cat

Concatenates files by rows or columns.

Write with --output. A trailing path is not a positional argument. Input files are positional; the output path is not.

# Concatenate files by rows (vertical; default --mode rows)
undatum cat file1.csv file2.csv --mode rows --output output.csv

# Concatenate files by columns (horizontal)
undatum cat file1.csv file2.csv --mode columns --output output.csv

Key options: --mode rows|columns (default rows), --output. Also accepts --table, --on-error, --error-log, and --quotechar (shared options).