Can nl number specific lines only?

Yes, the nl command can number specific lines only by using the -b option, which allows you to specify which lines to number based on certain criteria.

Options for Numbering Specific Lines

Number Non-Empty Lines: You can use -b p to number only non-empty lines. For example:
```
nl -b p filename.txt
```
Number Lines Matching a Pattern: You can specify a regular expression to match lines. For example, to number only lines that do not start with a # (often used for comments), you can use:
```
nl -b p'^[^#]' filename.txt
```

Example

Given a file config.txt with the following content:

# This is a comment
port=8080
max_connections=100

# Another comment
db_host=localhost

Running the command:

nl -b p'^[^#]' config.txt

Would produce:

 1 port=8080
 2 max_connections=100
 3 db_host=localhost

Summary

Using the -b option with nl allows you to focus on specific lines, making it easier to reference important configurations or settings while ignoring comments or empty lines. This feature is particularly useful in large configuration files or logs.

If you have more questions or need further examples, feel free to ask!