A script to test for traffic from a streaming server? - linux

Question

I have an instrument that streams data out a tcp port, and I'd like to use standard tools in a script to determine if the stream is available.
Manually I use ncat, which promptly exits if the data stream isn't available.
Here's my initial bash script:
#!/bin/bash
ncat somehost 1234 >/dev/null &
pid=$!
sleep 1
if [ -d /proc/$pid/ ]; then
kill -KILL $pid
echo "It's alive, ALIVE\!"
# Launch clients
else
echo "He's dead, Jim."
# Perform resurrection
fi
It works, but I'm wondering if there is a simpler or better way to accomplish this that doesn't rely on job control or procfs or even ncat. I'd also like to know how much data was sent before a timeout exires.
And, yes, in bash ncat host port can be replaced by cat </dev/tcp/host/port, but I'd also like to avoid bash-isms (so it can work under busybox).
Another approach would be to use wc to count the lines/chars output by ncat, since it outputs only one line and exits if the connection can't be made. But I can't wait forever for termination if the stream is up, so I'd need to use something like timeout, which has its own complications when trying to access the output of a command.
Is there a "plain" approach that works with minimal dependencies?
Or should I write a simple tool instead? It would return the number of bytes read if a connection was made to host/port, a negative errno otherwise, and would support a wait time and protocol spec (tcp/udp). A return value of 0 would mean the connection was made, but no data arrived before the wait time expired (a good thing to know).
Or maybe patch ncat to do the above?

Answer 1

I use netcat in FreeBSD, which has a -z option that simply checks whether a port is open. This eliminates the background and sleep you're using in your script.
-z Specifies that nc should just scan for listening daemons, without
sending any data to them.
The option exists in netcat on an old Ubuntu box here, so that might be an option for you.
$ nc -z somehost 1234 && echo "It's alive, ALIVE!"
I don't see an equivalent option in ncat. You may be able to compile netcat into busybox; it's much smaller than ncat, at least on my systems:
Linux:
$ ls -lH `which nc ncat`
-rwxr-xr-x 1 root root 31296 2010-02-21 01:32 /bin/nc
-rwxr-xr-x 1 root root 130448 2009-11-06 04:39 /usr/bin/ncat
FreeBSD:
ls -l `which nc ncat`
-r-xr-xr-x 1 root wheel 28112 Jan 15 14:53 /usr/bin/nc
-rwxr-xr-x 1 root wheel 182775 Mar 19 2012 /usr/local/bin/ncat
Of course, this doesn't help you check the amount of traffic that has come through the stream, or to analyse its content. For that, I think your solution is innovative and reasonable, though the challenge you've presented may not be resolvable easily.
You can use ps to avoid the procfs dependency, and you can store things in a temp file for analysis rather. Note that busybox should include a mktemp, but you should probably check the options. I haven't tested this:
#!/bin/sh
TMPFILE=`mktemp /tmp/str.XXXX`
trap "rm -f $TMPFILE" 0 1 2 3 15
nc somehost 1234 > $TMPFILE &
pid=$!
sleep 1
if ps $pid >/dev/null; then
kill -KILL $pid
echo -n "It's alive, "
if [ -s $TMPFILE ]; then
echo "ALIVE!"
else
echo "but unresponsive."
fi
else
echo "He's dead, Jim."
fi
That's all I've got. And it's not much different from what you already have.
Another option might be to build a custom tool that you might be able to compile into your own busybox, but that'll be a c question rather than a shell question. :-)