使用nc传输文件时文件为空

Question

I am struggling long time on the weird situation on file transferring using nc, I use the following bash script to transfer files from A machine to B machine, the script seems okay and the file is being transfer to the B machine, however the file's size is 0 byte and does not have any contents (which it really contains in machine A), then I run my script second time and this time machine B does contain content, may I know which part my script cause this problem? Million thanks for the help!

#!/bin/bash
IPADDRSTR="$1"
COUNT=0
for x in $IPADDRSTR
do
#server
    IPERRSTR[$COUNT]=$(sshpass -p pass ssh -o StrictHostKeyChecking=no -p 22 root@$x '$(/usr/bin/nc -l -p1234 > /root/a.txt)&' > /dev/null 2>&1 | echo -n $?)

        if [ ${IPERRSTR[COUNT]} -eq "0" ]
        then
            nc $x 1234 < ./a.txt
            echo "done"
        fi
    COUNT=$((COUNT+1))
done

Answer 1

rsync may be a more elegant utility here. I am just going to get you started. Your code seems okay to me, but they make good points above. Read about rsync here: https://www.digitalocean.com/community/tutorials/how-to-use-rsync-to-sync-local-and-remote-directories-on-a-vps

#!/bin/bash
IPADDRSTR="$1"
COUNT=0
for x in $IPADDRSTR
do
#server
    IPERRSTR[$COUNT]=$(sshpass -p pass ssh -o StrictHostKeyChecking=no -p 22 root@$x '$(/usr/bin/nc -l -p1234 > /root/a.txt)&' > /dev/null 2>&1 | echo -n $?)

        if [ ${IPERRSTR[COUNT]} -eq "0" ]
        then
            #######Place rysnc command here instead.######## 
            echo "done"
        fi
    COUNT=$((COUNT+1))
done

Answer 2

Thanks all guys, I think I have solved the problem, here is the cause and solution

The reason why the transferred file is 0 byte is that the transfer has not even started! As the nc client side is called right after the nc listening, there will be chance of the nc client start faster than the listening process, so I try to sleep sometime (~1s) after the nc -l command to make sure the listening process is started before the client start, then everything works fine now.

Thanks all for the reply!