Print Text Files with Line Numbers: nl, cat, and More

The nl command is purpose-built for adding line numbers to text files:

nl text-file

By default, nl numbers all non-empty lines and skips blank lines. This behavior is controlled by the body section style, which defaults to t (text only).

Common nl options

Number all lines including blanks:

nl -ba text-file

Start numbering at a specific value:

nl -v 100 text-file

Control number formatting:

# Right-aligned with leading zeros (width 6)
nl -nrz text-file

# Left-aligned
nl -nln text-file

# Left-aligned, no leading zeros
nl -nln text-file

Change the separator between number and content:

# Use colon and space instead of default tab
nl -s ': ' text-file

# Use pipe separator
nl -s ' | ' text-file

Set line number field width:

# Narrow field (4 characters)
nl -w 4 text-file

# Wide field (8 characters)
nl -w 8 text-file

Number only lines matching a pattern:

# Only number lines starting with uppercase letter
nl -bp'^[A-Z]' text-file

# Number lines containing "error"
nl -bp'error' text-file

Quick alternatives

cat -n for simple numbering:

cat -n text-file

This numbers all lines without blank-line distinction. It’s simpler but less flexible than nl. Good for quick one-offs.

grep -n to number while filtering:

grep -n "pattern" text-file

Restricts output to matching lines only. Useful when you need both filtering and line numbers in a single pass.

less -N for interactive viewing:

less -N /path/to/largefile

Displays line numbers within the pager. Press & in less to toggle line numbers on/off without exiting. Essential for navigating large log files.

awk for custom formatting:

awk '{print NR": "$0}' text-file

# With custom separators
awk '{printf "%5d | %s\n", NR, $0}' text-file

# Number only non-empty lines
awk 'NF {printf "%d: %s\n", NR, $0}' text-file

The awk approach gives you full control over output formatting and filtering logic.

sed for in-place numbering:

sed = text-file | paste -d ' ' - -

This uses sed to insert line numbers, then paste to align them. Less common but useful for scripting.

Practical examples

Number a log file and save output:

nl /var/log/syslog > syslog-numbered.txt

View a configuration file with line numbers, skipping blanks:

nl -s ': ' /etc/nginx/nginx.conf | less

Find error lines with context and line numbers:

nl -ba /var/log/app.log | grep -i error

Number a Python script with narrow field width:

nl -w 3 -s ' | ' script.py

Number specific line ranges:

# Lines 10-50 with numbers
sed -n '10,50p' file.txt | nl -v 10

Combine with head/tail for partial numbering:

# Number only the first 20 lines
head -20 file.txt | nl

# Number the last 30 lines
tail -30 file.txt | nl -v $(wc -l < file.txt | awk '{print $1-29}')

Compare files side-by-side with line numbers:

diff <(nl file1.txt) <(nl file2.txt)

Choosing the right tool

• Use nl when you need control over blank lines, formatting, and numbering patterns
• Use cat -n for quick numbering of entire files
• Use grep -n when you’re already filtering output
• Use less -N when viewing files interactively
• Use awk when you need complex custom formatting or conditional numbering