On 2025-11-02 01:24, Bruno Haible via GNU coreutils Bug Reports wrote:
We could document that users should not use environment variables that start with "__"
In addition to Pádraig's qualms, POSIX says[1] that if an environment variable's name consists solely of uppercase ASCII letters, '_', and non-initial digits, the variable might affect the behavior of standard utilities like 'cat'. So if we document environment variable gotchas, we should document that one too.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/V1_chap08.html#tag_08_01
