The following sequence of code has been observed in various programs: dup2(fd,0); dup2(fd,1); dup2(fd,2); if (fd > 2) close(fd); To see why the if test is needed, assume that fd is 1 and draw a picture of what happens to the three descriptor entries and the corresponding file table entry with each call to dup2. Then assume that fd is 3 and draw the same picture.

If fd is 1, then the dup2(fd, 1) returns 1 without closing descriptor 1. (Remember our discussion of this in Section 3.12.) After the three calls to dup2, all three descriptors point to the same file table entry. Nothing needs to be closed.
If fd is 3, however, after the three calls to dup2, four descriptors are pointing to the same file table entry. In this case, we need to close descriptor 3.