use java to write a linux tool

You can create a bash file /usr/bin/custom_program pointing to your jar:

#!/usr/bin/bash
java -jar /path/to/your/java.jar # add any custom command line switches here

Don't forget to mark it as executable with sudo chmod u+x /usr/bin/custom_program.

Your output would be given as System.out.println() calls in your program -- no need to return String from main.

Then, you can call your java program with custom_program with the help of the bash script above.

In addition of a shell script, you might also use binfmt_misc tricks.

I prefer the shell script, it is probably more portable (some Linux system might not load, or have available, the kernel module required to support binfmt_misc)

As the kernel's Documentation/java.txt explains:

2) You have to compile BINFMT_MISC either as a module or into the kernel (CONFIG_BINFMT_MISC) and set it up properly. If you choose to compile it as a module, you will have to insert it manually with modprobe/insmod, as kmod cannot easily be supported with binfmt_misc. Read the file 'binfmt_misc.txt' in this directory to know more about the configuration process.

3) Add the following configuration items to binfmt_misc (you should really have read binfmt_misc.txt now): support for Java applications:

 ':Java:M::\xca\xfe\xba\xbe::/usr/local/bin/javawrapper:'

support for executable Jar files:

 ':ExecutableJAR:E::jar::/usr/local/bin/jarwrapper:'

support for Java Applets:

 ':Applet:E::html::/usr/bin/appletviewer:'

or the following, if you want to be more selective:

 ':Applet:M::<!--applet::/usr/bin/appletviewer:'

Of course you have to fix the path names. The path/file names given in this document match the Debian 2.1 system. (i.e. jdk installed in /usr, custom wrappers from this document in /usr/local)

using gcj

Another possibility (which I don't recommend) would be to use gcj (the old Java compiler inside GCC).

gcj enables you to compile a Java program into a native Linux ELF binary executable. But I don't recommend it because:

• few (and less and less) people inside the GCC community are working on gcj
• few people are using gcj ; I only met only one occasional developer using it
• so gcj is becoming obsolete
• gcj generated code which allocates a lot does not perform very well (because it uses Boehm garbage collector which is much slower than the GC in other Java implementations). However on Java code with a low allocation rate it may run quite fast (because of the powerful optimizations of GCC done in GCC language-neutral middle-end).
• the Java language supported by gcj is a subset of some quite old Java standard.