This option is used to specify the file name of the archive tar works on. If you give an alternate login name, it will be given to the rsh . Starting with version 1.11.5, GNU tar uses standard input and standard output as the default device, and I will not try anymore supporting automatic device detection at installation time. This was failing really in too many cases, it was hopeless. Further, I think most actual usages of tar are done with pipes or disks, not really tapes, cartridges or diskettes.
Some users think that using standard input and output is running after trouble.
We had so many bug reports in that area of configuring default tapes automatically, and so many contradicting requests, that we finally consider the problem to be portably intractable. After having seen all this mess, using standard input and output as a default really sounds like the only clean choice left, and a very useful one too. GNU tar reads and writes archive in blocks, I suspect this is the main reason why block devices are preferred over character devices. Most probably, block devices are more efficient too. When this command is not used, the shell command found when the tar program was installed is used instead.