Practice 

Let consider the given command describes a method to take backup.

# tar cvf /dev/fd0 ./test*

./test

./test1

./test2

./test3

In the given example tar is the command that is used to take the backup, /dev/fd0 is the device used to take backup and. /test* is the directory.

c          -           It creates a new archive; writing starts at the beginning of the archive, alternate of after the last file.

v          -           Generally, tar does its work silently. The v (verbose) function modifier causes tar to show the name of every file it treats, proceeded through the function letter. With the t function, v provides more information about the archive entries than only the name.

f           -    It Causes tar to use the next argument as the name of the archive alter of the default  device listed in /etc/default/tar. tar writes to the standard output or reads from the standard input, whichever is appropriate if the name of the file is a dash (-). Therefore, tar can be used as the head or tail of a pipeline.

Note :

To use tape archive as device file use /dev/st0 (st0 - indicates the first tape drive) is to be used.

In Solaris, it is different.

/dev/rmt/0

/dev/rmt-logical device name of tape

0-denotes first tape drive