nl — Number lines

Linux / Basic Linux commands / nl.html

Prints a text file with line numbers added.

Basic Information

What is this command?

nl prints input with line numbers. It resembles cat -n but gives finer control over blank lines and number formatting.

-b selects which lines are numbered, -w sets the field width and -s the separator. It is useful when quoting code with line references.

What problems does it solve?

• Quoting code or configuration with line numbers
• Referring to a specific log line
• Numbering only non-empty lines
• Controlling number width and separator

Key features

Numbering

Adds a sequential number before each line.

• When to use: Citing documents or code
• What you get: You can reference exact lines.
• Limitations: The original file is not modified.

Blank line control

-b a or -b t selects all or non-empty lines.

• When to use: Reviewing files with many blank lines
• What you get: Numbering stays contiguous and readable.
• Limitations: By default only non-empty lines are numbered.

Formatting

-w and -s change width and separator.

• When to use: Producing a consistent output format
• What you get: Output stays aligned.
• Limitations: The options can feel numerous at first.

Installation and usage

Check whether it is installed

nl --version

sudo apt install coreutils # Debian/Ubuntu sudo dnf install coreutils # Fedora

Real-world examples

Basic numbering

Number non-empty lines.

nl config.yml | head

Number all lines

Number blank lines too.

nl -b a notes.txt | head

Custom format

Change width and separator.

nl -w 3 -s ': ' app.py | sed -n '1,10p'