Linux nl Command

Linux 命令大全Linux Command Reference

1. nl Command Overview

nlIt is a simple but practical command-line tool in the Linux system, with its full name being "number lines" (line numbering). Its main function is to read the file content, add a line number to each line, and then output the result to standard output.

1.1 Why Do We Need the nl Command?

In daily development or system administration, we often need to:

  • Quickly view specific lines of code or log files
  • Conveniently reference specific line numbers when discussing file content
  • Number file content for subsequent processing
  • Generate documents or reports with line numbers

Althoughcat -ncan also achieve similar functionality, butnlprovides richer line number control options, which can meet more complex needs.

2. Basic Syntax and Parameters

2.1 Basic Command Format

Example

nl [Option]... [File]...

2.2 Description of Common Options

Option Description
-b Specify the line number calculation rule (followed by a style)
-i Set the line number increment (default is 1)
-n Set the line number format (left-aligned/right-aligned/without zero padding)
-s Set the separator between the line number and the content
-v Set the starting line number (default is 1)
-w Set the line number field width (number of characters)
-p Do not reset line numbers at page breaks

2.3 Line Number Calculation Rules (-b Option)

-bThe option can be followed by the following styles:

Style Description
a Number all lines
t Number only non-empty lines (default)
n Do not number
p正则表达式 Number only lines matching the regular expression

3. Practical Example Demonstrations

3.1 Basic Usage: Adding Line Numbers to a File

Example

# Add line numbers to all non-empty lines of example.txt
nl example.txt

# Add line numbers to all lines (including empty lines)
nl -b a example.txt

3.2 Customizing Line Number Format

Example

# Set the line number width to 3, using right-aligned format
nl -n rz -w 3 example.txt

# Use "|" as the separator, line numbers left-aligned
nl -s " | " -n ln example.txt

3.3 Advanced Numbering Rules

Example

# Start numbering from 10 with an increment of 2
nl -v 10 -i 2 example.txt

# Number only lines containing "error"
nl -b perror example.log

3.4 Processing Multiple Files

Example

# Number multiple files consecutively
nl file1.txt file2.txt

# Number each file separately
nl -p file1.txt file2.txt

4. Comparison with Other Commands

4.1 nl vs cat -n

Feature nl cat -n
Default behavior Number only non-empty lines Number all lines
Format control Rich (alignment, width, separator, etc.) Limited
Numbering rules Customizable (regular expression matching, etc.) Fixed
Performance Slightly slower Slightly faster

4.2 nl vs grep -n

grep -nIt also displays line numbers, but its main purpose is search and filtering, and line numbers are just additional information.nlnl, on the other hand, is a tool specifically optimized for line number display.

5. Practical Application Scenarios

5.1 Code Review

Example

# Generate a code file with line numbers for easy discussion
nl -w 3 -n rz source.py > numbered_source.txt

5.2 Log Analysis

Example

# Number only log lines containing "ERROR"
nl -b pERROR app.log | less

5.3 Document Processing

Example

# Generate a Markdown document with line numbers
nl -s ". " -w 2 -n rz README.md

6. Notes and Tips

  1. Empty line handling: By default,nlempty lines are skipped; use-b ato force numbering for all lines.
  2. Large file handling: For very large files, consider combininglessorheadto use
  3. Pipeline usage: It can accept standard input, such ascat file | nl
  4. Format consistency: It is recommended to use a fixed set of parameters in scripts to ensure consistent output format.
  5. Performance considerations: For simple line numbering needs,cat -nmay be faster.

7. Summary and Exercises

7.1 Exercise Problems

  1. Create a test filetest.txtContaining 5 lines of text and 2 empty lines
  2. Usenlto display all line numbers (including empty lines)
  3. Set line numbers to start from 100 with an increment of 5
  4. Use "::" as the separator and a line number width of 4
  5. Redirect the result tonumbered.txt

7.2 Reference Answers

Example

# Create a test file
echo -e "First line\n\nSecond line\nThird line\n\nFourth line\nFifth line" > test.txt

# Execute the nl command
nl -b a -v 100 -i 5 -s "::" -w 4 test.txt > numbered.txt

# View the result
cat numbered.txt

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