macOS fix

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macOS’s BSD readlink is different from GNU readlink, says: illegal option -- f

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see also: #573

[darealshinji]

Maybe it's better to add a check before the elif readlink -f part.
Something like this:

elif readlink --version | head -1 | grep -q GNU && readlink -f "$SHELL" 2>&1 | grep -q -i 'busybox'; then

Does this work on Mac and suppress the readlink warning?

[zeilen]

Keep it simple. It’s simply not needed with my commit because the first if (line 1797) would become true in macOS too and so it would not run the elif and else code. Line 1799 runs fine.