String[] and exec() in Java

[tmiranda]

I'm launching a process in Java (from a Sage plugin) using this command:

process = Runtime.getRuntime().exec(command, env);

If I setup command like this it works:

String[] command = {"wine","/opt/sagetv/server/comskip/comskip.exe", "/opt/sagetv/server/comskip/SpongeBob.mpg"};

If I do this it fails: (It launches wine which in turn launches comskip but then comskip errors out saying the file supplied is not an MPEG.)

List<String> CommandList = new ArrayList<String>();
CommandList.add("wine");
CommandList.add("/opt/sagetv/server/comskip/comskip.exe");
CommandList.add("/opt/sagetv/server/comskip/SpongeBob.mpg");
String[] command = (String[])CommandList.toArray(new String[CommandList.size()]);

What am I doing wrong? I tried different variations converting the List to an array including a for loop like this:

for (int j=0; j<CommandList.size(); j++) {
   Command[j] = CommandList.get(j);
}

[tmiranda]

I sorted this one out. I simplified the code in my example above, the FileName parameters to comskip was actually a quoted String, like this:

String FileName = "SpongeBob.mpg";
String QuotedFileName = "\"" + FileName + "\"";

and that caused the problem. When I just used CommandList.add(FileName) it worked. So this leads me to another questions - How do I pass a quoted string to comskip? If I do not enclose the string in quotes any FileName that has a white space in it will fail because comskip uses a space as a delimiter.

[Slugger]

When you call exec() with an array like you are, Java takes care of quoting the args so white spaced args should work fine. You only need to worry about quoting white spaced args if you're using the single String arg version of exec() (i.e. passing the entire command + args as a single string argument to exec()).

[tmiranda]

OK, but I thought comskip itself needs to have the file name in quotes so it can tell it's one parameter, not two.

So if I pass an array {"wine","comskip","some file name.mpg"} won't comskip see three parameters (some, file, name.mpg) instead of one?

[jreichen]

It doesn't sound like it. I've never used that method before but it sounds easier. I know when I built the whole string myself I had to escape spaces with backslashes on Linux. Something like this:

s.replaceAll(" ", "\\ ");

Two backslashes are required because they're in a Java literal string and it will escape them too.

[Slugger]

Originally Posted by tmiranda

OK, but I thought comskip itself needs to have the file name in quotes so it can tell it's one parameter, not two.

So if I pass an array {"wine","comskip","some file name.mpg"} won't comskip see three parameters (some, file, name.mpg) instead of one?

Think of it this way: When you pass the command line as an array of strings, Java automatically quotes each element of the array. That's why you had problems when you quoted the string yourself; Java was quoting your quoted string. Hopefully that makes sense?

[tmiranda]

Yep, it works without doing anything special. I'm pleasantly surprised. Thanks for the help.

Now only this issue to sort out: http://forums.sagetv.com/forums/showthread.php?t=49935

[GKusnick]

Originally Posted by tmiranda

OK, but I thought comskip itself needs to have the file name in quotes so it can tell it's one parameter, not two.

The part you're missing is that command-line programs don't receive their arguments as one big unbroken string representing the entire command line. They receive them as an array of individual argument strings.

When you type in a command line manually from the console, you must quote the individual arguments so the shell (not the program being called) knows how to parse them into an array.

But when you pass an array of arguments into exec(), there is no such parsing, because the arguments are already in the form that the program expects. So no additional quoting of arguments is needed in that case.

[tmiranda]

Greg,

Now I understand. Thanks.

I was hesitant at first to use exec() with an array instead of one big String, but now I'm glad I did. It's much cleaner and generally avoids problems. It's especially nice that I can pass environment variables as well. That came in handy.

Tom