dup2 / dup - why would I need to duplicate a file descriptor ?
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I'm trying to understand the use of `dup2` and `dup` . 

From the man page : 

    DESCRIPTION
    
    dup and dup2 create a copy of the file descriptor oldfd.
    After successful return of dup or dup2, the old and new descriptors may be used interchangeably. They share locks, file position pointers and flags; for example, if the file position is modified by using lseek on one of the descriptors, the position is also changed for the other.
    
    The two descriptors do not share the close-on-exec flag, however.
    
    dup uses the lowest-numbered unused descriptor for the new descriptor.
    
    dup2 makes newfd be the copy of oldfd, closing newfd first if necessary.  
    
    RETURN VALUE
    
    dup and dup2 return the new descriptor, or -1 if an error occurred (in which case, errno is set appropriately).  

Why would I need that system call ? what is the use of duplicating the file descriptor ? 

If I have the file descriptor , why would I want to make a copy of it ? 

I'd appreciate if you could explain and give me an example where `dup2` / `dup` is needed . 

Thanks 