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running script in background thru Runtime.exec() hangs

 
shiva_try
New Member

running script in background thru Runtime.exec() hangs

Hi all,
I tried to ran a script in Linux (which has a background process) through the Java code using Runtime.exec(), it gets hanged. The same script is working fine when i ran it directly.

Is it a linux issue ? or any workaround is there ?
Any help would be highly appreciated.

thanks in advance
shiva.t
15 REPLIES 15
Bojan Nemec
Honored Contributor

Re: running script in background thru Runtime.exec() hangs

Hi,

I tried to write a short Java class on my Linux:

class Test
{
public static void main (String [] arg)
{
if (arg.length < 1)
System.exit (0);
try
{
Process p = Runtime.getRuntime().exec (arg[0]);
p.waitFor ();
} catch (Exception e)
{
e.printStackTrace ();
}
}
}

I run a script which puts another script in background. It works, except that I get no output on my terminal.

Linux is RedHat 8 and Java is Suns 1.4.1_01.

Bojan
shiva_try
New Member

Re: running script in background thru Runtime.exec() hangs

hi bojan,

thanks for your immediate response.

For your code pass the process to the stream, now you will get the output.
----
BufferedReader in = new BufferedReader(new InputStreamReader(p.getInputStream()));
while ((str = in.readLine()) != null) {
System.out.println(">>"+str);
}
}catch (FileNotFoundException fnfe) {
}catch (IOException fnfe) {}
-------

In my case, i am calling the script "startservs" which internally executes a java
code in background, which is used to start the server.
--
Process p=Runtime.getRuntime().exec(startservs);
--

Here it couldn't complete its process. So, it gets hanged. My aim is to complete the
process with the output.

thanks
shiva.t
Bojan Nemec
Honored Contributor

Re: running script in background thru Runtime.exec() hangs

shiva,

I misunderstood the java.lang.Process documentation (read to fast ;). With yours code pass the program runs ok and with output. I tested also with running another java code and it also work ok.

Maybe yours script or java code needs some input? If you look at the documentation http://java.sun.com/j2se/1.4.2/docs/api/java/lang/Process.html
there is a pass:
"Because some native platforms only provide limited buffer size for standard input and output streams, failure to promptly write the input stream or read the output stream of the subprocess may cause the subprocess to block, and even deadlock."

Bojan
Mohammed Saleem_1
New Member

Re: running script in background thru Runtime.exec() hangs

hi shiva,

i have the same problem.

have you found a solution for this? please share.

saleem
Mohammed Saleem_1
New Member

Re: running script in background thru Runtime.exec() hangs

Bojan,

Can you share the script which had the background process?

I think the issue comes when the background process is a continuous one.

Thanks,
Saleem
Bojan Nemec
Honored Contributor

Re: running script in background thru Runtime.exec() hangs

Hi,

I did this only for test. Substitute my test directory /home/bojan/tmp with yours. The first is a one line script (testa):

/home/bojan/tmp/testb &

The second script (testb):

echo `date` >> /home/bojan/tmp/test.log
ls /home/bojan/tmp/
pwd
java SubTest 1 2 3 4
echo `date` >> /home/bojan/tmp/test.log


Now the two Java classes, the first (rewised with Shivas suggestions):

class Test
{
public static void main (String [] arg)
{
if (arg.length < 1) System.exit (0);
try
{
Process p = Runtime.getRuntime().exec (arg[0]);
BufferedReader in = new BufferedReader (new InputStreamReader(p.getInputStream()));
String str;
while ((str = in.readLine()) != null) {
System.out.println(">>"+str);
}
p.waitFor ();
System.out.println ("Exit");
} catch (Exception e)
{
e.printStackTrace ();
}
}

And a dummy SubTest:

class SubTest
{
SubTest (String [] args)
{
try
{
for (int i = 0; i < args.length; i++) {
System.out.println ("arg " + i + " = " + args[i]);
Thread.currentThread().sleep(Long.parseLong (args[i]) * 1000);
}
} catch (Exception e)
{
e.printStackTrace ();
}
}
public static void main (String [] args)
{
new SubTest (args);
}
}

You run all with:
java Test /home/bojan/tmp/testa

Attached is this text, to save java indents.
Bojan
Mohammed Saleem_1
New Member

Re: running script in background thru Runtime.exec() hangs

In your testb script, the execution stops after a point of time. But for me, I am running a server and it will continue to run till I execute the stopscript. In this scenario, it hangs, because the execution doesn't stop.
Bojan Nemec
Honored Contributor

Re: running script in background thru Runtime.exec() hangs

Hi,

If I understand well, this is the same as if you modify my Test class in:

class Test
{
public static void main (String [] arg)
{
if (arg.length < 1) System.exit (0);
try
{
Process p = Runtime.getRuntime().exec (arg[0]);
} catch (Exception e)
{
e.printStackTrace ();
}
}

?

I try, it exit immediately after creating the process, the background script lives and do its job. The same is if I ommit testa and run directly testb or SubTest.

Bojan
shiva_try
New Member

Re: running script in background thru Runtime.exec() hangs

hi bojan,

Thanks for your frequent responses.

In your code, the loop will execute based on your arguments length. Anyway the process would complete with delay.

Whereas in our case the server will going to run continuously. So, i assume that checking with infinite loop in background will be useful

Any ideas will be hightly appreciated.
Thanks
Shiva