dup fileId [targetFileId]
Duplicates an open file. A new file ID is opened that addresses the same file as fileId.
If targetFileId is specified, then the file is dup to
this specified file ID. Normally this is stdin, stdout, or stderr.
The dup command will handle flushing output and
closing this file. The target file should be open if it is one of
stdin, stdout, or stderr and the process is not going to do an execl.
Otherwise internal C code that uses one of these files via direct
access to stdio FILE struct may behave strangely or fail.