What is the nl command used for?

The nl command adds line numbers to text files. It's useful for referencing specific lines in documents, code files, or any text content where line numbers would be helpful.

How do I number all lines in a file?

Use 'nl filename' to number all lines, or 'cat filename | nl' to pipe the content through nl. The command will add line numbers to the left of each line.

Can I customize the line numbering format?

Yes, you can use options like -w (width), -s (separator), -n (number format), and -v (starting number) to customize the appearance and format of line numbers.

nl

Add line numbers to text files

Syntax

nl [options] [file]

Basic Usage

Number lines in a file

nl filename.txt

Adds line numbers to the specified file.

Number lines from stdin

cat filename.txt | nl

Number lines with custom starting number

nl -v 10 filename.txt

Common Options

-b style: Body numbering style (a=all, t=non-empty, n=none, p=regex)
-f style: Footer numbering style
-h style: Header numbering style
-i number: Increment line numbers by specified value
-l number: Join blank lines (default is 1)
-n format: Number format (ln=left, rn=right, rz=right with zeros)
-p: Don't reset line numbers at logical page breaks
-s string: String to separate line numbers from text
-v number: Starting line number
-w number: Width of line number field

Practical Examples

Basic line numbering

nl document.txt

Number only non-empty lines

nl -b t document.txt

Custom separator and width

nl -s " | " -w 3 document.txt

Right-aligned numbers with zeros

nl -n rz -w 4 document.txt

Start numbering from 100

nl -v 100 document.txt

Increment by 5

nl -i 5 document.txt

Number with custom format

nl -s " --> " -w 2 -n ln document.txt

Number only specific sections

nl -b p'^[A-Z]' document.txt

Numbering Styles

Body Numbering (-b)

a: Number all lines
t: Number only non-empty lines
n: No numbering
p regex: Number only lines matching regex

Number Format (-n)

ln: Left-justified, no leading zeros
rn: Right-justified, no leading zeros
rz: Right-justified, with leading zeros

Best Practices

When to Use nl

Adding line numbers to code files for reference
Numbering document lines for easy citation
Creating numbered lists from text files
Preparing text for documentation or reports
Debugging and troubleshooting text processing

Important Notes

nl modifies the output but doesn't change the original file
Use redirection (>) to save numbered output to a new file
Blank lines are counted by default unless using -b t
Line numbers are left-aligned by default
Consider using cat -n for simpler line numbering

nl