UNIX for Dummies Questions & Answers

hi Everybody

I am trying to capture process id of a process which has been started through the execution of a shell script

Code:

CLASSPATH=.
CLASSPATH=${CLASSPATH}:sampleswing.jar
java -cp ${CLASSPATH} Sample
echo $! > pid.sh

above is the simple script file called swings.sh

When i execute swings.sh , script internally creates another java process.
I need the process id of the newly created java process and not the process id of the shell script.

Actually when i execute shell script, java process does not return until the java application is closed thru the application's exit button.so the command next to " java -cp ${CLASSPATH} Sample " does not get executed until the invoked java process is closed thru application's exit button.

In this case how can i get the process id of the java process simultaneoulsy when executing the shell script. (something like... java process should be daemon process and next line should be executed once the java process is started)

Suppose if i execute following command alone independently after running the above script without the last line in a different console , I get the process id of the newly created java process.

Code:

echo $!

My requirement is to execute shell script as well as get the process id of the java process on a single run of the shell script.

Kindly shed some light on getting the process id of the java process when executing the script.

I am storing this PID in another file for the very reason ,when java application hangs up, sometimes even after pressing ctrl+C, the process is not exiting

$! is the process ID of the last command run in the background not the last command run. So if you do:

Code:

java -cp ${CLASSPATH} Sample &

Then it should work. Either that or change your script to:

Code:

CLASSPATH=.
CLASSPATH=${CLASSPATH}:sampleswing.jar
echo $$ > pid.sh
exec java -cp ${CLASSPATH} Sample

The effect of the exec command is to cause the command after it to be run in place of the current shell, but no new process is created. Thus it will have the same process ID as the shell it is run from.

Thanks a lot Unbeliver
Ur first command worked out to be suitable for my requirement.
Now comes another problem, when i execute below script

Code:

CLASSPATH=.
CLASSPATH=${CLASSPATH}:sampleswing.jar
java -cp ${CLASSPATH} Sample &
echo "kill -9 "$! > killer.sh
chmod 777 killer.sh

output of the application "Sample" is getting displayed on the console from where i have executed the shell script.

Kindly help me in not directing the output and error to the console from where i have started the shell script.i do not want to handle those logs ... it can also be made nullified.

I also tried nohup command before
In this case, all logs were transferred to some file called nohup_out ...even i do not want that to happen beacuse internally i have used log4j to redirect my logs to some other files in another directory and also some error logs are still directed to the console from where i started the shell script.

Reply wud be very much appreciated

If I read your reply correctly, I think you want to throw away all the output, not have it written to nohup.out.

If you use nohup then the only way to stop it creating a nohup.out file is to make sure you redirect both stdout and stderr to somewhere else. So for instance if your script was called 'run_sample.sh' you would use:

Code:

nohup ./run_sample.sh > /dev/null 2>&1 &

If this is not what you want then please explain again.

Once again Thanks very much unbeliver

my requirement is to throw away the logs sent to the stdout and stderr .It seems it is working with ur command.

can u please explain the whole command as what you are tyring to do with this command with the split of each part in that command.

In the above u are directing to /dev/null ... means what
2>&1 ... means what and also why & infront of 1

& .... to make the entire operation to be background process
Basically, run_sample.sh is going to contain the following commands

Code:

CLASSPATH=.
CLASSPATH=${CLASSPATH}:sampleswing.jar
java -cp ${CLASSPATH} Sample &
echo "kill -9 "$! > killer.sh
chmod 777 killer.sh

already i am using '&' inside shell script to make the java process run as background.

Do we still need to make the shell script run as background in order to achieve whatever i said .. basically i want final command that can go for prod enviroment.

please do give me final and consolidated command ....

Thanks once again and i very much appreciate

Code:

nohup ./run_sample.sh > /dev/null 2>&1 &

The two '&' characters have very different meanings. Let me go through the line bit by bit.

nohup: This command allows you to run other commands in such a way that they ignore the SIGHUP signal. Most importantly this means if you exit the shell that they are started from they *do not* exit (ie. if youlogout from the system they continue to run).

./run_sample.sh: This, of course, is your script.

> /dev/null: '>' is short for '1>' and tells the shell to redirect file descriptor 1 (standard out). In this case it is redirected to /dev/null (ie. it it is thrown away).

2>&1: This is another command to the shell to redirect another file descriptor. In this case it tells the shell to redirect file descriptor 2 (thats the '2>' bit) which is standard error, to file descriptor '1' (thats the '&1' bit), File descriptor 1 has already been pointed at /dev/null. So all output to standard error goes to /dev/null as well.

&: At the end of the line this indicates to the shell that the process should be run in the background

Please note the order is important.

Code:

my_command > /dev/null 2>&1
  and
my_command 2>&1 > /dev/null

Have very different results. I'll leave you to work out the difference

If you wish just the java process to run uninteruptted in the background then you can just move the 'nohup inside the shell script:

Code:

CLASSPATH=.
CLASSPATH=${CLASSPATH}:sampleswing.jar
nohup java -cp ${CLASSPATH} Sample > /dev/null 2>&1 &
echo "kill -9 "$! > killer.sh
chmod 777 killer.sh

Excellent explaination !!!!

Thanks a lot for ur replies, unbeliver

Now shell script have got some good shape in executing those java process.
Next concern is , i want to do preliminary check before executing java process inside the shell script ...

Code:

CLASSPATH=.
CLASSPATH=${CLASSPATH}:sampleswing.jar
nohup java -cp ${CLASSPATH} Sample > /dev/null 2>&1 &
echo "kill -9 "$! > killer.sh
chmod 777 killer.sh

Preliminary check is necessary to make sure that any already spawned java process are killed completely before starting another process with the shell script. For this purpose only , i have stored the process ID of the java process in a file called Killer.sh which contains command
Basically i want to execute killer.sh as a first command before executing
Suppose for the first time , if there is no killer.sh available , then there should not be any problem when i execute it.

Kindly get me a code that can be included before executing
which kills the already running java process, also it indirectly make sure that only one java process can be run at any time.Code construct also shud handle if that file 'killer.sh' is not available.

Reply wud be very much appreciated