What the standards say supports that assertion even less than what the other manual pages say. There is a standard for this. It is IEEE 1003.1 a.k.a. The Sing

What the standards say supports that assertion even less than what the other manual pages say. There is a standard for this. It is IEEE 1003.1 a.k.a. The Single Unix Specification. * http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap03.html#tag_03_437 http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_... * http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap03.html#tag_03_282 http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_... Valid portable user names can have digits and letters intermixed. They are simply restricted to not starting with a minus. * https://github.com/systemd/systemd/issues/6237 https://github.com/systemd/systemd/issues/6237 (https://news.ycombinator.com/item?id=14681377 https://news.ycombinator.com/item?id=14681377) In the actual bug report, as opposed to Mattias Geniar's article headlined here, this is already mentioned.