Processes Interaction

Processes cannot interact by default.  However they can communicate using

Pipes, shared memory, semaphores, and other IPC (interprocess communication) mechanisms
     $ ls -l | grep "Apr 19" | awk '{print $9}'

FIFOs, sockets, network sockets, files, etc
     $ mknod /tmp/named_pipe p
     $ ls -l /tmp/named_pipe
     prw-------  1 reegen reegen   0 Apr 19 16:03 /tmp/named_pipe
     $ echo "Hello World!" >> /tmp/named_pipe &
     $ cat /tmp/named_pipe
     Hello World!

Signals (HUP, TERM, USR1, KILL, etc)
     $ killall -HUP syslogd