nl Command: Number Lines in Linux
Summary
The nl command numbers the lines of a file or standard input, offering control over the line numbering format and criteria.
Introduction
The nl command in Linux is a utility that reads lines from a file (or standard input) and numbers them. Unlike some simpler line-numbering tools, nl provides a high degree of customization over which lines are numbered and how the numbering is formatted. This makes it useful for preparing documents, scripts, or code listings where line numbers are required for reference or analysis.
Use Case and Examples
Numbering all lines in a file
This command will number all non-empty lines inmyfile.txt, prepending the line number to each line. Empty lines will not be numbered by default. Numbering only lines containing specific text
This command will number only lines inmyfile.txt that contain the string "pattern". The -b p"pattern" option specifies that the numbering should only apply to lines matching the given regular expression. Numbering all lines including empty lines
This command numbers all lines, including empty ones, inmyfile.txt. The -b a option specifies that all lines should be numbered. Changing the starting line number
This will start the numbering at 100 instead of 1 formyfile.txt. Changing the numbering increment
This will number the lines with an increment of 5 (1, 6, 11, ...).Customizing the number format
This example changes the number format to left-justified with leading zeros using the-n ln option. Other options for -n are rn (right-justified, leading zeros) and rz (right-justified, space padding). Changing the delimiter between number and text
This changes the default delimiter (usually a tab) to ": " for the line numbers inmyfile.txt. Commonly used flags
| Flag | Description | Example |
|---|---|---|
-b STYLE | Specifies which lines to number, where STYLE can be a (all lines), t (non-empty lines), n (no lines), or p followed by a regular expression (lines matching the regular expression). | nl -b a myfile.txt (numbers all lines) |
-n FORMAT | Specifies the number format, where FORMAT can be ln (left-justified, no leading zeros), rn (right-justified, leading zeros), or rz (right-justified, space padding). | nl -n rn myfile.txt (right-justified numbers with leading zeros) |
-v STARTNUM | Sets the initial value of the line number to STARTNUM. | nl -v 100 myfile.txt (starts numbering at 100) |
-i INCREMENT | Sets the increment value for line numbers. | nl -i 5 myfile.txt (increments line numbers by 5) |
-s STRING | Sets the separator string used between the line number and the corresponding line text. The default is a tab. | nl -s ": " myfile.txt (uses ": " as the separator) |
-w WIDTH | Sets the width of the line number field. Default is 6. | nl -w 4 myfile.txt (uses a field width of 4 for the line numbers) |