The meaning of copyleft is just that those who use the license have to typically also use the same or a similar license. There is no restriction to who can use it, just to the terms under which they can use it and have to license it.
GPLv3 for example does not allow derivative works to be closed source, while MIT does. In practice this makes some companies hesitant to use GPL-licensed works, but they can, and many do, they just have to abide by its rules. :)
I use simple old Busybox ash, which comes as default with Alpine Linux :)
No fancy features at all, no bashisms. Not everyone's cup of tea I'm sure, but I like the extreme simplicity.
That is answered in the original post, the author is just doing this for fun and learning. :)
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@mander.xyz