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    Files that are copied (e.g. by cp) are in fact newly created. If they don't inherit the group permission, the copying program is playing games, like copying to a temporary file and then moving it to the target directory. Commented Aug 4, 2015 at 15:30
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    @Kaz Good point. I updated the answer to clarify that it is cp -p that overrides the setgid setting. Commented Aug 4, 2015 at 17:55
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    But does cp -p override the setgid setting? On every single Unix implementation in existence? POSIX says that it is unspecified whether a failure to copy the user ID or group ID under cp -p results in a diagnostic message! However, the S_SUID and S_SGID bits, respectively, are required to be cleared in that situation (i.e. if a file is setuid bob, but bob's ownership can't be copied so that the file is owned by janet, don't make it setuid janet.) Commented Aug 4, 2015 at 20:58
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    "does cp -p override the setgid setting?" According to the POSIX spec, that is what it is supposed to do. It does so on all the Unix systems that I have used. You have quoted the part of the spec regarding what to do to protect security in the case when the group ID cannot be duplicated. I have never run into such a "cannot" situation, have you? Commented Aug 5, 2015 at 6:22
  • Some part of this answer is inaccurate. For example, cp -p does have the setgid effect on the file. Commented Apr 12, 2022 at 14:57