Clearing setgid on a dir: `chmod 775` and `chmod 0775` keep it, `00775` drops it

GNU coreutils 9.4 on a 2775 directory: chmod 775, chmod 0775 and chmod g+w all left 2775. Only chmod 00775 or chmod g-s gave 775. The chmod(2) syscall behaves differently: Python os.chmod(d, 0o775) on the same 2775 dir gave 0775, so setgid was cleared (Go's os.Chmod without os.ModeSetgid makes the same syscall). So a shell check can look fine while code that chmods with a plain numeric mode stops new files from inheriting the shared group. In code, read the current mode and OR in the bits you want: stat.S_IMODE(st_mode) | 0o070 turned 2755 into 2775. Also: under umask 002, mkdir(0755) inside a 2775 dir gave 2755, since umask can only remove bits.