It's okay to send the text on stderr, when invalid option is given: /usr/bin/Xephyr -xyzabc But not, when help is asked: /usr/bin/Xephyr --help | less Please change the -help option to send text to stdout, not stderr. See GNU cp(1), ls(1) etc. for reference.
This appears to be a problem with the X server in general, not just Xephyr. The function for printing the help message can be found at [1] and as you can see it writes everything to stderr regardless of the situation. It is called by both the normal X server and Xephyr. Perhaps this should be reopened as a general X server issue? [1]: https://cgit.freedesktop.org/xorg/xserver/tree/os/utils.c?id=fdc79fe72bc0b97776df2c3a664076c60e08a87c#n516
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