Bash - Use automatic file descriptor creation instead of fifo

Question

At the beginning, I created a small server with netcat and it worked well:

#!/bin/bash

PORT="1234";

startServer(){
    fifo="fifo/$1";
    mkfifo "$fifo";

    connected="0";

    netcat -q 0 -l -p "$PORT" < "$fifo" | while read -r line; do
        if [ "$connected" == "0" ];then
            #listen for a new connection
            startServer $(($1+1))&
            connected="1";
        fi

        #server logic goes here

    done > "$fifo";

    rm "$fifo";
}

mkdir fifo;
startServer 0;

It uses fifos to redirect the main loop output back to netcat.

Every call to startServer increases the value of $1 and creates a fifo file fifo/$1.

But I want to use bash's automatic file descriptor creation to get rid of fifo files and the parameter of startServer.

After multiple attempts, here is my current attempt at modifying the startServer function. I can receive lines from the client but unfortunately it does not receive anything back. I am confused about what is wrong.

startServer(){
    connected="0";

    #create file descriptor and read output from main loop
    exec {fd}> >( netcat -q 0 -l -p "$PORT" | while read -r line; do
        if [ "$connected" == "0" ];then
            #listen for a new connection
            startServer&
            connected="1";
        fi

        #server logic goes here

    done ) >&$fd;

    #close file descriptor
    exec {fd}<&-;
    exec {fd}>&-;
}

Also, I cannot use netcat's "-e" option since it is not available in the netcat-openbsd package.

Allocating a file descriptor doesn't allocate a buffer for it to refer to. — chepner, Nov 18 '17 at 18:42
@chepner The redirection >( ... ) or <( ... ) should allocate a buffer. — nd97, Nov 18 '17 at 22:28
That's not a redirection; it's a process substitution, which is effectively just special syntax for a fifo. — chepner, Nov 18 '17 at 22:40

meuh · Accepted Answer · 2017-11-18T21:33:07.050

8

You cannot set and use fd in the same command; you are effectively doing exec {fd}> ... >&$fd. What can work is creating the bash fifo/pipe first, using some simple command like :. Eg:

startServer(){
    local connected=0 fd
    exec {fd}<> <(:)
    nc -q 0 -l -p "$PORT" <&$fd | 
    while read -r line
    do  if [ "$connected" == "0" ]
        then startServer $(($1+1)) &
             connected="1"
        fi
        echo server $1 logic goes here
    done >&$fd
    exec {fd}<&-
}

edited Nov 18 '17 at 21:33

answered Nov 18 '17 at 19:49

meuh

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Nice! no-op FTW – nhed Aug 21 '20 at 17:30

Bash - Use automatic file descriptor creation instead of fifo

1 Answers1