Pseudo-Terminals (ptys) are a type of interprocess communication with a master and a slave end. One program (in our case nullmodem) opens the master sides of connections and therefore provides connectible slave sides of connections. If nullmodem receives some data on one of these connections it retransmits them on all other connections. It can be seen as a network with star-type wiring. Because the names of the slave side ptys are dynamic, nullmodem creates symbolic links to filenames of your choice. e.g. the command nullmodem /tmp/loop0 /tmp/loop1 /tmp/loop2 creates a pty-network with 3 slave sides linked to the names /tmp/loop0, /tmp/loop1 and /tmp/loop2. If you close a slave side connection nullmodem will detect this and provide a new stub to connect to and update the symbolic link.