Introduction to the `nl` Command (with examples)

1: Number non-blank lines in a file

Code:

nl path/to/file

Motivation: Numbering lines in a file can be useful for reference or organization purposes. By using the nl command, we can easily add line numbers to a file.

Explanation: The nl command is used to number lines in a file or from standard input (stdin). By running nl path/to/file, the command will number all non-blank lines in the specified file. Non-blank lines contain printable text and are separated by the newline character.

Example Output:

 1  First line of text
 2  Second line of text
 3  Third line of text

2: Read from stdout

Code:

cat path/to/file | nl options -

Motivation: Sometimes, we might want to read the contents of a file from standard output rather than directly from a file. This can be useful when working with processes that produce output that needs to be piped into the nl command for numbering.

Explanation: The cat command is used to concatenate and display the contents of files. By combining cat with the nl command, we can read the contents of a file using the pipe (|) operator and then number the lines using nl options -. The - after the options indicates that the input should be read from standard input.

Example Output:

 1  First line of text
 2  Second line of text
 3  Third line of text

3: Number only the lines with printable text

Code:

nl -t path/to/file

Motivation: In some cases, we might only be interested in numbering the lines that contain printable text. This can be useful when working with files that contain a mix of blank lines and actual content, and we want to maintain accurate line numbering for the non-blank lines.

Explanation: By using the -t option with the nl command, we can number only the lines that have printable text. Blank lines will be excluded from the numbering. This allows us to maintain a logical sequence of line numbers when working with files that have empty lines.

Example Output:

 1  First line of text
 2  Second line of text
 3  Third line of text

4: Number all lines including blank lines

Code:

nl -b a path/to/file

Motivation: Sometimes, we may want to preserve line numbers for all lines in a file, including blank lines. This can be useful when we need to maintain the original structure of the file and accurately reference specific lines.

Explanation: Using the -b a option with the nl command will number all lines in the file, including blank lines. This ensures that line numbers are assigned to every line, regardless of whether it contains printable text or not.

Example Output:

 1
 2  First line of text
 3
 4  Second line of text
 5  Third line of text

5: Number only the body lines that match a basic regular expression (BRE) pattern

Code:

nl -b p'FooBar[0-9]' path/to/file

Motivation: There may be situations where we only want to number lines that match a specific pattern, such as lines containing a certain word or phrase. This can be useful when we want to quickly identify and reference specific lines in a file based on their content.

Explanation: Using the -b p'FooBar[0-9]' option with the nl command allows us to number only the lines that match the specified basic regular expression (BRE) pattern. In this example, the pattern is 'FooBar[0-9]', which will match lines containing “FooBar” followed by a single digit (0-9).

Example Output:

 1  FooBar1
 2  FooBar2
 3  FooBar3

In conclusion, the nl command provides a convenient way to number lines in a file or from standard input. By using various options, we can customize the behavior of the command to meet our specific needs, whether it’s numbering only non-blank lines, preserving blank lines, or numbering lines based on a specific pattern.