How can I access the STDIN of a subprocess? - linux

Question

I want to run the command:
nc localhost 9998
Then I want my script to monitor a file and echo the contents of the file to this sub process whenever the file changes.
I can't work out the re-direction scheme. How can get access to the STDIN of the subprocess?

Answer 1

How about
tail -f $file |nc localhost 9998
Edit:
Since you already have a buffer, then you can try something like this:
while [ 1 ]; do
# Your stuff here.
buf=yourfunctionhere
buffer=$buffer$buf
if [ ! -z $buffer ]; then
echo $buffer |nc localhost 9998
# Empty buffer on success.
if [ $? -eq 0 ]; then
buffer="";
fi
fi
done

Answer 2

mkfifo X
some_program <X >output &
create_input >X
some_program will block reading X until create_input writes to it.

Answer 3

Two solutions that I found acceptable:
1) use coprocess, this way we have access to stdin and stdout of the process created by the coprocess command via the COPROC[0/1] array.
2) What I ultimately did is separate my application into two code blocks as shown below. The first block writes to stdout, that is then piped to the stdin of the second block. This gives me a clean way to buffer data when there are issues with netcat in the second code block:
{ while true;
write to STDOUT; } |
{ while true
nc localhost 9998 }
(in actuality the script is far more complex as the second command provides to-disk buffering when netcat is unable to connect, but the use of the pipe provides buffering so that data isn't lost when a network issue interrupts netcat)

Answer 4

I found a solution using diff and a simple bash script.
The following script execute cat $file > $namedpipe when file change. This is the script I made check-file.sh:
#!/bin/bash
file=$1
tmp=`mktemp`
cp "$file" "$tmp"
namedpipe=`mktemp`
rm -rf $namedpipe
mkfifo $namedpipe
function cleanup() {
echo "end of program"
rm -rf $tmp
rm -rf $namedpipe
exit 1;
}
trap cleanup SIGINT
tail -f $namedpipe 2> /dev/null | netcat localhost 9998 &
while true; do
diff=$(diff "$file" "$tmp")
if [ ! -z "$diff" ]; then
cat $file > $namedpipe
cp $file $tmp
fi
sleep 1
done
This script take as an input the name of a file. For example try these commands in your environment (whit netcat -l 9998 running):
touch /tmp/test
bash check-file.sh /tmp/test &
echo "change 1" > /tmp/test
sleep 1
echo "change 2" > /tmp/test
sleep 1
echo "change 3" > /tmp/test
Note: The temp file get cleaned up by the trap, so you can interrupt this script gracefuly.