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-   -   Writing to and reading from a socket from bash script. (http://www.linuxquestions.org/questions/programming-9/writing-to-and-reading-from-a-socket-from-bash-script-794812/)

vouser 03-11-2010 05:00 PM

Writing to and reading from a socket from bash script.
 
Hello all.
Here the description of the issue I am having.
I am writing a bash test script which reads lines from a file, builds ISO messages, sends them to a server,
reads the response with response code and reports the result of the test to a file or on the screen.

The message that I need to send is 94 characters long.
Here's the portion of a code that I initially wrote:

###############################################

#~ Open socket.
exec 3<>/dev/tcp/172.26.0.25/9991

#~ Send msg.
echo "$msg_out" >&3 # OK but adds EOL character which confuses the server.


#~ Receive msg.
read -r msg_in <&3
echo "msg_in: $msg_in"

###############################################

It works OK with the "echoserver".
However, it does not on a real server.
The symptom is that the program is blocked on "read" statement.
One of the facts that is confirmed by server people is that the message received by a server is actually 95 bytes long.

So, based on this information I have 2 questions:

1. Is there any other way to do what I do in line "echo "$msg_out" >&3" to avoid End Of Line character?
2. Am I using the correct way to read from socket? "read -r msg_in <&3".

Thanks for your help,
vouser.

theNbomr 03-11-2010 05:09 PM

Never tried using your method of using sockets in a shell script. When confronted by such tasks, I typically turn to netcat (nc, on most installations). Perhaps your newline is coming from the echo command, and can be supressed with the -n switch.
Code:

echo -n "$msg_out" >&3
--- rod.

tuxdev 03-11-2010 05:58 PM

1. Use printf(1), not echo.
2. Use the -u argument to read.

vouser 03-11-2010 06:30 PM

Rod:
thanks a lot for your post.
It took care of my question #1.
With "-n" switch the server people confirmed that they get exactly 94 bytes.
I still have a problem reading from the server, which is my question #2.
What's interesting is that with "-n" switch even "echoserver" can't read the msg back (it could before).
Looks like I need a better way to read.
Thanks again,
vouser.

ntubski 03-11-2010 08:11 PM

You need -n for read as well, otherwise it waits for a newline.

x29a 06-08-2010 06:37 PM

just in case anybody was wondering, i ended up using dd:

Code:

RETURN=`dd bs=$1 count=1 <&5 2> /dev/null`
with $1 beeing the amount of bytes to read. this wont block while reading the result.

the complete script can be found here: http://blog.chris007.de/?p=238

shearerg 05-10-2012 05:13 PM

Hi all,

I found that the method outlined at the top of this post works fine ... as long as you use printf instead of echo, as recommended by 'tuxdev'. I've also found this in other non-socket related scripts. Result is quite an elegant solution ... as long as a blocked read is ok.

Greg


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