Working with Named Pipes
Pipes allow processes to communicate with each other. A pipe may also be known as a "FIFO" (First In, First Out).
The advantage over using files as a means of communication is, that processes are synchronized by pipes: a process writing to a pipe blocks if there is no reader, and a process reading from a pipe blocks if there is no writer.
The following is an example on how to create a named pipe, named here as pipe1.
cd /tmp mkfifo pipe1
To send a message to the pipe, use:
echo "hello" > pipe1
The process will appear to be hung at this point. There is no other process running to collect the data, so the kernel suspends the process. The process is said to be "blocked" at this stage.
On another terminal, it is possible to collect the data from the pipe, as follows:
The data from the pipe will now be read by cat (and written to the terminal), and the "blocked" writer process will be free to resume.
For some more information, see Bash FAQ #85.
Synchronous bidirectional Client - Server example
Here is a small example of a server process communicating with a client process. The server sends commands to the client, and the client acknowledges each command:
# server - communication example # Create a FIFO. Some systems don't have a "mkfifo" command, but use # "mknod pipe p" instead mkfifo pipe while sleep 1 do echo "server: sending GO to client" # The following command will cause this process to block (wait) # until another process reads from the pipe echo GO > pipe # A client read the string! Now wait for its answer. The "read" # command again will block until the client wrote something read answer < pipe # The client answered! echo "server: got answer: $answer" done
# client # We cannot start working until the server has created the pipe... until [ -p pipe ] do sleep 1; # wait for server to create pipe done # Now communicate... while sleep 1 do echo "client: waiting for data" # Wait until the server sends us one line of data: read data < pipe # Received one line! echo "client: read <$data>, answering" # Now acknowledge that we got the data. This command # again will block until the server read it. echo ACK > pipe done
Write both examples to files server and client respectively, and start them concurrently to see it working:
$ chmod +x server client $ server & client & server: sending GO to client client: waiting for data client: read <GO>, answering server: got answer: ACK server: sending GO to client client: waiting for data client: read <GO>, answering server: got answer: ACK server: sending GO to client client: waiting for data [...]