nl

What it does

nl reads a file and prepends line numbers. Unlike cat -n, it allows control over which lines are numbered (e.g., non-blank lines only, all lines, or specific logical sections).

How it works (mechanical)

nl processes input line by line and prefixes a formatted number field. It can define logical sections (header/body/footer) separated by markers, and apply numbering rules to each section independently.

• -b control numbering of body lines
• -h control numbering of header lines
• -f control numbering of footer lines
• -w set width of number field
• -s set separator between number and text

10 Practical Examples

# 1) Number all non-blank lines (default)
nl file.txt

# 2) Number ALL lines (including blanks)
nl -ba file.txt

# 3) Number only non-empty lines (explicit)
nl -bt file.txt

# 4) Set custom number width
nl -w 4 file.txt

# 5) Change separator to " | "
nl -s " | " file.txt

# 6) Number lines in a pipeline
grep ERROR logfile | nl

# 7) Start numbering at a specific value
nl -v 100 file.txt

# 8) Increment by 5
nl -i 5 file.txt

# 9) Restart numbering per logical page
nl -p file.txt

# 10) Compare with cat -n
cat -n file.txt

Notes & Gotchas

• Blank lines: default behavior does not number blank lines.
• Section markers: logical sectioning requires special delimiters.
• cat -n vs nl: cat -n is simpler; nl is more configurable.
• Formatting width: large files may require wider number fields.

Historical Context

nl has been part of Unix text processing tools for decades. It reflects the classic design philosophy: small, composable utilities that perform one job well and integrate into pipelines.

Related Commands

• cat -n — simple line numbering
• awk '{print NR, $0}' — programmable numbering
• sed '=' — print line numbers separately
• pr — formatted printing with headers