To cite package ‘getopt’ in publications use:

  Davis T, Day A (2024). _getopt: C-Like 'getopt' Behavior_. R package
  version 1.20.4, <https://github.com/trevorld/r-getopt>.

A BibTeX entry for LaTeX users is

  @Manual{,
    title = {getopt: C-Like 'getopt' Behavior},
    author = {Trevor L Davis and Allen Day},
    year = {2024},
    note = {R package version 1.20.4},
    url = {https://github.com/trevorld/r-getopt},
  }