Linux provides various commands for text formatting and editing, and one particularly useful command is the nl command. This tool allows you to add line numbers to each line of a text file or input, making it easier to read and organize content.
What Does the nl Command Do?
The nl command displays the contents of a text file with line numbers added to non-empty lines by default. It’s especially useful when working with scripts or reviewing large files where tracking line numbers is critical for debugging or editing.
Example: Using the nl Command
Suppose you have a file named hello-world.sh containing the following script:
#!/bin/bash
#
#This is a demo script that greets the world
#Author: Jose Navarro Varela
#Usage: ./hello-world.sh
clear
echo hello world
exit 0
To display the file with line numbers, simply run:
nl hello-world.sh
The output will look like this:
1 #!/bin/bash
2 #
3 #This is a demo script that greets the world
4 #Author: Jose Navarro Varela
5 #Usage: ./hello-world.sh
6 clear
7 echo hello world
8 exit 0
By default, the nl command numbers only non-empty lines.
Numbering All Lines (Including Empty Lines)
To include empty lines in the numbering, use the -b a option:
nl -b a hello-world.sh
The output will look like this:
1 #!/bin/bash
2 #
3 #This is a demo script that greets the world
4 #Author: Jose Navarro Varela
5 #Usage: ./hello-world.sh
6
7 clear
8 echo hello world
9 exit 0
Adding a Custom Separator
The nl command allows you to customize the separator between the line numbers and the text using the -s option. For example, to add ... as the separator:
nl -s "..." hello-world.sh
The output will look like this:
1...#!/bin/bash
2...#
3...#This is a demo script that greets the world
4...#Author: Jose Navarro Varela
5...#Usage: ./hello-world.sh
6...
7...clear
8...echo hello world
9...exit 0
Additional Options
The nl command offers more options for customization, such as:
-w: Specify the width of the line number field.-n: Define the numbering format (right-aligned, left-aligned, or zero-padded).
You can explore all available options by checking the manual page with:
man nl
Common Scenarios for Using the nl Command
The nl command is widely used in various situations. Below are some of the most common scenarios:
1. Debugging Shell Scripts
When debugging a shell script, you may need to refer to specific lines where errors occur. Using the nl command to add line numbers makes it easier to pinpoint the problematic lines.
Example:
nl my-script.sh
2. Preparing Documentation or Code Reviews
When sharing scripts or code snippets for review, adding line numbers makes the content more readable and easier to reference during discussions.
Example:
nl config.conf > numbered-config.conf
3. Reviewing Large Text Files
When working with large log files, configuration files, or data files, adding line numbers helps you navigate and reference specific sections.
Example:
nl /var/log/syslog
4. Comparing Files with Line Numbers
Adding line numbers to files before comparing them makes it easier to identify differences.
Example:
nl file1.txt > numbered-file1.txt
nl file2.txt > numbered-file2.txt
diff numbered-file1.txt numbered-file2.txt
5. Formatting Output for Reports
The nl command can be used to format text files for reports or presentations where numbered lines are required for clarity.
Example:
nl report.txt > numbered-report.txt
6. Viewing Scripts with Empty Line Numbers
Sometimes scripts contain empty lines that are significant for readability. You can use the -b a option to number all lines, including empty ones.
Example:
nl -b a script.sh
7. Customizing Line Numbers for Specific Formats
When presenting numbered text files, you might need custom separators or formatting.
Example:
nl -s " -> " textfile.txt
Why Use the nl Command?
The nl command is a simple yet powerful tool for:
- Debugging scripts by identifying specific lines.
- Adding line numbers to text files for easier navigation.
- Preparing formatted output for documentation or sharing.