java program in background

nephilim · 10-17-2003, 01:26 AM

I wrote a java program that reads from standard input.

However, if the program is launched in background, it crashes because a background process can't read from standard input. The message I get is:

[1] + Stopped (SIGTTIN) name_of_my_program

So, the obvious question to ask: is it possible to check if my java program is launched in the background so I can redirect standard input?

UltimaGuy · 10-17-2003, 02:11 AM

Yes, you can check whether the command line option includes '&' and if so, then you can use this to redirect the stdin. You have to check whether any of the args in main is '&'.

nephilim · 10-17-2003, 02:33 AM

Thank you for the answer, but that won't work for me. The java command line is part of a script and the script is launched in the background. So there's no "&" in the JVM arguments.

I could define my own JVM parameter, but I would rather check in the code if the program is launched in the background. Is this possible?

UltimaGuy · 10-17-2003, 06:18 AM

Well, you have to check for the & in the script command line parameters. Are you using a bash script, or perl? Both these support command line parameters, and you have to just check for them. And if '&' is present, just call java with an option line "BACK", which you can check for in the args string, and redirect the stdin.

nephilim · 10-17-2003, 06:31 AM

I see, that should do the trick indeed. Could you give me an example how a bash script can check for the "&" command line parameter?

nephilim · 10-17-2003, 06:47 AM

What I actually mean is, how can I check if $1 (the first command line parameter) exists?

In other words, how to verify if there are command line parameters?

UltimaGuy · 10-17-2003, 06:51 AM

If there are no parameters, the value of the $1 will be ""(NULL), so you have no problem in checking for that. And also if there is no $1, there will be no $2 etc..

I am not very good in bash scripting, but if you want, you can build a perl script, which I have some experience, and which will be easier to customize.

But if what you do is simple, you must stick with bash.

nephilim · 10-17-2003, 07:04 AM

Yes, it works now! Thank you VERY much, UltimaGuy!

nephilim · 10-17-2003, 07:18 AM

I was too quick, it doesn't work...

I tested it with a bogus parameter and that worked, but it seems that "&" doesn't count as a command line parameter...

If I do ./myscript & and in the script I echo $#, the output is 0.

Now what?

UltimaGuy · 10-17-2003, 07:59 AM

In your script, check for '&' using '\&', as you have to escape system variables before checking with them.

And I don't know what echo $# does

nephilim · 10-17-2003, 08:12 AM

echo $# displays the number of command line parameters. This is 0 because apparantly the & isn't considered to be a command line parameter.

I'm beginning to think I have to create 2 different scripts to solve my problem...

UltimaGuy · 10-20-2003, 06:49 AM

Whatever command line parameters I give, 'echo $#' just outputs 0(zero). I don't think that has anything to do with your problem. And, yes '&' is certainly a parameter, and I think you can check for it. I'll try this one with some of my friends and come back.