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Java hangs even though shell script’s execution is completed

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Old 08-14-2010
Error Java hangs even though shell script’s execution is completed
I'm trying to execute a script from within my java code. The execution of the script is over(it's pid is no more), but java is stuck on waitFor() method of the shell script process!. And yes, I'm reading output and error streams in 2 separate threads. Yes, they are being joined at the end(after waitFor()).

The aspect which makes this question relevant to this forum is, if I put the following line in the shell script(at the top), everything seems to work fine.

Code:
exec 3>&1 > /tmp/some_log 2>&1

And no, interestingly enough, the script doesn't generate any output!. Zero chars!. So putting exec statement there makes no sense!

But still, magically enough, putting exec statement in the script makes java work!. Why??

How can I avoid that illogical exec statement in the script?.

exec has any other functionality here, other than redirection?

P.S: This definitely has got something to do with exec command, hence, this question is meant for shell/script guys and not necessarily for Java guys.
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LEARN ABOUT CENTOS

java-wrappers

JAVA-WRAPPERS(7)						   Java wrappers						  JAVA-WRAPPERS(7)

NAME

       java-wrappers - capacities shared by java wrapper scripts

DESCRIPTION

       Most  Java programs are not run directly, but through small shell scripts that take care of various things, such as finding a suitable java
       environment and looking for libraries.

       To ease the task of the maintainers, they rely on a library providing runtime detection, library detection and other features. This  manual
       page  describes	the  common  features of all those scripts, and to which point you can configure them. This is mainly done via environment
       variables.

ENVIRONMENT VARIABLES

       java-wrappers understands some environment variables:

       JAVA_CMD
	      The java command that will be run. If this variable is set, it disables all lookup for a java runtime.

       JAVA_BINDIR
	      Specifies a directory that will be looked for a java or a jdb executable (depending on the setting of JAVA_DEBUGGER).  It has prece-
	      dence over JAVA_HOME but not over JAVA_CMD.

       JAVA_HOME
	      A  path  to a java runtime. If this variable is set, all lookup for a java runtime is disabled, except that if no java executable is
	      found in the path, the command java is used.

       JAVA_FLAVOR
	      A probably more easy-to-use version of the JAVA_HOME variable: instead of specifying the full path of the java runtime, you name it.
	      List of available flavors can be found in the file /usr/lib/java-wrappers/jvm-list.sh.  See examples below.

       JAVA_DEBUGGER
	      If  this	is  set,  the wrapper will try to pick up a java debugger rather than a java interpreter. This will fail if the jbd of the
	      runtime found is a stub.

       JAVA_CLASSPATH
	      Additional classpath, will have priority over the one found by the wrapper.

       JAVA_ARGS
	      Additional arguments to the java command. They will come before all other arguments.

       FORCE_CLASSPATH
	      If this variable is set, it will be the only classpath.  You'd better know what you are doing.

       DEBUG_WRAPPER
	      This is probably the most important variable; if it set, the wrapper will print out useful information as it goes by  its  business,
	      such as which runtime it did find, and which command is run eventually.

       JAVA_JARPATH
	      The  path  where	the  wrappers  will  go  looking  for  jar archives. If not set, the wrapper will look into the default directory,
	      /usr/share/java.	Warning : the wrapper will not look anywhere else than in JAVA_JARPATH.  Setting it incorrectly will most probably
	      result in early crashes.

EXAMPLES

       The examples all rely on rasterizer(1), from the package libbatik-java, but they really apply to all scripts that use java-wrappers.

       Print out debugging information:

       DEBUG_WRAPPER=1 rasterizer

       Limit rasterizer's memory to 80 MB:

       JAVA_ARGS=-Xmx80m rasterizer

       Force rasterizer to run with kaffe(1):

       JAVA_HOME=/usr/lib/kaffe rasterizer

       The same, but using JAVA_BINDIR:

       JAVA_BINDIR=/usr/lib/kaffe/bin rasterizer

       Force rasterizer to run with openjdk:

       JAVA_FLAVOR=openjdk rasterizer

       Debug rasterizer with Sun's debugger, while printing debugging information from the wrapper:

       DEBUG_WRAPPER=1 JAVA_CMD=/usr/lib/jvm/java-6-sun/bin/jdb rasterizer

BUGS

       Care has been taken to make the wrappers bug-free. If that was not the case, please file a bug report against the java-wrappers package.

       If  you	wish  to submit any problem with a java executable relying on java-wrappers, please also submit the output of the command run with
       DEBUG_WRAPPER=1.  It will save one mail exchange and therefore potentially reduce the time it takes to fix the bug.

DEVELOPERS

       There is currently no documentation about writing a wrapper script save the comments in	/usr/lib/java-wrappers/java-wrappers.sh.   If  you
       have  to  write	one,  we  suggest  you base yourself upon, for instance, the rasterizer wrapper script, or any other one (just pick up any
       direct reverse dependency of java-wrappers and look for scripts).

SEE ALSO

       java(1), jdb(1)

       /usr/lib/java-wrappers/java-wrappers.sh

AUTHOR

       java-wrappers and its documentation were written by Vincent Fourmond <fourmond@debian.org>

Version 0.1.16							    2010-05-04							  JAVA-WRAPPERS(7)