
will not fit on a 
single terminal 
line and needs to 
be wrapped.

fold vs fold -s:

Without -s (breaks mid-word):
This is a very long
line of text that wi
ll not fit on a sing
le terminal line

With -s (breaks at spaces):
This is a very long
line of text that
will not fit on a
single terminal line

More examples:

# Wrap a long line at exactly 40 characters
$ echo "abcdefghijklmnopqrstuvwxyz0123456789ABCDEFGHIJ" | fold -w40
abcdefghijklmnopqrstuvwxyz0123456789ABCDEF
GHIJ

# Fold a file at 60 columns
$ fold -w60 document.txt

# Fold with space-breaking
$ fold -s -w60 document.txt

# Fold code (break at exact width — no word awareness)
$ fold -w72 script.py

# Fold binary data (byte counting)
$ fold -b -w40 data.bin

# Fold with multibyte characters
$ echo "café résumé naïve" | fold -c -w10

When to use fold:

Wrapping code or URLs where mid-token breaks are acceptable
Fitting text into a fixed-width column
Wrapping text where you control the width exactly
Handling text that isn’t natural-language prose

Note: fold doesn’t understand paragraphs. It processes each line independently and doesn’t reflow across line boundaries. For paragraphs, use fmt.

c – fmt command

fmt formats text paragraphs. It automatically adjusts the length of lines to fit within width margins. The default length is 75 characters. Unlike fold, fmt reflows entire paragraphs — it moves words around to make lines fit naturally.

Syntax:

fmt [options] [file...]

Common options:

Option	Purpose
`-w N`	Set line width to N (default 75)
`-c`	Crown margin mode — consistent paragraph width, break at natural stopping points
`-s`	Split long lines but don’t join short ones
`-u`	Uniform spacing (one space between words, two after sentences)
`-p PREFIX`	Only reformat lines starting with PREFIX
`-t`	Tagged paragraph mode

Examples:

# Look at the unformatted text
$ cat format.txt
This is a paragraph of text that is written on one very long line without any consideration for width or readability. It just keeps going and going.

# Default formatting (75 columns)
$ fmt format.txt
This is a paragraph of text that is written on one very long line without
any consideration for width or readability. It just keeps going and going.

# Format at 20 columns
$ fmt -w20 format.txt
This is a paragraph
of text that is
written on one very
long line without
any consideration
for width or
readability. It
just keeps going and
going.

# Format at 20 columns with crown mode
$ fmt -w20 -c format.txt
This is a
paragraph of text
that is written on
one very long line
without any
consideration for
width or
readability. It
just keeps going
and going.

How fmt differs from fold:

Aspect	fold	fmt
Break points	Exactly at width	At word boundaries
Paragraphs	Line-by-line	Reflows whole paragraphs
Joining lines	No	Yes — joins short lines
Best for	Code, URLs	Prose, documents

fmt reflows — fold doesn’t:

Input (two short lines):
The quick brown fox
jumps over the lazy dog.

fold -w80 (unchanged):
The quick brown fox
jumps over the lazy dog.

fmt -w80 (reflowed):
The quick brown fox jumps over the lazy dog.

More examples:

# Reformat a file to 60 columns
$ fmt -w60 document.txt

# Format with crown mode (consistent paragraph width)
$ fmt -w60 -c document.txt

# Split long lines but don't join short ones
$ fmt -s document.txt

# Uniform spacing
$ fmt -u document.txt

# Format only lines starting with "> " (quoted text)
$ fmt -p '> ' email.txt

# Format a paragraph from stdin
$ echo "This is a long sentence that should be reformatted to fit within a narrower margin for readability." | fmt -w30
This is a long sentence
that should be reformatted
to fit within a narrower
margin for readability.

Default width of fmt:

# Default is 75 characters
$ fmt format.txt
This is a paragraph of text that is written on one very long line without
any consideration for width or readability. It just keeps going and going.
#                                                                         ^
#                                                              75 columns |

More on crown mode (-c):

Without -c, fmt may produce slightly different line lengths, especially for the last line of each paragraph. With -c, it tries to make all lines consistent and break at a more natural stopping point.

$ fmt -w20 format.txt
This is a paragraph
of text that is
written on one very
long line without
any consideration
for width or
readability. It
just keeps going and
going.

$ fmt -w20 -c format.txt
This is a
paragraph of text
that is written on
one very long line
without any
consideration for
width or
readability. It
just keeps going
and going.

Tip: Use fmt for README files, plain-text documentation, or when you want to reflow prose to a specific width. It’s especially useful after editing a document with a text editor that produces uneven line lengths.

Complete Example Session

# ============================================
# PART 1: NL — NUMBER LINES
# ============================================

$ cat filename.txt
first line
second line
third line
fourth line

$ nl filename.txt
     1	first line
     2	second line
     3	third line
     4	fourth line

$ nl -n rz filename.txt
000001	first line
000002	second line
000003	third line
000004	fourth line

$ nl -n ln filename.txt
1     	first line
2     	second line
3     	third line
4     	fourth line

$ nl -n rn filename.txt
     1	first line
     2	second line
     3	third line
     4	fourth line

$ nl -i 2 filename.txt
     1	first line
     3	second line
     5	third line
     7	fourth line

# ============================================
# PART 2: NL — SKIP BLANK LINES
# ============================================

$ cat mixed.txt
first

second

third

$ nl -b t mixed.txt
     1	first

     2	second

     3	third

# ============================================
# PART 3: NL — CUSTOM SEPARATOR
# ============================================

$ nl -s ': ' filename.txt
     1: first line
     2: second line
     3: third line
     4: fourth line

$ nl -b a -n rz -s ': ' filename.txt
000001: first line
000002: second line
000003: third line
000004: fourth line

# ============================================
# PART 4: FOLD — WRAP AT FIXED WIDTH
# ============================================

$ cat longtext.txt
This is a very long line of text that will not fit on a single terminal line and needs to be wrapped.

$ fold longtext.txt
This is a very long line of text that will not fit on a single terminal line a
nd needs to be wrapped.

$ fold -w20 longtext.txt
This is a very long 
line of text that wi
ll not fit on a sing
le terminal line and
 needs to be wrapped
.

# ============================================
# PART 5: FOLD — BREAK AT SPACES
# ============================================

$ fold -s -w20 longtext.txt
This is a very long 
line of text that 
will not fit on a 
single terminal 
line and needs to 
be wrapped.

# ============================================
# PART 6: FMT — REFLOW PARAGRAPHS
# ============================================

$ cat format.txt
This is a paragraph of text that is written on one very long line without any consideration for width or readability. It just keeps going and going.

$ fmt format.txt
This is a paragraph of text that is written on one very long line without
any consideration for width or readability. It just keeps going and going.

$ fmt -w20 format.txt
This is a paragraph
of text that is
written on one very
long line without
any consideration
for width or
readability. It
just keeps going and
going.

$ fmt -w20 -c format.txt
This is a
paragraph of text
that is written on
one very long line
without any
consideration for
width or
readability. It
just keeps going
and going.

# ============================================
# PART 7: FMT VS FOLD
# ============================================

# Two short lines
$ printf "The quick brown fox\njumps over the lazy dog.\n"

# fold — leaves them alone
$ printf "The quick brown fox\njumps over the lazy dog.\n" | fold -w80
The quick brown fox
jumps over the lazy dog.

# fmt — joins them
$ printf "The quick brown fox\njumps over the lazy dog.\n" | fmt -w80
The quick brown fox jumps over the lazy dog.

# ============================================
# PART 8: COMBINING NL, FOLD, FMT
# ============================================

# Number the reflowed output
$ fmt -w40 document.txt | nl

# Wrap a long line and number it
$ fold -s -w30 longtext.txt | nl -n rz

# Reflow a file and save it
$ fmt -w72 messy.txt > clean.txt

# Number non-blank lines of a reflowed file
$ fmt -w60 document.txt | nl -b t

# ============================================
# PART 9: REAL-WORLD USES
# ============================================

# Number lines of source code for a code review
$ nl -b a -n rz script.sh | head -20
000001	#!/bin/bash
000002	
000003	# A simple script
...

# Wrap a long URL list
$ fold -w80 urls.txt

# Reflow a plain-text README to 72 columns
$ fmt -w72 README.txt > README.formatted.txt

# Number lines of a log after filtering
$ grep ERROR app.log | nl -n rz
000001	2024-01-15 ERROR: connection refused
000002	2024-01-15 ERROR: timeout
000003	2024-01-15 ERROR: disk full

Quick Reference

nl

Command	Purpose
`nl FILE`	Number all lines (right-aligned)
`nl -b a FILE`	Number all lines
`nl -b t FILE`	Number non-blank lines
`nl -n ln FILE`	Left-aligned numbers
`nl -n rn FILE`	Right-aligned numbers
`nl -n rz FILE`	Zero-padded numbers
`nl -i N FILE`	Increment by N
`nl -s STR FILE`	Custom separator
`nl -w N FILE`	Number column width
`nl -v N FILE`	Start at N

fold

Command	Purpose
`fold FILE`	Wrap at 80 columns
`fold -w N FILE`	Wrap at N columns
`fold -s FILE`	Break at spaces
`fold -s -w N FILE`	Both
`fold -b FILE`	Count bytes
`fold -c FILE`	Count characters

fmt

Command	Purpose
`fmt FILE`	Reflow at 75 columns
`fmt -w N FILE`	Reflow at N columns
`fmt -c FILE`	Crown mode (consistent width)
`fmt -s FILE`	Split only, don’t join
`fmt -u FILE`	Uniform spacing
`fmt -p PREFIX FILE`	Only lines with prefix

Tool Comparison

Tool	Purpose	Wraps at	Reflows?
`nl`	Number lines	—	—
`fold`	Hard-wrap lines	Exact width	No
`fmt`	Reflow paragraphs	Word boundaries	Yes
`cat -n`	Number lines	—	—

Default Widths

Tool	Default
`fold`	80 columns
`fmt`	75 columns

Best Practices

✅ Do This:

# Use nl for line numbers in reviews
nl -b a -n rz script.sh              # ✅

# Use fold -s to avoid mid-word breaks
fold -s -w72 document.txt            # ✅

# Use fmt for prose
fmt -w72 README.txt                  # ✅

# Use fmt -c for consistent paragraph width
fmt -w72 -c document.txt             # ✅

# Combine nl with other tools
grep ERROR app.log | nl              # ✅

# Save formatted output
fmt -w72 messy.txt > clean.txt       # ✅

# Use fold for code/URLs
fold -w80 urls.txt                   # ✅

❌ Don’t Do This:

# Don't use fold for prose — it breaks mid-word
fold -w72 article.txt                # ❌ use fmt instead

# Don't use fmt for code — it reflows
fmt -w72 script.py                   # ❌ use fold

# Don't forget -s with fold
fold -w20 text.txt                   # ❌ breaks mid-word

# Don't use nl when cat -n is enough
nl -b a file.txt                     # ⚠️  cat -n is simpler

# Don't expect fold to join lines
fold -w80 short-lines.txt            # ❌ use fmt

# Don't forget the default width
fmt document.txt                     # ⚠️  75, not 80

Common Pitfalls

Pitfall	Problem	Solution
`fold` breaks mid-word	Ugly output	Add `-s`
`fmt` joins lines unexpectedly	Lost structure	Use `fold`
`nl` shows blank lines	Numbering all	Use `-b t`
Wrong number format	Misaligned	Use `-n rn` / `-n rz`
Default width wrong	Too narrow/wide	Set `-w N`
`fmt` on code	Reflows code	Use `fold`
Tabs in output	Confusing alignment	Use `-s ' '`
`fold` on UTF-8	Byte splitting	Use `-c`

Real-World Examples

1. Number Lines for Code Review

$ nl -b a -n rz script.sh | head -10
000001	#!/bin/bash
000002	
000003	# Script to back up files
000004	
000005	SRC="/home/kronos"
000006	DST="/backup"
000007	
000008	rsync -av --delete "$SRC" "$DST"
000009	
000010	echo "Backup complete"

2. Number Non-Blank Lines

$ nl -b t document.txt
     1	Chapter 1

     2	Introduction

     3	This is the first paragraph.

3. Zero-Padded Numbering

$ nl -n rz file.txt
000001	line one
000002	line two
000003	line three

4. Wrap a Long URL

$ echo "https://example.com/very/long/path/to/a/resource/that/needs/wrapping" | fold -w40
https://example.com/very/long/path/to/a/
resource/that/needs/wrapping

5. Wrap at Spaces

$ echo "The quick brown fox jumps over the lazy dog" | fold -s -w20
The quick brown fox
jumps over the lazy
dog

6. Reflow a Paragraph

$ cat messy.txt
This   is  a paragraph
with   uneven   spacing
and   broken   lines.

$ fmt -w40 messy.txt
This is a paragraph with uneven
spacing and broken lines.

7. Reflow at 60 Columns

$ fmt -w60 article.txt > article.formatted.txt

8. Reflow with Crown Mode

$ fmt -w60 -c document.txt

9. Format Quoted Text in an Email

$ fmt -p '> ' email.txt

10. Combine `fmt` and `nl`

$ fmt -w60 document.txt | nl -b t
     1	This is the first line of the reflowed text
     2	which now fits nicely within sixty columns
     3	instead of running off the edge.

11. Number Log Errors

$ grep ERROR app.log | nl -n rz
000001	2024-01-15 ERROR: connection refused
000002	2024-01-15 ERROR: timeout
000003	2024-01-15 ERROR: disk full

12. Prepare Text for a Fixed-Width Display

$ fold -w32 -s menu.txt

13. Number and Wrap a File

$ fold -s -w40 longtext.txt | nl -n rz
000001	This is a very long line of text
000002	that will not fit on a single
000003	terminal line and needs to be
000004	wrapped.

14. Fix a File with Long Lines

$ fmt -w72 longlines.txt > fixed.txt

15. Check the Default Width

$ echo "The default width is 75 characters for fmt and 80 for fold. This line is designed to be long enough to see where the break happens." | fmt
The default width is 75 characters for fmt and 80 for fold. This line is
designed to be long enough to see where the break happens.

16. Format a Man Page Source

$ fmt -w72 mytool.1 > mytool.1.formatted

17. Add Line Numbers to `diff` Output

$ diff -u old.txt new.txt | nl
     1	--- old.txt
     2	+++ new.txt
     3	@@ -1,3 +1,3 @@
     4	 line one
     5	-old line two
     6	+new line two
     7	 line three

18. Reflow README to 72 Columns

$ fmt -w72 README.md > README.formatted.md

Visual: nl, fold, fmt

┌──────────────────────────────────────────────┐
│                  nl                          │
│                                              │
│  Input:          Output:                     │
│  first line      1    first line             │
│  second line     2    second line            │
│  third line      3    third line             │
│                                              │
│  Adds line numbers                           │
│                                              │
└──────────────────────────────────────────────┘

┌──────────────────────────────────────────────┐
│                 fold -w20                    │
│                                              │
│  Input:                                      │
│  This is a very long line of text            │
│                                              │
│  Output:                                     │
│  This is a very long  ← exactly 20 chars     │
│  line of text         ← exactly 20 chars     │
│                                              │
│  Hard break at exact width                   │
│                                              │
└──────────────────────────────────────────────┘

┌──────────────────────────────────────────────┐
│                 fmt -w20                     │
│                                              │
│  Input:                                      │
│  This is a very long line of text            │
│                                              │
│  Output:                                     │
│  This is a very long  ← 20 chars             │
│  line of text         ← reflowed             │
│                                              │
│  Soft break at word boundaries,              │
│  reflows whole paragraphs                    │
│                                              │
└──────────────────────────────────────────────┘

Summary

Command	Purpose	Example
`nl FILE`	Number all lines	`nl filename.txt`
`nl -b a FILE`	Number all lines	`nl -b a filename.txt`
`nl -b t FILE`	Number non-blank	`nl -b t filename.txt`
`nl -n ln FILE`	Left-aligned	`nl -n ln filename.txt`
`nl -n rn FILE`	Right-aligned	`nl -n rn filename.txt`
`nl -n rz FILE`	Zero-padded	`nl -n rz filename.txt`
`nl -i N FILE`	Increment by N	`nl -i 2 filename.txt`
`nl -s STR FILE`	Custom separator	`nl -s ': ' filename.txt`
`fold FILE`	Wrap at 80	`fold longtext.txt`
`fold -w N FILE`	Wrap at N	`fold -w20 longtext.txt`
`fold -s FILE`	Break at spaces	`fold -s longtext.txt`
`fold -s -w N FILE`	Both	`fold -s -w20 longtext.txt`
`fmt FILE`	Reflow at 75	`fmt format.txt`
`fmt -w N FILE`	Reflow at N	`fmt -w20 format.txt`
`fmt -c FILE`	Crown mode	`fmt -w20 -c format.txt`
`fmt -s FILE`	Split only	`fmt -s format.txt`
`fmt -u FILE`	Uniform spacing	`fmt -u format.txt`
`fmt -p P FX`	Prefix mode	`fmt -p '> ' email.txt`

Key takeaways:

nl numbers lines — use -b t for non-blank only, -n rz for zero-padded, -i N for increments
fold does a hard wrap at an exact column width — use -s to break at spaces
fmt does a soft wrap — it reflows whole paragraphs at word boundaries
fold is for code, URLs, and fixed-width output; fmt is for prose
The default width is 80 for fold and 75 for fmt
fmt -c (crown mode) produces more consistent paragraph widths
fmt -s splits long lines without joining short ones
fmt -p PREFIX reformats only lines starting with a prefix (great for quoted email)
Combine them: fmt -w60 document.txt | nl -b t — reflow then number
Use nl instead of cat -n when you need format control

Remember: nl numbers, fold wraps hard, fmt reflows soft. If you’re formatting prose — READMEs, documentation, email — use fmt. If you’re formatting code, URLs, or fixed-width output — use fold. And when you need line numbers for a review or a report, nl gives you the control that cat -n doesn’t. These three are small tools, but they turn raw text into something readable.

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a – nl command

b – fold command

c – fmt command

Complete Example Session

Quick Reference

nl

fold

fmt

Tool Comparison

Default Widths

Best Practices

Common Pitfalls

Real-World Examples

1. Number Lines for Code Review

2. Number Non-Blank Lines

3. Zero-Padded Numbering

4. Wrap a Long URL

5. Wrap at Spaces

6. Reflow a Paragraph

7. Reflow at 60 Columns

8. Reflow with Crown Mode

9. Format Quoted Text in an Email

10. Combine fmt and nl

11. Number Log Errors

12. Prepare Text for a Fixed-Width Display

13. Number and Wrap a File

14. Fix a File with Long Lines

15. Check the Default Width

16. Format a Man Page Source

17. Add Line Numbers to diff Output

18. Reflow README to 72 Columns

Visual: nl, fold, fmt

Summary

Ads Blocker Detected!!!

10. Combine `fmt` and `nl`

17. Add Line Numbers to `diff` Output