Number Lines in a File with the Linux nl Command

Learn how to use the Linux nl command to number non-blank or every line, customize increments and separators, match lines with regular expressions, and format output.

The Linux nl command reads text and writes it to standard output with line numbers. It is useful for inspecting files, comparing text, and sending numbered content through a shell pipeline.

nl normally displays a numbered copy of the input; it does not modify the original file in place.

Basic nl Syntax

nl [options] [file]

For example:

nl notes.txt

When a file is not supplied, nl reads from standard input:

printf 'first\nsecond\n' | nl

The numbered text is printed to standard output, so it can be viewed directly, piped to another command, or redirected to a different file.

Default Line-Numbering Behavior

With no options, nl numbers ordinary non-blank body lines. A blank line is a line with no visible text. Blank lines are normally left without numbers and do not advance the displayed number sequence.

Suppose notes.txt contains:

alpha

beta
gamma

Run:

nl notes.txt

The result is similar to:

     1  alpha

     2  beta
     3  gamma

The first numbered body line starts at 1, and each subsequent numbered body line normally increases by 1.

Number Every Line with `-b a`

The -b option selects the body-line numbering style. The style a means “all,” so every physical input line receives a number, including blank lines.

nl -b a notes.txt

Using the same file, the output is similar to:

     1  alpha
     2
     3  beta
     4  gamma

This differs from the default because the blank line receives number 2, and the following lines continue with 3 and 4.

Change the Numbering Increment

An increment is the amount added after each numbered line. Use -i NUMBER to change it.

nl -i 5 notes.txt

Numbered lines use a sequence such as 1, 6, 11, and 16. Other examples include:

nl -i 2 notes.txt
nl -i 10 notes.txt

The increment affects the generated numbers, not the physical spacing or content of the source lines.

Change the Number Separator

A separator is the string inserted between the formatted number and the original line content. Set it with -s STRING.

nl -s ': ' notes.txt

Output resembles:

     1: alpha

     2: beta
     3: gamma

Other separators are possible:

nl -s '. ' notes.txt
nl -s $'\t' notes.txt
nl -s ' -- ' notes.txt

The shell syntax $'\t' produces a tab in shells that support ANSI-C quoting, such as Bash.

Number Only Lines Matching a Regular Expression

A regular expression is a pattern used to select text. The form -b pREGEXP tells nl to number only body lines matching the expression.

To number only lines whose first character is an uppercase T:

nl -b 'p^T' notes.txt

The ^ anchor means “the beginning of the line.” Therefore, ^T matches a line starting with T. Nonmatching lines remain visible but have no generated number.

Quote the pattern when it contains spaces, wildcard-like characters, parentheses, brackets, or other shell-special characters. Quoting prevents the shell from changing the expression before nl receives it.

nl -b 'p^Task: ' notes.txt
nl -b 'p^[A-Z][a-z]' notes.txt

If a pattern does not select the expected lines, first check whether it matches the actual text. Differences in capitalization, leading spaces, and punctuation commonly cause a mismatch.

Format the Line Numbers

The -n FORMAT option controls line-number alignment and zero padding. A line-number format changes the presentation of the number, not the source text.

nl -n ln notes.txt
nl -n rn notes.txt
nl -n rz notes.txt

Format — Alignment — Leading Zeros — Illustrative Output

ln — Left-justified — None — 1 text

rn — Right-justified — None — 1 text

rz — Right-justified — Yes — 000001 text

ln places numbers at the left side of their number field without leading zeros. rn aligns numbers on the right without leading zeros and is commonly used by the default presentation. rz aligns numbers on the right and pads them with zeros.

The active number width determines how many positions are available. With rz, the padding makes numbers line up consistently for machine-oriented or fixed-width output.

Common nl Options

Option — Purpose — Example — Result

No option — Number non-blank body lines — nl notes.txt — Sequential numbers on non-blank lines

-b a — Number every body line — nl -b a notes.txt — Blank lines receive numbers too

-b pREGEXP — Number matching body lines — nl -b 'p^T' notes.txt — Only lines beginning with T are numbered

-i NUMBER — Set the increment — nl -i 5 notes.txt — Numbers advance by 5

-s STRING — Set the separator — nl -s ': ' notes.txt — A colon and space separate numbers from text

-n ln — Left-justify without padding — nl -n ln notes.txt — Numbers are left-aligned

-n rn — Right-justify without padding — nl -n rn notes.txt — Numbers are right-aligned

-n rz — Right-justify with zero padding — nl -n rz notes.txt — Numbers contain leading zeros

Use nl in Pipelines

nl accepts both named files and standard input. This makes it useful with commands that generate text.

printf 'first\n\nthird\n' | nl -b a

Expected output is similar to:

     1  first
     2
     3  third

You can combine numbering with another command through a pipe:

printf 'alpha\nbeta\n' | nl -s ': ' | less

When you need a saved numbered copy, redirect output to a different file:

nl -b a notes.txt > numbered-notes.txt

This leaves notes.txt unchanged and writes the numbered output to numbered-notes.txt.

Important safety rule: Do not redirect output to the same file that nl is reading.

nl notes.txt > notes.txt

The shell usually opens the output file before starting nl. Opening it for writing can truncate the file before nl reads it. Use a distinct destination, inspect the result, and replace the original only as a separate deliberate step.

Troubleshooting nl

Blank lines do not have numbers

This is the normal default behavior: nl numbers non-blank body lines only. Use:

nl -b a FILE

A pattern does not number the expected lines

The regular expression may not match the actual line content, or the shell may have interpreted part of the pattern. Check capitalization, leading spaces, and anchors. Quote the pattern:

nl -b 'p^T' FILE

Numbers have unexpected spacing or no leading zeros

The -n setting controls alignment and padding. Select the required style:

nl -n ln FILE
nl -n rn FILE
nl -n rz FILE

The original file becomes empty

This usually happens when output was redirected to the input file. Write to a distinct file instead:

nl FILE > numbered-FILE

The command is not found

Check whether the executable is available and whether your PATH is configured correctly:

command -v nl

If no path is printed, install or restore the operating system's standard core command-line utilities package.

Exam-Relevant Notes

nl FILE numbers non-blank body lines by default and normally starts at 1.
-b a numbers all lines, including blank lines.
-i NUMBER changes the amount added between successive numbered lines.
-s STRING changes the text between the number and the source content.
-b pREGEXP numbers only body lines matching a regular expression.
-n ln, -n rn, and -n rz select left alignment, right alignment, and right alignment with zero padding respectively.
nl writes to standard output and does not edit the input file in place.
Redirect numbered output to a different file to avoid truncating the input.

Related Linux Skills

For more command-line context, see Linux command-line topics, Bash, and finding the full path of shell commands.