nl

Adds line numbers to TSV rows. This is a simplified, TSV-aware version of the Unix nl program with support for treating the first input line as a header.

Behavior:

• Prepends a line number column to each row of input data.
• Line numbers increase by 1 for each data line, continuously across all input files.
• Header lines are never numbered.
• Completely empty files are skipped and do not consume line numbers.
• Supports custom delimiters between the line number and line content.

Input:

• Reads from files or standard input.
• Files ending in .gz are transparently decompressed.
• When multiple files are given, lines are numbered continuously across files.
• Empty files (including files with only blank lines) are skipped.

Output:

• By default, output is written to standard output.
• Use --outfile / -o to write to a file instead.
• Each output line starts with the line number, followed by a delimiter, then the original line content.

Header behavior:

• --header / -H: Treats the first line of the input as a header. The header is written once at the top of the output with the line number column header prepended.
• --header-string / -s: Sets the header text for the line number column (default: “line”). This option implies --header.
• When using header mode with multiple files, only the header from the first non-empty file is written; subsequent header lines are skipped and not numbered.

Numbering:

• --start-number / -n: The number to use for the first line (default: 1, can be negative).
• Numbers increase by 1 for each data line across all input files.

Examples:

1. Number lines of a TSV file tva nl data.tsv

2. Number lines with a header for the line number column tva nl --header --header-string LINENUM data.tsv

3. Number lines starting from 100 tva nl --start-number 100 data.tsv

4. Number multiple files, preserving continuous line numbers tva nl input1.tsv input2.tsv

5. Read from stdin cat input1.tsv | tva nl

6. Use a custom delimiter tva nl --delimiter ":" data.tsv